Mips implementation

University of Ulster at Jordanstown
University of Applied Sciences, Augsburg

Master of Engineering
VLSI Design Project Report

Processor
Implementation
in VHDL
According to Computer Organisation & Design
by David A. Patterson and John L. Hennessy

Author(s): M. Linder
M. Schmid

Supervisor(s): J. Färber
A. Eder

Submitted: 06/07/07

Document Revision History, Designers
Department of Electrical Engineering

Document Revision History
Rev. Date Author Description
0.1 15/05/2007 M. Schmid First draft release
0.2 15/05/2007 M. Linder Features of the project
0.3 29/05/2007 M. Linder Target Spec. (2.1, 2.2)
0.4 10/06/2007 M. Linder Target Spec. (2.3)
0.5 30/06/2007 M. Linder - include jump instruction to Target Spec.
- Module Spec. of Control
0.6 02/07/2007 M. Linder Module Spec. of Data
0.6.1 02/07/2007 M. Schmid Module Spec. of ALU and Memory
0.6.2 03/07/2007 M. Schmid Design Tasks
0.7 04/07/2007 M. Linder - Module Spec. of Datapath
- Synthesis Results
- References
0.8 05/07/2007 M. Linder, M. Schmid - Synthesis Results
- Source Code
- Conclusion
1.0 05/07/2007 M. Linder, M. Schmid Final release

Designer(s)
M. Linder michael-linder@web.de
M. Schmid martin-werner.schmid@gmx.de

Contact
Michael Linder
Angerstraße 8a
86356 Neusäß, Germany
Phone: +49 (0) 176 22 93 58 30
Mail: michael-linder@web.de

Martin Schmid
Fichtenstraße 2
86500 Kutzenhausen, Germany
Phone: +49 (0) 160 92 94 91 54
Mail: martin-werner.schmid@gmx.de

M. Linder, M. Schmid II

Contents

Contents

1 Introduction................................................................................ 1
1.1 Starting from a Simple Implementation Scheme...................................1
1.2 Using Multicycle Implementations.........................................................2
1.3 Enhancing Performance with Pipelining................................................2

2 Target Specification................................................................... 3
2.1 Building a Datapath............................................................................... 3
2.1.1 Major Components.................................................................................... 3
2.1.2 Components for Arithmetic and Logic Functions....................................... 4
2.1.3 Load word (lw) and store word (sw) instructions........................................ 5
2.1.4 Branch on equal instruction....................................................................... 6
2.1.5 Jump Instruction........................................................................................ 6
2.2 Simple Implementation Scheme............................................................7
2.2.1 Creating a Single Datapath....................................................................... 7
2.2.2 ALU Control............................................................................................... 8
2.2.3 Main Control.............................................................................................. 9
2.2.4 Disadvantages of a Single-Cycle Implementation................................... 10
2.3 Multicycle Implementation...................................................................11
2.3.1 Additions and Changes in the Scheme.................................................... 11
2.3.2 Execution of Instructions in Clock Cycles................................................ 14
2.3.3 Defining the Control by a Finite State Machine........................................ 18

3 Design Tasks............................................................................ 21
4 Module Specification............................................................... 22
4.1 ALU......................................................................................................22
4.1.1 Functional Description............................................................................. 22
4.1.2 Block Diagram......................................................................................... 23
4.1.3 Simulation Results................................................................................... 26
4.1.4 Design Files............................................................................................. 26
4.2 Memory................................................................................................27
4.2.2 Block Diagram......................................................................................... 28
4.2.4 Design Files............................................................................................. 29

M. Linder, M. Schmid III

Contents

4.3 Control..................................................................................................30
4.3.2 State Diagram......................................................................................... 31
4.3.3 Block Diagram......................................................................................... 32
4.3.5 Design Files............................................................................................. 33
4.4 Data Path.............................................................................................34
4.4.1 Instruction Fetch...................................................................................... 34
4.4.1.1 Functional Description.................................................................................... 34
4.4.1.2 Block Diagram................................................................................................ 34
4.4.1.3 Design Files.................................................................................................... 35
4.4.2 Instruction Decode.................................................................................. 35
4.4.2.2 Block Diagram................................................................................................ 35
4.4.2.3 Design Files.................................................................................................... 36
4.4.3 Execution................................................................................................. 36
4.4.3.2 Block Diagram................................................................................................ 37
4.4.3.3 Design Files.................................................................................................... 38
4.4.4 Memory Writeback.................................................................................. 39
4.4.4.2 Block Diagram................................................................................................ 40
4.4.4.3 Design Files.................................................................................................... 41
4.4.5 Data Path................................................................................................ 42
4.4.5.1 Block Diagram................................................................................................ 42
4.4.5.2 Design Files.................................................................................................... 42
4.5 Processor and Memroy........................................................................43
4.5.2 Block Diagram......................................................................................... 43
4.5.3 Design Files............................................................................................. 44

5 Synthesis Results.................................................................... 45
6 Results of Prototype Testing.................................................. 47
6.1 Description...........................................................................................47
6.2 Simulation Result.................................................................................49

7 Conclusion................................................................................ 50
7.1 Our own experiences...........................................................................50

M. Linder, M. Schmid IV

Contents

7.2 Annotations to “Computer Organization & Design” [PaHe98].............50
7.3 Further work on the project..................................................................51

8 Appendix................................................................................... 52
8.1 Design files..........................................................................................52
8.1.1 Project Entities........................................................................................ 52
8.1.2 Project Architectures............................................................................... 58
8.1.3 Package.................................................................................................. 79
8.1.4 Testbenches............................................................................................ 80
8.2 References...........................................................................................91

M. Linder, M. Schmid V

Contents

List of Figures

Figure 1.1: Simple block diagram with datapaths [PaHe98] p. 352....................... 1
Figure 1.2: Multicycle Datapath [PaHe98] p. 414...................................................2
Figure 1.3: Pipelined Version of the Datapath [PaHe98], p. 452........................... 2
Figure 2.1: Instruction Memory, Program Counter and Adder [PaHe98], p 344....3
Figure 2.2: Datapath for fetching instructions and incrementing the PC
[PaHe98] p. 345......................................................................................................3
Figure 2.3: Register and ALU [PaHe98] p. 346......................................................4
Figure 2.4: Datapath for R-type Instructions [PaHe98] p. 347...............................4
Figure 2.5: Data Memory and Sign extension unit [PaHe98] p. 348......................5
Figure 2.6: Load or Store Word instruction field.....................................................5
Figure 2.7: Datapath for Load Word and Store Word [PaHe98] p. 348.................5
Figure 2.8: Datapath for a branch instruction [PaHe98] p. 350..............................6
Figure 2.9: Completed Simple Datapath [PaHe98] p. 353.....................................7
Figure 2.10: MIPS field...........................................................................................8
Figure 2.11: Table for ALU Control.........................................................................8
Figure 2.12: Datapath with ALU Control Unit [PaHe98] p. 358..............................9
Figure 2.13: Meaning of the main control signals [PaHe98] p. 359....................... 9
Figure 2.14: The simple datapath with the control unit [PaHe98] p. 360.............10
Figure 2.15: Truth table of the main control unit [PaHe98] p. 361.......................10
Figure 2.16: Abstract view of a multicycle desing [PaHe98] p. 378..................... 11
Figure 2.17: Complete Datapath for multicycle design [PaHe98] p. 383............. 13
Figure 2.18: Actions of 1-bit control signals [PaHe98] p. 384..............................14
Figure 2.19: Actions of 2-bit control signals [PaHe98] p. 384..............................14
Figure 2.20: Summary of the multicycle steps [PaHe98] p. 389..........................18
Figure 2.21: Complete finite state machine control [PaHe98] p. 396.................. 19
Figure 2.22: Setting of Control Signals.................................................................20
Figure 4.1: ALU 1/3...............................................................................................23
Figure 4.2: ALU 2/3...............................................................................................24
Figure 4.3: ALU 3/3...............................................................................................25
Figure 4.4: Simulation Results of ALU..................................................................26
Figure 4.5: Memory...............................................................................................28

