ee 3610: digital systems 1 - weber state universityfaculty.weber.edu/snaik/ece3610/13lec13.pdf ·...

EE 3610 Digital Systems Suketu Naik

Microprocessor

without

Interlocked

Pipeline

Stages

EE 3610: Digital Systems

2MIPS: Application

The MIPS is used in

Embedded systems,

Cisco routers

Nintendo 64

Sony PlayStation , Sony PlayStation 2

and Sony PlayStation Portable

Small computing devices

Small consumer electronics and

appliances

Google's Honeycomb (Android 3)

tablet

3Why MIPS?

The MIPS is an example of RISC (Reduced Instruction Set

Computing)

RISC uses simplified instruction set (as opposed to a

complex set which is used in CISC)

Provides higher performance architecture capable of

executing those instructions using fewer microprocessor

cycles per instruction

RISC Processors include,

DEC Alpha, AMD 29k, ARC, ARM (used in tablets, smart

phones), Atmel AVR, Blackfin, Intel i860 and i960, MIPS,

Motorola 88000, PA-RISC, PowerPC, SuperH, and SPARC (open

source)

4CISC vs. RISC

Memory

Through

Microcoding

through

simple

instructions

5Realm of Microprocessors

Babbage: Difference and Analytical Machines

Automatic calculation based on finite differences

Wheels on shaft: data memory

Cards in deck: instructions

Harvard Architecture

Electromechanical computer: separation of data and

instructions

punched paper tape: instructions

loops: join the end of the paper tape containing the

program back to the beginning of the tape

Harvard Architecture

Physically separate signals and storage for code and data

memory

Program memory is read-only and data memory is read-

Impossible for program contents to be modified by the

program itself

CPUInstruction

Memory

Von Neumann Architecture

Shared signals and memory for code and data

The program can be easily modified by itself since it is

stored in read-write memory

CPUInstruction

9Caches

Cache is fast memory used to reduce

the time required to fetch an instruction or

When a piece of data or instruction is

fetched, the same data or instruction will

be accessed in the near future or data and

instructions nearby will be accessed.

Instead of the CPU spending time

accessing program or data memory for

every piece of data or instruction, it can

check cache first and then main memory

if the desired data or instruction is not

available in cache

Registers

10Register: Set of D Flipflops

Q’CLK

11CPU DESIGN

Heart of CPUs:

Register File and ALU (Arithmetic and Logic Unit)

1) At each clock cycle, the register will load the data

present at its inputs

2) This data is used by ALU to carry out operations

opcode

13ALU: An Example

Functions

(selected by F):

ADD SUBTRACT

LEFT SHIFT

RIGHT SHIFT

14CPU DESIGN

Problem: Where to start?

No way to get the data into the register file initially

Solution: Add a path for 'immediate' values

XImmediate

values1

Imm or offset

15R Instructions

R-Format (32-bits): ALU (core) Instructions

0 RS RT Shift F_CodeRd

Source

Registers

Destination

Register

Amount

How does CPU know what function to perform? A: Opcodehttp://alumni.cs.ucr.edu/~vladimir/cs161/mips.html

opcode=0

(indicates type

of instruction)

ADD, SUB,

AND, OR, etc

16MIPS Instruction Encoding (Data)

R-Format (32-bits)

opcode RS RT Shift Function

function

(Immediate value)

O (offset)

17R Instructions

R-Format (32-bits)

0 RS RT Shift F_CodeRd

Function (F_Code)(same op-code, different

arithmetic operation)

add (add)

addi (add immediate)

addu (add unsigned)

addiu (add imm unsigned)

sub (subtract)

subu (subtract unsigned)

and (and)

andi (and immediate)

or (or)

ori (or immediate)

sll (shift left logical)

srl (shift right logical)

18R Instructions: Examples

19R Instructions: Examples

20I Instructions

I-Format (32-bits): Load/Store, Immediate ALU, Branch

Source

Registers

offset/immediate

op code RS RT C (sign extended)

opcode

21I Instructions

I-Format (32-bits): Load/Store, Immediate ALU, Branch

op code

op code RS RT C (sign extended)

22I: Load/Store

Load/Store Register file is limited (32 registers)

We must get data to and from memory

DIn Dout

Data Memory

23I Instructions: Examples

word: 32 bits

byte: 8-bits

24JUMP Instructions

J-Format (32-bits)

word: 32 bits

byte: 8-bits

Offset (26-bits)

op code Jump Address

opcode

25JUMP Instructions

J-Format (32-bits) Jump offset (target address) is specified as a word address (32-bits)

MIPS uses byte (8-bits) address: multiply the offset by 4, then concatenated

with the highest 4 bits of the program counter (PC) to get the target address

Program counter: register that contains the address of the next instruction,

is always incremented by 4 (so that 4 bytes or a word is incremented)

pc_sel

0DOAddrPC

clk Instruction

to decode

Offset is 26 bits long

bits 25:0 (shifted by 2 =

multiply by 410: 1002 )

word: 32 bits

byte: 8-bits

26JUMP Instructions

ExampleThe 26-bit field in a jump instruction is 0x0100080. What is the

actual address the jump instruction refers to if the top four bits

of the Program Counter are 0000?

