MIPS Data Path
Ellen Spertus
MCS 111
October 23, 2001
2
Readings and lectures
• HP has been growing in importance through the semester
• Now it is vital
• Recommendation– Read before lecture– Ask questions in lecture– Review as needed
3
Big Picture
• Given devices that implement simple boolean functions…
• Understand how a computer works– Build an actual computer that you will
program– Study the MIPS architecture
• Write assembly language programs• Learn the implementation of a MIPS subset
4
Implementing MIPS subset
• Review: Mite
• Definition of MIPS subset (5.1)
• Elements of the data path (5.2)
• Control of elements (5.3)
• Optimization (6)
5
Review: Mite
B
bus
ALU
A
3s
LDB
in
out0
out1
LDA
F3F2F1F0
C'M
s6 s5 s4 s3 s2 s1 s0s7
DRALUDRCONST
6
MIPS subset
• Word size: 32 bits
• Number of registers: 32
• Instruction length: 32 bits
• Types of instructions:– Memory: lw, sw– Arithmetic-logical: add, sub, and, or, slt
– Control flow: beq, jump
Mite
7
Clocking methodology
• Defines when signals can be read and when they can be written.
• Write values on rising edge of clock
Clock cycle
Stateelement
1Combinational logic
Stateelement
2
8
Data path elements
• Memory: PC, instruction, registers, data
• ALU• Which of these does Mite have?
PC
InstructionMemory
read address
instruction
read reg 1
read reg 2
write register
write data
read data 1
read data 2
Register File
AL
U
Data Memory
address
write data
read data
9
Closer look at program counter (PC)
PC
InstructionMemory
read address
instruction
4
Ad
d
10
Closer look at register file
• Sample instruction: – add $t1, $t2, $t3– add $9, $10, $11
write register
write data
Register File
read data 1
read data 2
read reg 1
read reg 2
11
Register file: read
Register 0
Register 1
read reg 1
read reg 2
read data 1
read data 2
Register 2
Register 3
12
Register file: write
write register
data
Register 0
Register 1
Register 3
Register 2
13
Data path usage: arithmetic-logical
• Increment PC• Read registers• Use ALU
add $t1, $t2, $t3
add $9, $10, $11
PC
InstructionMemory
read address
instruction
read reg 1
read reg 2
write register
write data
read data 1
read data 2
Register File
AL
U
1011
9
14
Data path usage: memory
• Increment PC• Read registers• Use ALU
lw $t0, 1($s0)
lw $8, 1($16)
Registers
Register #
Data
Register #
Datamemory
Address
Data
Register #
PC Instruction ALU
Instructionmemory
Address
15
Data path usage: memory
• Increment PC• Read registers• Use ALU
lw $t0, 1($s0)
lw $8, 1($16)
PC
InstructionMemory
read address
instruction
read reg 1
read reg 2
write register
write data
read data 1
read data 2
Register File
AL
U
Data Memory
address
write data
read data
16
A closer look at load/storelw $8, 1($16)
PC
InstructionMemory
read address
instruction
read reg 1
read reg 2
write register
write data
read data 1
read data 2
Register File
AL
U
Data Memory
address
write data
read data
5
5
5
16
32
32
17
Mite: six bits to eight
B
bus
ALU
A
3s
LDB
in
out0
out1
LDA
F3F2F1F0
C'M
s6 s5 s4 s3 s2 s1 s0s7
DRALUDRCONST
18
Assembly of conditional branches
• Assembly codeslt $t0, $a0, 2
bne $t0, $zero, recur
mov $v0, $t0
jr $ra
recur:
add $sp, $sp, 12
sw $ra, 8($sp)
...
• Machine code0: slti $8, $4, 2
4: bne $8, $0, __
8: add $2, $0, $8
12: jr $31
recur:
14: addi $29, $29, 12
16: sw $31, 8($29)
...
19
• Increment PC• Read registers• Use ALU• Add offset to PC
beq $t1, $t2, label
Data path usage: control flow
PC
InstructionMemory
read address
instruction
read reg 1
read reg 2
write register
write data
SignExtend
Shiftleft 2
read data 1
read data 2
Register File
4
Ad
d
Ad
d
AL
U
zero?
20
PC
InstructionMemory
read address
instruction
read reg 1
read reg 2
write register
write data
SignExtend
Shiftleft 2
mux
read data 1
read data 2
Register File
mux4
Ad
d
Ad
d
AL
U
Data Memory
address
write data
read data mux
The Simple Datapath
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