M. Linder, M. Schmid VI

Contents

Figure 4.6: Simulation Results of Memory (registered outputs)...........................28
Figure 4.7: Simulation Results of Memory (unregistered outputs).......................29
Figure 4.8: Control Finite State Machine..............................................................31
Figure 4.9: Control FSM........................................................................................32
Figure 4.10: ALU Control......................................................................................32
Figure 4.11: Control..............................................................................................33
Figure 4.12: Simulation Results of the Control FSM............................................33
Figure 4.13: Instruction Fetch...............................................................................34
Figure 4.14: Instruction Decode............................................................................35
Figure 4.15: Execution..........................................................................................37
Figure 4.16: Memory Writeback........................................................................... 40
Figure 4.17: Processing Unit (Datapath & Controlpath).......................................43
Figure 4.18: Processing Unit & Memory...............................................................43
Figure 5.1: Analysis & Synthesis Summary..........................................................45
Figure 5.2: Analysis & Synthesis Settings............................................................46
Figure 5.3: Compilation History............................................................................ 46
Figure 6.1: Simulation Results of MIPS and Memory.......................................... 49

M. Linder, M. Schmid VII

Contents

List of VHDL-Source

VHDLSource 8.1: e_control_ControlFSM.vhd......................................................52
VHDLSource 8.2: e_control_ALUControl.vhd...................................................... 52
VHDLSource 8.3: e_control.vhd...........................................................................52
VHDLSource 8.4: e_tempreg.vhd.........................................................................53
VHDLSource 8.5: e_pc.vhd..................................................................................53
VHDLSource 8.6: e_instreg.vhd...........................................................................53
VHDLSource 8.7: e_regfile.vhd............................................................................54
VHDLSource 8.8: e_alu_vhd................................................................................54
VHDLSource 8.9: e_data_fetch.vhd.....................................................................54
VHDLSource 8.10: e_data_decode.vhd...............................................................55
VHDLSource 8.11: e_data_execution.vhd...........................................................55
VHDLSource 8.12: e_data_memwriteback.vhd................................................... 56
VHDLSource 8.13: e_data.vhd.............................................................................56
VHDLSource 8.14: e_ram.vhd..............................................................................56
VHDLSource 8.15: e_memory.vhd.......................................................................57
VHDLSource 8.16: e_mips.vhd............................................................................57
VHDLSource 8.17: e_procmem.vhd.....................................................................57
VHDLSource 8.18: a_control_ControlFSM.vhd....................................................60
VHDLSource 8.19: a_control_ALUControl.vhd.................................................... 61
VHDLSource 8.20: a_control.vhd.........................................................................62
VHDLSource 8.21: a_tempreg_behave.vhd.........................................................63
VHDLSource 8.22: a_pc_behave.vhd..................................................................63
VHDLSource 8.23: a_instreg_behave.vhd...........................................................64
VHDLSource 8.24: a_regfile_behave.vhd............................................................64
VHDLSource 8.25: a_alu_behave.vhd.................................................................65
VHDLSource 8.26: a_data_fetch.vhd...................................................................67
VHDLSource 8.27: a_data_decode.vhd...............................................................69
VHDLSource 8.28: a_data_execution.vhd...........................................................69
VHDLSource 8.29: a_data_memwriteback.vhd................................................... 70
VHDLSource 8.30: a_data.vhd.............................................................................73
VHDLSource 8.31: a_ram_rtl.vhd.........................................................................73

M. Linder, M. Schmid VIII

Contents

VHDLSource 8.32: a_memory_behave.vhd.........................................................75
VHDLSource 8.33: a_mips.vhd............................................................................77
VHDLSource 8.34: a_procmem.vhd.....................................................................78
VHDLSource 8.35: p_procmem_definitions.vhd.................................................. 79
VHDLSource 8.36: t_alu_fileio.vhd.......................................................................83
VHDLSource 8.37: t_memory.vhd........................................................................86
VHDLSource 8.38: t_procmem.vhd......................................................................87

M. Linder, M. Schmid IX

1 Introduction

1 Introduction
“The performance of software systems is dramatically affected by how well soft-
ware designers understand the basic hardware technologies at work in a sys-
tem.” According to the book “Computer Organization & Design” written by David
A. Patterson and John L. Hennessy the hardware and behaviour of a micropro-
cessor is implemented in VHDL.

1.1 Starting from a Simple Implementation Scheme

In the first section starting from a simple implementation scheme of a MIPS sub-
set the basic hardware of the microcontroller´s datapath and its control is devel-
oped step by step and implemented in VHDL. Testbenches will verify the correct
implementation of the arithmetic-logical instructions (add, sub, and, or and slt),
the memory-reference instructions (load word and store word) and the branch in-
structions (beq and jump).

Figure 1.1: Simple block diagram with datapaths [PaHe98] p. 352

M. Linder, M. Schmid, 07/07 1

1.2 Using Multicycle Implementations

1.2 Using Multicycle Implementations

Figure 1.2: Multicycle Datapath [PaHe98] p. 414

Establishing that the efficiency of a long single-cycle implementation is not likely
to be very good the processor´s speed is improved by using multicycle imple-
mentations. Then, instructions are allowed to take different numbers of clock cy-
cles and functional units can be shared within the execution of single instruc-
tions.

1.3 Enhancing Performance with Pipelining

In order to enhance the performance and to get very fast processors another im-
plementation technique called pipelining is introduced. Multiple instructions are
overlapped in execution so that some stages are working in parallel.

Figure 1.3: Pipelined Version of the Datapath [PaHe98], p. 452

2 M. Linder, M. Schmid, 07/07

2 Target Specification

2 Target Specification

2.1 Building a Datapath

2.1.1 Major Components

At first we look at the elements required to execute the MIPS instructions and
their connection.
The first element needed is a place to store the program instructions. This In-
struction Memory is used to hold and supply instructions given an address.
The address must be kept in the Program Counter (PC), and in order to incre-
ment the PC to the address of the next instruction, we also need an Adder.
All these elements are shown in figure 2.1.

Figure 2.1: Instruction Memory, Program Counter and Adder [Pa-
He98], p 344

After fetching one instruction from the instruction memory, the program counter
has to be incremented so that it points to the address of the next instruction 4
bytes later.
This is realised by the datapath shown in figure 2.2.