27JUMP Instructions

Special Cases Sometimes we compute jump address or need a jump address > 228

pc_sel

to pcData 1 (from reg file)

I25:0 x 4

PC + 4

pc_sel

to pcData 1

I25:0 x 4

PC + 4

28J Instructions: Examples

29A Subset of MIPS Instructions

30Instructions Encoding

31MIPS: example program

Write MIPS Assembly Language Program for the following task:

for i= 1, 100, i++ ; repeat 100 times

y(i) = x(i) + y(i) ; add each element of the arrays and store it in y

Assume that x and y start at locations 400010 and 800010

Answer:

andi $3 ,$3, 0 ;initialize loop counter $3 to 0

andi $2, $2, 0 ; clear register for loop end

addi $2, $2, 100 ; loop end

$label: lw $15, 4000($3) ;load x(i) to Register#15

lw $14, 8000($3) ;load y(i) to Register#14

add $24, $15, $14 ;x(i) + y(i)

sw $24, 8000 ($3) ;save new y(i)

addi $3, $3, 4 ;update address reg, addr=addr+4

bne $3,$2,$label ;check if the loop counter=loop end

x(i)+y(i)

y(i)=x(i)+y(i)

32MIPS: example program Write MIPS Machine Code given the following assembly program:

andi $3,$3, 0 ;initialize loop counter $3 to 0

andi $2, $2, 0 ; clear register for loop end

addi $2, $2, 100 ; loop end

$label: lw $15, 4000($3) ;load x(i) to R15

lw $14, 8000($3) ;load y(i) to R14

add $24, $15, $14 ;x(i) + y(i)

sw $24, 8000 ($3) ;save new y(i)

addi $3, $3, 4 ;update address reg, addr=addr+4

bne $3,$2,$label ;check if the loop counter=loop end

Answer

MIPS using VHDL

34MIPS Controller: Sequential

Decode

Execute

Writeback

Memory

__/load_pc and

write reg_file

opcode=beq

or opcode

bne/load_pc

opcode=jr or j

/load_pc opcode≠jr /_

opcode=jw

or sw /_

opcode=sw/mem_

write and load_pc

Fetch:

PC outputs to memory

Instruction is loaded at the end of the state

Decode: Registers accessed

Execute: ALU computes the result

opcode≠sw/_

35Simplified MIPS Microprocessor in VHDLFetch Stageinst_address <= std_logic_vector(F_PC);

process (clk, reset)

if reset = '1' then

F_PC <= X"00000000"; -- start executing instr at address 0

D_PC <= X"00000004"; -- normally P_PC + 4

D_IR <= X"00000000"; -- instruction register

elsif rising_edge(clk) and not D_stall then

D_IR <= X"00000000"; -- no-op instruction in case of jump/branch

D_PC <= F_PC + 4; -- pc transferred to decode stage for branch &

case D_pc_select is

when pc_next => F_PC <= F_PC+4; D_IR <= unsigned(inst_in);

when pc_jump => F_PC <= D_PC(31 downto 28) & D_IR(25 downto 0)

& "00"; -JUMP by contactenating w 1st 4 bits of PC and sll by 2

when pc_branch => F_PC <= D_PC + (D_imm(29 downto 0) & "00");

when pc_jreg => F_PC <= D_T;

end case;

end if;

end process;

36Simplified MIPS Microprocessor in VHDLExecute StageE_result <= E_Op1 + E_Op2 when E_alu_op = alu_add else -- add

E_Op1 - E_Op2 when E_alu_op = alu_sub else -- subtract

E_Op1 and E_Op2 when E_alu_op = alu_and else -- and

E_Op1 or E_Op2; -- or

process (clk, reset)

if reset = '1' then

M_read_mem <= false;

M_write_mem <= false;

M_write_reg <= false;

elsif rising_edge(clk) then

M_read_mem <= E_read_mem;

M_write_mem <= E_write_mem;

M_write_reg <= E_write_reg;

M_T <= E_T; -- for store instruction

M_result <= E_result;

M_Rd <= E_Rd;

end if;

end process;

More Simplified VHDL on the class page

ee 3610: digital systems 1 - weber state universityfaculty.weber.edu/snaik/ece3610/13lec13.pdf ·...

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