Figure 2.2: Datapath for fetching instructions and incrementing the PC
[PaHe98] p. 345


2.1 Components for Arithmetic and Logic Functions

2.1.2 Components for Arithmetic and Logic Functions

The instructions we use all read two registers, perform an ALU operation and
write back the result.
These arithmetic-logical instructions are also called R-type instructions
([PaHe98] p. 154). This instruction class considers add, sub, slt, and and or.
The 32 registers of the processor are stored in a Register File. To read a data-
word two inputs and two outputs are needed. The inputs are 5 bits wide and
specify the register number to be read, the outputs are 32 bits wide and carry the
value of the register.
To write the result back two inputs are needed: one to specify the register num-
ber and one to supply the data to be written. The Register is shown in Figure 2.3.

Figure 2.3: Register and ALU [PaHe98] p. 346
To process the data from the Register, an ALU with two data inputs is used.
Figure 2.4 shows the combination of Register and ALU to operate on R-type in-
structions.

Figure 2.4: Datapath for R-type Instructions [PaHe98] p. 347


2.1 Load word (lw) and store word (sw) instructions

2.1.3 Load word (lw) and store word (sw) instructions

Two more elements are needed to implement the sw- and lw-instructions: the
Data Memory and the Sign Extension Unit.

Figure 2.5: Data Memory and Sign extension unit [PaHe98] p. 348

The sw- and lw-instructions compute a memory address by adding a register val-
ue to the 16-bit signed offset field contained in the instruction.
Because the ALU has 32-bit values, the instruction offset field must be sign ex-
tended from 16 to 32 bits simply by concatenating the sign-bit 16 times to the
original value.
The instruction field for a lw- or sw-instruction is shown in figure 2.6:

op rs rt address
6 bits 5 bits 5 bits 16 bits

Figure 2.6: Load or Store Word instruction field

Figure 2.7: Datapath for Load Word and Store Word [PaHe98] p. 348


2.1 Branch on equal instruction

2.1.4 Branch on equal instruction

The beq instruction has three operands, two registers that are compared for
equality, and a 16-bit offset used to compute the branch target address relative
to the branch instruction address.

Figure 2.8: Datapath for a branch instruction [PaHe98] p. 350

Figure 2.8 shows the datapath for a branch on equal instruction. This datapath
must do two operations: compare the register contents and compute the branch
target.
Therefore two things must be done: The address field of the branch instruction
must be sign extended from 16 bits to 32 bits and must be shifted left 2 bits so
that it is a word offset.
The branch target address is computed by adding the address of the next in-
struction (PC + 4) to the before computed offset.

2.1.5 Jump Instruction

The jump instruction is similar to the branch instruction, but computes the target
PC differently and not conditional.
The destination address for a jump is formed by concatenating the upper 4 bits
of the current PC + 4 to the 26-bit address field in the jump instruction (see figure
2.10 on page 8) and adding “00” as the last two bits.


2.2 Simple Implementation Scheme

2.2 Simple Implementation Scheme

The simplest possible implementation of the MISP Processor contains the datap-
ath segments explained above added by the required control lines.

2.2.1 Creating a Single Datapath

The simplest datapath might attempt to execute all instructions in one clock cy-
cle. This means that any element can be used only once per instruction. So
these elements have to be duplicated.

If possible datapath elements can be shared by different instruction flows. There-
fore multiple connections to the input must be realised. This is commonly done
by a multiplexer.

Figure 2.9 shows the combined datapath including a memory of instructions and
one for data, the ALU, the PC-unit and the mentioned multiplexers.

Figure 2.9: Completed Simple Datapath [PaHe98] p. 353


2.2 ALU Control

2.2.2 ALU Control

The MIPS field that contains the information about the instruction has the follow-
ing structure:
op rs rt rd shamt funct
6 bits 5 bits 5 bits 5 bits 5 bits 6 bits

Figure 2.10: MIPS field
The meaning of the fields are:
• op: basic operation
• rs: first register source
• rt: second register source
• rd: register destination
• shamt: shift amount
• funct: function

Instruction Instruction Desired ALU control
opcode ALUOp operation Funct field ALU action input
LW 00 load word XXXXXX add 010
SW 00 store word XXXXXX add 010
Branch equal 01 branch equal XXXXXX subtract 110
R-type 10 add 100000 add 010
R-type 10 subtract 100010 subtract 110
R-type 10 AND 100100 and 000
R-type 10 OR 100101 or 001
R-type 10 set on less than 101010 set on less than 111

Figure 2.11: Table for ALU Control

Figure 2.11 shows in the last column the 3-bit ALU control input.
It depends on the 6-bit funct field of the MIPS instruction and the 2-bit ALUOp
signal generated form the Main Control Unit (see Chapter 2.2.3).
Figure 2.12 shows the datapath including the ALU Control Unit.


2.2 ALU Control

Figure 2.12: Datapath with ALU Control Unit [PaHe98] p. 358

2.2.3 Main Control

The main control unit generates the control bits for the multiplexers, the data
memory and the ALU control unit.
The input of the main control unit is the 6-bit op-field of the MIPS instruction field
(see figure 2.9 on page 7).
Figure 2.13 shows the meaning of the several control signals.
Signal name Effect when deasserted Effect when asserted
RegDst The register destination number for the Write re- The register destination number for the Write re-
gister comes from the rt field (bits 20-16). gister comes from the rd field (bits 15-11).
RegWrite None The register on the Write register input is written
with the value on the Write data input.
ALUSrc The second ALU operand comes from the se- The second ALU operand is the sign-extended,
cond register file output (Read data 2). lower 16 bits of the instruction.
PCSrc The PC is replaced by the output of the adder The PC is replaced by the output of the adder
that computes the value of PC + 4. that computes the branch target.
MemRead None Data memory contents designated by the ad-
dress input are put on the Read data output.
MemWrite None Data memory contents designated by the ad-
dress input are replaced by the value on the Wri-
te data input.
MemtoReg The value fed to the register Write data input co- The value fed to the register Write data input co-
mes from the ALU. mes from the data memory.

Figure 2.13: Meaning of the main control signals [PaHe98] p. 359


2.2 Main Control

The connection of the main control unit is shown in figure 2.14. This and the
meaning of the signals described in figure 2.13 leads directly to the truth table for
the main control unit shown in figure 2.15.

Figure 2.14: The simple datapath with the control unit [PaHe98] p. 360

Memto- Reg Mem Mem
Instruction RegDst ALUSrc Reg Write Read Write Branch ALUOp1 ALUOp2
R-format 1 0 0 1 0 0 0 1 0
lw 0 1 1 1 1 0 0 0 0
sw X 1 X 0 0 1 0 0 0
beq X 0 X 0 0 0 1 0 1

Figure 2.15: Truth table of the main control unit [PaHe98] p. 361

2.2.4 Disadvantages of a Single-Cycle Implementation

In modern designs a single cycle implementation of a processor is not used, be-
cause it is inefficient.
A clock cycle must have the same length for every instruction and therefore it is
determined by the longest possible path. Almost this is the path of the load word
instruction which uses five functional units in series: the instruction memory, the
register file, the ALU, the data memory and the register file again.
However a single cycle implementation can be used for a small instruction set.
But if the machine gets more powerful there can be used thousands of functional
units and then the longest path causes the cycle time.


2.3 Multicycle Implementation

2.3 Multicycle Implementation

To avoid the disadvantages of the single cycle implementation described in the
section before, a multicycle implementation is used.
This technique divides each instruction into steps and each step is executed in
one clock cycle.
The multicycle implementation allows a functional unit to be used more than
once in a instruction, so that the number of functional units can be reduced.
The major advantage of a multicycle design is the ability to share functional units
within an execution.

2.3.1 Additions and Changes in the Scheme

Figure 2.16 shows a abstract design of a multicycle datapath.

Figure 2.16: Abstract view of a multicycle desing [PaHe98] p. 378

Comparing to the single-cycle datapath the differences are that only one memory
unit is used for instructions and data, there is only one ALU instead of an ALU
and two adders and several output registers are added to hold the output value
of a unit until it is used in a later clock cycle.
The instruction register (IR) and the memory data register (MDR) are added to
save the output of the memory. The registers A and B hold the register operands
read form the register file and the ALUOut holds the output of the ALU.
With exception of the IR all these registers hold data only between a pair of
adjacent clock cycles.


2.3 Additions and Changes in the Scheme

Because the IR holds the value during the whole time of the execution of a
instruction, it requires a write control signal.

The reduction from former three ALUs to one causes also the following changes
in the datapath:
An additional multiplexer is added for the first ALU input to choose between the
A register and the PC.
The multiplexer at the second ALU input is changed from a two-way to a four-
way multiplexer. The two new inputs are a constant 4 to increment the PC and
the sign-extended and shifted offset field for the branch instruction.

In order to handle branches and jumps more additions in the datapath are
required.
The three cases of R-type instructions, branch instruction and jump instruction
cause three different values to be written into the PC:
• The output of the ALU which is PC + 4 should be stored directly to the PC.
• The register ALUOut after computing the branch target address.
• The lower 26 bits of the IR shifted left by two and concatenated with the
upper 4 bits of the incremented PC, when the instruction is jump.
If the instruction is branch, the write signal for the PC is conditional. Only if the
the two compared registers are equal, the computed branch address has to be
written to the PC.
Therefore the PC needs two write signals, which are PCWrite if the write is
unconditional (value is PC + 4 or jump instruction) and PCWriteCond if the write
is conditional.


2.3 Additions and Changes in the Scheme

Figure 2.17 shows the completed datapath for a multicycle implementation
including the whole control.
It also shows that the write signal for the PC is combined form the ALU zero bit
and the two write signals PCWrite and PCWriteCond by an AND gate and OR
gate.

Figure 2.17: Complete Datapath for multicycle design [PaHe98] p. 383


2.3 Execution of Instructions in Clock Cycles

2.3.2 Execution of Instructions in Clock Cycles

The execution of an instruction is broken into clock cycles, that means that each
instruction is divided into a series of steps.
Therefore the setting of the control signals are shown in figures 2.18 and 2.19.

Signal name Effect when deasserted Effect when asserted
RegDst The register file destination number for the Write The register file destination for the Write register
register comes from the rt field comes from the rd field
RegWrite None The general-purpose register selected by the Wri-
te register number is written with the value of the
Write data input.
ALUSrcA The first ALU operand is the PC The first ALU operand comes from the A register
MemRead None Content of memory at the location specified by the
Address input is put on Memory data output.
MemWrite None Memory contents at the location specified by the
Address input is replaced by value on Write data
input.
MemtoReg The value fed to the register file Write data input The value fed to the register file Write data input
comes from ALUOut. comes from the MDR.
IorD The PC is used to supply the address to the me- ALUOut is used to supply the address to the me-
mory unit. mory unit.
IRWrite None The output of the memory is written into the IR.
PCWrite None The PC is written; the source is controlled by
PCSource.
PCWriteCond None The PC is written if the Zero output from the ALU
is also active.

Figure 2.18: Actions of 1-bit control signals [PaHe98] p. 384
Signal name Value Effect
ALUOp 00 The ALU performs an add operation.
01 The ALU performs an subtract operation.
10 The function field of the instruction determines the ALU operation.
ALUSrcB 00 The second input to the ALU comes from the B register.
01 The second input to the ALU is the constant 4.
10 The second input to the ALU is the sign-extended, lower 16 bits of the IR.
11 The second input to the ALU is the sign-extended, lower 16 bits of the IR shifted left
2 bits.
PCSource 00 Output of the ALU (PC + 4) is sent to the PC for writing.
01 The contents of ALUOut (the branch target address) are sent to the PC for writing.
10 The jump target address (IR[25-0] shifted left 2 bits and concatenated with
PC +4[31-28]) is sent to the PC for writing.

Figure 2.19: Actions of 2-bit control signals [PaHe98] p. 384



The execution of an instruction is divided into maximal five steps.
Different elements of the datapath can work in parallel during one clock cycle,
whereas others can only be used in series.
So there must be sure, that after one step the values computed are stored either
in the memory or in one of the registers.

The operation steps are:
1. Instruction fetch step
Fetch the instruction from the memory and computed the address of the
sequential instruction:
IR = Memory[PC]
PC = PC + 4

Control signal setting:
MemRead = 1
IRWrite = 1
IorD = 0
ALUSrcA = 1
ALUSrcB = 01
ALUOp = 00
PCSource = 00
PCWrite = 1

2. Instruction decode and register fetch step
It is still unknown what the instruction is, so there can only be performed
actions that are applicable for all instructions or are not harmful.
The registers indicated by the rs and rd field of the instruction are read
and store into the A and B register, and the potential branch target is
computed and stored into the ALUOut register.
A = Reg[IR[25-21]]
B = Reg[IR[20-16]]
ALUOut = PC + (sign-extend (IR[15-0]) << 2)

ALUSrcA = 0
ALUSrcB = 11
ALUOp = 00



3. Execution, memory address computation or branch completion
In this step the instruction is known and the operation depends on what
the instruction is. One of these four functions is executed:
1. Memory reference:
ALUOut = A + sign-extend(IR[15-0])

ALUSrcA = 1
ALUSrcB = 10
ALUOp = 00

2. Arithmetic-logical instruction:
ALUOut = A op B

ALUSrcA = 1
ALUSrcB = 00
ALUOp = 10

3. Branch:
if (A == B) PC = ALUOut

ALUSrcA = 1
ALUSrcB = 00
ALUOp = 01
PCWriteCond = 1
PCSource = 01

4. Jump:
PC = PC[31-28] & (IR[25-0] << 2)

PCWrite = 1



4. Memory access or R-type instruction completion step
In this step a load or store instruction accesses memory or a arithmetic-
logical instruction writes its result.
1. Memory reference:
MDR = Memory [ALUOut]

or
Memory [ALUOut] = B

MemRead = 1 or MemWrite = 1
IorD = 1

2. Arithmetic-logical instruction:
Reg[IR[15-11]] = ALUOut

RegDst = 1
RegWrite = 1
MemtoReg = 0

5. Memory read completion step
The load instruction is completed by writing back the value from the
memory:
Reg[IR[20-16]] = MDR

MemtoReg = 1
RegWrite = 1
RegDst = 0

These five steps are summarised in figure 2.20.



Step name Action for R-type Action for memory- Action for Action for
instructions reference instructions branches jumps
Instruction fetch IR = Memory[PC]
PC = PC + 4
Instruction decode A = Reg[IR[25-21]]
register fetch B = Reg[IR[20-16]]
ALUOut = PC + (sign-extend(IR[15-0] << 2)
Execution, address ALUOut = A op B ALUOut = A + sign-extend if (A == B) then PC = PC[31-28] ||
computation, (IR[15-0]) PC = ALUOut (IR[25-0] << 2)
branch/jump completi-
on
Memory access or R- Reg[IR[15-11]] = Load: MDR = Memory[ALUOut]
type completion ALUOut or
Store: Memory[ALUOut] = B
Memory read comple- Load: Reg[IR[20-16]] = MDR
tion

Figure 2.20: Summary of the multicycle steps [PaHe98] p. 389

2.3.3 Defining the Control by a Finite State Machine

In the single step implementation the control was defined by simple truth tables
that set the control signals depending on the instruction.
This does not work for a mulitcycle datapath.
The control is more complex, because it must specify both the signals to be set
in any step and the next step in the sequence.
Therefore a finite state machine is used.

Figure 2.21 shows the finite state machine for the control of the multicycle
datapath implementation.


2.3 Defining the Control by a Finite State Machine

Figure 2.21: Complete finite state machine control [PaHe98] p. 396

The setting of the control signals is also shown in figure 2.21.

All unused signals have to be deasserted or keep their value during the next
states until they are set again.

All signal settings in all states is shown in figure 2.22.


2.3 Defining the Control by a Finite State Machine

Signal name State
0 1 2 3 4 5 6 7 8 9
RegDst 0 0 0 0 0 0 0 1 0 0
RegWrite 0 0 0 0 1 0 0 1 0 0
ALUSrcA 0 0 1 1 1 1 1 1 1 0
MemRead 1 0 0 1 0 0 0 0 0 0
MemWrite 0 0 0 0 0 1 0 0 0 0
MemtoReg 0 0 0 0 1 0 0 0 0 0
IorD 0 0 0 1 0 1 0 0 0 0
IRWrite 1 0 0 0 0 0 0 0 0 0
PCWrite 1 0 0 0 0 0 0 0 0 1
PCWriteCond 0 0 0 0 0 0 0 0 1 0
ALUOp 00 00 00 00 00 00 10 10 01 00
ALUSrcB 01 11 10 10 10 10 00 00 00 11
PCSource 00 00 00 00 00 00 00 00 01 10

Figure 2.22: Setting of Control Signals


3 Design Tasks

3 Design Tasks
• Block Diagram of first hierarchy levels

• Register Transfer Level Models implemented in pure VHDL

• VHDL Testbench of important RTL Models

• Implementation in Altera Target Technology

• Prototype Testing

• Simulation Tool: ModelSim

• Synthesis Tool: Altera Quartus

• Milestone Presentations

• Design Project Report in OpenOffice Document Format

• Design Directory Structure is mandatory according to the following table:

Object Description
toplevel Root directory for a VHDL design project
toplevel/src directory for VHDL source code
toplevel/work directory for VHDL working library, contains compiled object code of
ModelSim VHDL compiler
toplevel/simulation simulation results
toplevel/stimuli stimuli files of extended simulation runs should be stored in
this directory
toplevel/pnr data produced after a place&route run can be found in this directory
toplevel/scripts scriptfiles for automated batch processing of the design steps
should be placed here
toplevel/log log files of the different design steps
toplevel/doc directory for project documentation, data sheets, etc.


4 Module Specification

4 Module Specification

4.1 ALU

4.1.1 Functional Description

The arithmetic-logic unit (ALU) performs basic arithmetic and logic operations
which are controlled by the opcode. The result of the instruction is written to the
output. An additional zero-bit signalizes an high output if the result equals zero.

At the present time, the basic arithmetic operations add and sub and the logic
operations and, or and slt can be applied to inputs. The inputs are 32 bit wide
with type unsigned. A detection of overflow or borrow is not supported at the mo-
ment.


4.1 Block Diagram

4.1.2 Block Diagram

Figure 4.1: ALU 1/3


4.1 Block Diagram

Figure 4.2: ALU 2/3


4.1 Block Diagram

Figure 4.3: ALU 3/3


4.1 Simulation Results

4.1.3 Simulation Results

Figure 4.4: Simulation Results of ALU

4.1.4 Design Files

File Name File Type Description
e_alu.vhd VHDL Source Files Arithmetic-logic unit
a_alu_behave.vhd
t_alu.vhd VHDL Testbench File Testbench for single operations
t_alu_fileio.vhd VHDL Testbench File Testbench using file I/O


4.2 Memory

4.2 Memory


Data is synchronously written to or read from the memory with a data bus width
of 32 bit. The memory consists of four ram blocks with 8 bit data width each.

A control signal enables the memory to be written, otherwise data is only read. In
order to store data to the memory the data word is subdivided into four bytes
which are separately written to the ram blocks. Vice versa, the single bytes are
concatenated to get the data word back again.

At the moment, it is only possible to read and write data words. An addressing of
half-words or single bytes is not allowed. In order to write or read data words, all
ram blocks have to be selected. Hence, the lowest two bit are not examined for
chip-select logic.

Data is addressed by the MIPS-processor with an address width of 32 bit, while
the address width of a ram block is 8 bit each. All ram blocks are connected to
the same address, namely from mem_address(9 downto 2). Since we do not use
the full address width for addressing and chip selects, data words are addressed
by multiple addresses.

Unfortunately, some problems occurred during simulation of the memory unit.
According to the MIPS design shown in literature [PaHe98], there should be im-
plemented a memory unit with an unregistered output. The Altera Quartus
MegaWizard Plug-In Manager yielded a ram block with a synchronous output
(a_ram_syn.vhd) , although the output was defined as unregistered.

In order to get an unregistered memory output, another ram block was defined in
VHDL code (a_ram_rtl.vhd). There, the output directly yields the data being ad-
dressed by the unregistered input address. Unfortunately, the synthesizer does
not support memory initialisation files in the RTL-code for setting data to the
memory. Hence, it was not possible to implement the memory in real hardware.


4.2 Block Diagram

4.2.2 Block Diagram

Figure 4.5: Memory


Figure 4.6 shows the simulation results with registered data output.

Figure 4.6: Simulation Results of Memory (registered outputs)


4.2 Simulation Results

Figure 4.7 shows the simulation results with unregistered output. Note that the
simulation contains unknown values, because the memory initialisation files are
not supported.

Figure 4.7: Simulation Results of Memory (unregistered outputs)

4.2.4 Design Files

e_ram.vhd VHDL Source Files Ram block used as component for
a_ram_rtl.vhd memory instantiation
a_ram_syn.vhd
a_ram_lpm.vhd
e_memory.vhd VHDL Source Files Instantiation and connection of ram
a_memory_behave.vhd blocks
t_memory.vhd VHDL Testbench Files Test memory read. write and address
./simulation/ram0_256x8.hex Intel Hex Format Files Used for memory initialisation
./simulation/ram1_256x8.hex (a_ram_syn.vhd)
./simulation/ram2_256x8.hex
./simulation/ram3_256x8.hex


4.3 Control

4.3 Control


The control of the processor is realised by a Finite State Machine described in
section 2.3.3.

The input to the State Machine are the upper 6 bits of the function field contain-
ing the instruction.

The outputs of the state machine are the control signals of the single functional
units of the processor implementation especially the multiplexers of the datapath.

The Operation Code of the ALU is stored in a truth table and the corresponding
Opcode is produced depending on the ALUOp signal of the state machine and
the lower 6 bits of the function field containing the information which of the arith-
metic or logic instruction is to use.


4.3 State Diagram

4.3.2 State Diagram

ErrorState

Figure 4.8: Control Finite State Machine

An additional Error State is inserted which is a deadlock. If any unknown instruc-
tion occurs the Error State is entered.


4.3 Block Diagram

4.3.3 Block Diagram

Figure 4.9: Control FSM

Figure 4.10: ALU Control


4.3 Block Diagram

Figure 4.11: Control


Figure 4.12: Simulation Results of the Control FSM

4.3.5 Design Files

e_control_ControlFSM.vhd VHDL Source Files Finite State Machine for Control
a_control_ControlFSM.vhd
e_control_ALUControl.vhd VHDL Source Files Truth Tabel for ALU Control
a_control_ALUControl.vhd
e_control.vhd VHDL Source Files Controlpath
a_control.vhd


4.4 Data Path

4.4 Data Path

The datapath is divided into four sections with respect to the pipelining structure
of a processor. The four parts are the Instruction Fetch, Instruction Decode, Exe-
cution and Memory Writeback.

These sections are synthesized of their own and then combined to the Data
Block.

4.4.1 Instruction Fetch

4.4.1.1 Functional Description

The Instruction Fetch Block contains the PC the Instruction Register and the
Memory Data Register.

This part provides the data and instruction form the memory.

4.4.1.2 Block Diagram

Figure 4.13: Instruction Fetch


4.4 Instruction Fetch

4.4.1.3 Design Files

e_pc.vhd VHDL Source Files Program Counter
a_pc_behave.vhd
e_tempreg.vhd VHDL Source Files Temporary Memory Data Register
a_tempreg_behave.vhd
e_instreg.vhd VHDL Source Files Instruction Register
a_instreg_behave.vhd
e_data_fetch.vhd VHDL Source Files Instruction Fetch Block
a_data_fetch_behave_vhd

4.4.2 Instruction Decode


The Instruction Decode Block writes the instruction of the Instruction Register to
the Register File and computes the second operand for a Branch Instruction or a
sw- or lw-instruction.


Figure 4.14: Instruction Decode


4.4 Instruction Decode


e_regfile.vhd VHDL Source Files Register File
a_regfile_behave.vhd
e_tempreg.vhd VHDL Source Files Temporary Memory Data Register
e_data_decode.vhd VHDL Source Files Data Decode Block
a_data_decode_behave.vhd

4.4.3 Execution


The Execution contains the ALU as main element and computes the desired re-
sult of the instruction.

It also computes the jump target address and provides it for the Memory Write-
back Block.

The operands loaded to the ALU are chosen by two multiplexers which are sen-
sible to the signals ALUSrcA and ALUSrcB.


4.4 Execution


Figure 4.15: Execution


4.4 Execution


e_alu.vhd VHDL Source Files ALU
a_alu_behave.vhd
e_data_execution.vhd VHDL Source Files Execution Block
a_data_execution.vhd


4.4 Memory Writeback

4.4.4 Memory Writeback


The Memory Writeback Block consists of the ALUOut register and a multiplexer
with source signal PCSource.

This block leads the result of the computation either back to memory or to the
register file.

The multiplexer leads back the next PC value depending on the PCSource sig-
nal.




Figure 4.16: Memory Writeback




e_tempreg.vhd VHDL Source Files Temporary ALUOut Register
e_data_memwriteback.vhd VHDL Source Files Memory Writeback Block
a_data_memwriteback.vhd


4.4 Data Path

4.4.5 Data Path



e_data.vhd VHDL Source Files Datapath
a_data_vhd
e_data_fetch.vhd VHDL Source Files Data Fetch Block
a_data_fetch.vhd
a_data_decode.vhd
e_data_execution.vhd VHDL Source Files Data Execution Block
a_alu_behave.vhd
a_pc_behave.vhd


4.5 Processor and Memroy

4.5 Processor and Memroy


The both parts Datapath and Controlpath are combined to the processing unit.
Together with the Memory the whole processor is completed.

4.5.2 Block Diagram

Figure 4.17: Processing Unit (Datapath & Controlpath)

Figure 4.18: Processing Unit & Memory


4.5 Design Files

4.5.3 Design Files

e_control_ControlFSM.vhd VHDL Source Files Finite State Machine for Control
a_control_ControlFSM.vhd
e_control_ALUControl.vhd VHDL Source Files Truth Tabel for ALU Control
a_control_ALUControl.vhd
e_control.vhd VHDL Source Files Controlpath
a_control.vhd
e_data.vhd VHDL Source Files Datapath
a_data.vhd
e_data_fetch.vhd VHDL Source Files Data Fetch Block
a_data_fetch.vhd
a_data_decode.vhd
e_data_execution.vhd VHDL Source Files Data Execution Block
a_alu_behave.vhd
a_pc_behave.vhd
e_ram.vhd VHDL Source Files Ram block used as component for
a_ram_rtl.vhd memory instantiation
a_ram_syn.vhd
a_ram_lpm.vhd
t_procmem.vhd VHDL Testbench Files Testbench for testing the processor
t_procmem_init.vhd


5 Synthesis Results

5 Synthesis Results
+------------------------------------------------------------------------------+
; Analysis & Synthesis Summary ;
+------------------------------------+-----------------------------------------+
; Analysis & Synthesis Status ; Successful - Thu Jul 05 11:15:33 2007 ;
; Quartus II Version ; 7.0 Build 33 02/05/2007 SJ Full Version ;
; Revision Name ; procmem ;
; Top-level Entity Name ; procmem ;
; Family ; Cyclone II ;
; Total logic elements ; 0 ;
; Total combinational functions ; 0 ;
; Dedicated logic registers ; 0 ;
; Total registers ; 0 ;
; Total pins ; 2 ;
; Total virtual pins ; 0 ;
; Total memory bits ; 0 ;
; Embedded Multiplier 9-bit elements ; 0 ;
; Total PLLs ; 0 ;
+------------------------------------+-----------------------------------------+

Figure 5.1: Analysis & Synthesis Summary

+--------------------------------------------------------------------------------------------------------------+
; Analysis & Synthesis Settings ;
+--------------------------------------------------------------------+--------------------+--------------------+
; Option ; Setting ; Default Value ;
+--------------------------------------------------------------------+--------------------+--------------------+
; Device ; EP2C20F484C7 ; ;
; Top-level entity name ; procmem ; procmem ;
; Family name ; Cyclone II ; Stratix ;
; Restructure Multiplexers ; Auto ; Auto ;
; Create Debugging Nodes for IP Cores ; Off ; Off ;
; Preserve fewer node names ; On ; On ;
; Disable OpenCore Plus hardware evaluation ; Off ; Off ;
; Verilog Version ; Verilog_2001 ; Verilog_2001 ;
; VHDL Version ; VHDL93 ; VHDL93 ;
; State Machine Processing ; Auto ; Auto ;
; Safe State Machine ; Off ; Off ;
; Extract Verilog State Machines ; On ; On ;
; Extract VHDL State Machines ; On ; On ;
; Ignore Verilog initial constructs ; Off ; Off ;
; Add Pass-Through Logic to Inferred RAMs ; On ; On ;
; DSP Block Balancing ; Auto ; Auto ;
; NOT Gate Push-Back ; On ; On ;
; Power-Up Don't Care ; On ; On ;
; Remove Redundant Logic Cells ; Off ; Off ;
; Remove Duplicate Registers ; On ; On ;
; Ignore CARRY Buffers ; Off ; Off ;
; Ignore CASCADE Buffers ; Off ; Off ;
; Ignore GLOBAL Buffers ; Off ; Off ;
; Ignore ROW GLOBAL Buffers ; Off ; Off ;
; Ignore LCELL Buffers ; Off ; Off ;
; Ignore SOFT Buffers ; On ; On ;
; Limit AHDL Integers to 32 Bits ; Off ; Off ;
; Optimization Technique -- Cyclone II ; Balanced ; Balanced ;
; Carry Chain Length -- Stratix/Stratix GX/Cyclone/MAX II/Cyclone II ; 70 ; 70 ;
; Auto Carry Chains ; On ; On ;
; Auto Open-Drain Pins ; On ; On ;
; Perform WYSIWYG Primitive Resynthesis ; Off ; Off ;
; Perform gate-level register retiming ; Off ; Off ;
; Allow register retiming to trade off Tsu/Tco with Fmax ; On ; On ;
; Auto ROM Replacement ; On ; On ;
; Auto RAM Replacement ; On ; On ;
; Auto Shift Register Replacement ; On ; On ;
; Auto Clock Enable Replacement ; On ; On ;
; Allow Synchronous Control Signals ; On ; On ;
; Force Use of Synchronous Clear Signals ; Off ; Off ;
; Auto RAM to Logic Cell Conversion ; Off ; Off ;
; Auto Resource Sharing ; Off ; Off ;
; Allow Any RAM Size For Recognition ; Off ; Off ;
; Allow Any ROM Size For Recognition ; Off ; Off ;
; Allow Any Shift Register Size For Recognition ; Off ; Off ;
; Ignore translate_off and synthesis_off directives ; Off ; Off ;
; Show Parameter Settings Tables in Synthesis Report ; On ; On ;
; Ignore Maximum Fan-Out Assignments ; Off ; Off ;


5 Synthesis Results

; Retiming Meta-Stability Register Sequence Length ; 2 ; 2 ;
; PowerPlay Power Optimization ; Normal compilation ; Normal compilation ;
; HDL message level ; Level2 ; Level2 ;
; Suppress Register Optimization Related Messages ; Off ; Off ;
; Number of Removed Registers Reported in Synthesis Report ; 100 ; 100 ;
; Use smart compilation ; Off ; Off ;
+--------------------------------------------------------------------+--------------------+--------------------+

Figure 5.2: Analysis & Synthesis Settings

+-----------------------------------------------------+
; Compilation Hierarchy ;
+-----------------------------------------------------+
; |procmem ;
; |mips ;
; |control:inst_control| ;
; |ALUControl:inst_ALUControl| ;
; |ControlFSM:inst_ControlFSM| ;
; |data:inst_data| ;
; |data_decode:inst_data_decode| ;
; |regfile:inst_regfile| ;
; |tempreg:A| ;
; |tempreg:B| ;
; |data_execution:inst_data_execution| ;
; |alu:alu_inst| ;
; |data_fetch:inst_data_fetch| ;
; |instreg:instr_reg| ;
; |pc:proc_cnt| ;
; |tempreg:mem_data_reg| ;
; |data_memwriteback:inst_data_memwriteback| ;
; |tempreg:tempreg_inst| ;
; |memory ;
; |ram:mem_block0| ;
; |altsyncram:ram_block_rtl_0| ;
; |altsyncram_ia61:auto_generated| ;
+-----------------------------------------------------+

Figure 5.3: Compilation History


6 Results of Prototype Testing

6 Results of Prototype Testing

6.1 Description

For the first test of the completed processor and the memory a simple addition of
two numbers was done.

Therefore at first the memory has to be loaded with the instructions and data by
using *.mif-files to write the information into the memory blocks before starting
the simulation.

The instructions written into the memory are:

Memory Instruction Field
Address Instruction op rs rt rd shamt funct
000 lw $s0, 128($zero) 100011 00000 10000 0000000010000000
004 lw $s1, 132($zero) 100011 00000 10001 0000000010000100
008 add $s2, $s0, $s1 000000 10000 10001 10010 00000 100000
012 sw $s2, 136($zero) 101011 00000 10010 0000000010001000
016 sub $s3, $s1, $s0 000000 10001 10000 10011 00000 100010
020 sw $s3, 140($zero) 101011 00000 10011 0000000010001100
024 and $s4, $s1, $s0 000000 10001 10000 10100 00000 100100
028 sw $s4, 144($zero) 101011 00000 10100 0000000010010000
032 or $s5, $s1, $s0 000000 10001 10000 10101 00000 100101
036 sw $s5, 148($zero) 101011 00000 10101 0000000010010100
040 slt $s6, $s1, $s0 000000 10000 10001 10110 00000 101010
044 sw $s6, 152($zero) 101011 00000 10110 0000000010011000
048 beq $s0, $s4, 56 000100 10000 10100 0000000000000001
052 UNDEFINED UUUUUU UUUUU UUUUU UUUUU UUUUU UUUUUU
056 j8 000010 00000000000000000000000010

The data written to the memory are:

Memory
Address Data (dec) Data (bin)
128 379 00000000 00000000 00000001 01111011
132 383 00000000 00000000 00000001 01111111


6.1 Description

The expected values stored back into the memory are:

Memory
Address Data (dec) Data (bin)
136 762 00000000 00000000 00000010 11111010
140 4 00000000 00000000 00000000 00000100
144 379 00000000 00000000 00000001 01111011
148 383 00000000 00000000 00000001 01111111
152 1 00000000 00000000 00000000 00000001

The simulation starts at memory address 000 with a load word instruction. The
value of memory address 128 is written into register $s0. The PC is incremented
and the next instruction of memory address 004 is executed. It is also an load
word instruction which loads the value of memory address 132 to register $s1.

Then an add instruction follows which adds the two operands written into the reg-
isters $s0 and $s1 and writes the result to register $s2.

Then a store word instruction writes the content of register $s2 to the memory at
address 136.

The following instructions are for subtract, add, or, slt, beq and jump. The result
of a computation is always stored to the memory by a store word instruction.

Note:

For description of the register numbers and names used for the test see Figure
3.13 of [PaHe98] p. 140.

The used assembler instructions are not completely declared in this report.
For information on the machine language see [PaHe98] Chapter 3, especially fig-
ure 3.14 on page 141.


6.2 Simulation Result

6.2 Simulation Result

Figure 6.1: Simulation Results of MIPS and Memory


7 Conclusion

7 Conclusion

7.1 Our own experiences

While working on our miniproject, we applied a lot of knowledge learned in the
lecture VHDL. Furthermore, we gained a lot of experience in using the simulation
and synthesis tools. It was very interesting and exciting to describe real hardware
and to see the expected results in simulation and the block diagrams after syn-
thesis.

Our miniproject implementing a processor in VHDL has been a real challenge.
The complexity was not located in the single components, but rather in the imple-
mentation of the synchronous operation of the whole control and datapath. Due
to an intensive preparation of the desired hardware according to the literature
“Computer Organization & Design” [PaHe98], we prevented unintended design
errors. Since our project transcends a pure implementation of VHDL code, we
were able to gain experience in hierarchical design with component instantiation
and package design.

Additionally, while implementing a microprocessor, we could refresh our knowl-
edge in processor operations, memory addressing and MIPS instruction coding.

7.2 Annotations to “Computer Organization & Design” [PaHe98]

Since the design of our MIPS processor is closely connected to the literature
[PaHe98], we read the chapter 5 in detail. Overall, we adjusted the design as
conform as possible to the description in [PaHe98]. There are some passages
which do not provide a full description, e.g. the output signals of the control FSM
are not listed completely for each state. Nevertheless, “Computer Organization &
Design” by Patterson and Hennessy provides a brilliant composition describing
the control and datapath of a processor implementation.


7.3 Further work on the project

7.3 Further work on the project

Although we spent much more time than scheduled, we did not reach all our
aims. For further work on the project, we recommend our successors to continue
the following tasks:

– Verify the synthesis results (with VHDL code created by Quartus) with desired
behavior implemented in RTL and seen in testbench simulation in order to
obtain the desired unregistered memory output mentioned in chapter 4.2.

– Realise an hardware implementation of processor and memory in order to
verify the behavior of the desired hardware on the Cyclone II Development &
Education Board, e.g. debug the memory data.

– Introduce the pipelining of instructions described in chapter 6 [PaHe98] to im-
prove the performance of the MIPS processor.


8 Appendix

8 Appendix

8.1 Design files

8.1.1 Project Entities

LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;

ENTITY ControlFSM IS
PORT (clk, rst_n : IN std_ulogic;
instr_31_26 : IN std_ulogic_vector(5 downto 0);
RegDst, RegWrite, ALUSrcA, MemRead, MemWrite, MemtoReg, IorD, IRWrite, PCWrite,
PCWriteCond : OUT std_ulogic;
ALUOp, ALUSrcB, PCSource : OUT std_ulogic_vector(1 downto 0)
);
END ControlFSM;
VHDLSource 8.1: e_control_ControlFSM.vhd

LIBRARY IEEE;

ENTITY ALUControl IS
PORT (instr_15_0 : IN std_ulogic_vector(15 downto 0);
ALUOp : IN std_ulogic_vector(1 downto 0);
ALUopcode : OUT std_ulogic_vector(2 downto 0)
);
END ALUControl;
VHDLSource 8.2: e_control_ALUControl.vhd

LIBRARY IEEE;

ENTITY control IS
PORT (clk, rst_n : IN std_ulogic;
zero : IN std_ulogic;
ALUopcode : OUT std_ulogic_vector(2 downto 0);
RegDst, RegWrite, ALUSrcA, MemRead, MemWrite, MemtoReg, IorD, IRWrite : OUT
std_ulogic;
ALUSrcB, PCSource : OUT std_ulogic_vector(1 downto 0);
PC_en : OUT std_ulogic
);
END control
VHDLSource 8.3: e_control.vhd


8.1 Project Entities

LIBRARY IEEE;
USE IEEE.numeric_std.ALL;
-- use package
USE work.procmem_definitions.ALL;

ENTITY tempreg IS
PORT (
clk : IN STD_ULOGIC;
rst_n : IN STD_ULOGIC;

reg_in : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
reg_out : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0) );

END tempreg
VHDLSource 8.4: e_tempreg.vhd

LIBRARY IEEE;
-- use package

ENTITY pc IS
PORT (
pc_in : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
PC_en : IN STD_ULOGIC;
pc_out : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0) );
END pc;
VHDLSource 8.5: e_pc.vhd

LIBRARY IEEE;
-- use package

ENTITY instreg IS
PORT (
memdata : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
IRWrite : IN STD_ULOGIC;

instr_31_26 : OUT STD_ULOGIC_VECTOR(5 DOWNTO 0);
instr_15_0 : OUT STD_ULOGIC_VECTOR(15 DOWNTO 0) );

END instreg;
VHDLSource 8.6: e_instreg.vhd



LIBRARY IEEE;
-- use package

ENTITY regfile IS
PORT (clk,rst_n : IN std_ulogic;
wen : IN std_ulogic; -- write control
writeport : IN std_ulogic_vector(width-1 DOWNTO 0); -- register input
adrwport : IN std_ulogic_vector(regfile_adrsize-1 DOWNTO 0);-- address write
adrport0 : IN std_ulogic_vector(regfile_adrsize-1 DOWNTO 0);-- address port 0
adrport1 : IN std_ulogic_vector(regfile_adrsize-1 DOWNTO 0);-- address port 1
readport0 : OUT std_ulogic_vector(width-1 DOWNTO 0); -- output port 0
readport1 : OUT std_ulogic_vector(width-1 DOWNTO 0) -- output port 1
);
END regfile;
VHDLSource 8.7: e_regfile.vhd

LIBRARY IEEE;
-- use package

ENTITY alu IS
PORT (
a, b : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
opcode : IN STD_ULOGIC_VECTOR(2 DOWNTO 0);

result : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
zero : OUT STD_ULOGIC);
END alu;
VHDLSource 8.8: e_alu_vhd

LIBRARY IEEE;
-- use package

ENTITY data_fetch IS
PORT (
-- inputs
pc_in : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
alu_out : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
mem_data : IN std_ulogic_vector(width-1 DOWNTO 0);
-- control signals
PC_en : IN STD_ULOGIC;
IorD : IN STD_ULOGIC;
IRWrite : IN STD_ULOGIC;
-- outputs
reg_memdata : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
mem_address : OUT std_ulogic_vector(width-1 DOWNTO 0);
pc_out : OUT std_ulogic_vector(width-1 DOWNTO 0)
);

END data_fetch;
VHDLSource 8.9: e_data_fetch.vhd



LIBRARY IEEE;
-- use package

ENTITY data_decode IS
PORT (
-- inputs
instr_25_21 : IN STD_ULOGIC_VECTOR(4 DOWNTO 0);
reg_memdata : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
alu_out : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
-- control signals
RegDst : IN STD_ULOGIC;
RegWrite : IN STD_ULOGIC;
MemtoReg : IN STD_ULOGIC;
-- outputs
reg_A : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
reg_B : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
instr_15_0_se : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
instr_15_0_se_sl : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0)
);

END data_decode;
VHDLSource 8.10: e_data_decode.vhd

LIBRARY IEEE;
-- use package

ENTITY data_execution IS
PORT (instr_25_21 : IN std_ulogic_vector(4 downto 0);
ALUSrcA : IN std_ulogic;
ALUSrcB : IN std_ulogic_vector(1 downto 0);
ALUopcode : IN std_ulogic_vector(2 downto 0);
reg_A, reg_B : IN std_ulogic_vector(width-1 downto 0);
pc_out : IN std_ulogic_vector(width-1 downto 0);
instr_15_0_se : IN std_ulogic_vector(width-1 downto 0);
instr_15_0_se_sl : IN std_ulogic_vector(width-1 downto 0);

jump_addr : OUT std_ulogic_vector(width-1 downto 0);
alu_result : OUT std_ulogic_vector(width-1 downto 0);
zero : OUT std_ulogic
);
END data_execution;
VHDLSource 8.11: e_data_execution.vhd


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