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Computer System Design
Lecture 2
Instruction Set Architecture
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What’re the component of ISA?

Machine Instruction Set
 Instruction format
 Nature of the fetch through execute
narat:
hine instruction set :make use of storage cells, formats, and result of fetch/execute , register transfer
uction format : size and meaning of field within instruction
nature of the fetch-execute cycle : things that are done before the operation code is known.
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Varies Programming Model
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What must an instruction specify?

Which Operation is perform?
ADD
r1,r2,r3
 Where to find the operands
ADD
r1,r2,r3
 Place to store the result
ADD
r1,r2,r3
 Location of next instruction
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wannarat:
Basic ISA Class
add A, B ; A = A + B
add A B C; A = B + C

Accumulator (1 register)
1 address
add A; acc <= acc + mem[A]
1 + x address addx A; acc <= acc + mem[A+x]
 Stack :
0 address
add tos <= tos + next
 General Purpose Register
2 address
add A, B
3 address
add A, B, C
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Basic ISA Classes(con’t)

Load/Store
load Ra, Rb
Store Ra, Rb
Ra <= mem[Rb]
mem[Rb] <= Ra
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Compare number of instruction
Stack
Push A
Push B
Add
Pop C
Code Sequence for C = A + B
Accumulator
Register
Register
(reg. - mem)
(load/store)
Load A
Add B
Store C
Load R1,A
Add R1,B
Store C, R1
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Load R1,A
Load R2,B
Add R3,R1,R2
Store C,R3
CPU Register
Stack Register
Arithmetic Register &
Address Register
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nnarat:
General Purpose Register
er for compiler to use : (A*B) - (C*D) - (E*F) can do multiplies in any order vs. stack
d variables: memory traffic is reduced (program speed up), code density improves ( register named w
er bits than memory location)
1975 - 1995 all machines use general purpose
registers.
 Advanced of Registers
- faster than memory
- easier for compiler to use
- hold variables

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Summary Instruction Set Class

Data Movement Instructions
- Load
- Store
 Arithmetic and Logic (ALU) Instruction
- Add, Sub, Shift …
 Branch Instructions
- Br, Brz, …
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wannarat:
3-Address Machine and ISA
Address of next instruction kept in Processor state register (PC)
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2-Address Machine ISA
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1-Address Machine and ISA
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0-Address Machine and ISA
k in CPU,
stack for both operands and result,
have 1-address instruction to push and pop operands to and
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Examples
a = (b+c)*d - e
3-address
add a,b,c
mpy a,a,d
sub a,a,e
2-address
load a,b
add a,c
mpy a,d
sub a,e
1-address
load b
add c
mpy d
sub e
store a
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stack
push b
push c
add
push d
mpy
push e
sub
pop a
Real Machine

Have mixture of 3, 2, 1 or 0 address instructions
 if ALU instructions only use registers for
operands and result, machine type is load-store
 mix of register-memory and memory-memory
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Break 5 Minutes
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Addressing Mode
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Addressing Mode
Addressing Mode Examples
Meaning
Register
Add r4,r3
r4 <= r4 + r3
Immediate Add r4,#3
r4 <= r4 + 3
Displacement Add r4,100(r1)r4 <= r4 + mem[100+r1]
indirect(r) Add r4,(r1)
r4 <= r4 + mem[r1]
index+base Add r3,(r1+r2)
r3 <= r3 + mem[r1+r2]
Direct
Add r1,(1001)
r1 <= r1 + mem[1001]
indirect(m) Add r1,@(r3)
r1 <= r1 + mem[mem[r3]]
auto-incre Add r1,(r2)+
r1 <= r1+mem[r2];r2=r2+d
auto-decre Add r1,-(r2)
r2 <=r2-d,r1<=r1+mem[r2]
scaledAdd r1,100(r2)[r3]
r1 240-334
<=r1+mem[100+r2+r3*d]
by Wannarat
MIPS Registers
31 x 32-bit GPR (R0 = 0)
 32 x 32-bit FP register
 PC
 lo hi-multiplier output register

R0
R1
PC
lo
hi
R31
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Memory Addressing

Since 1980, Most machine uses address
to level of 8-bits (byte)

How do byte address map onto words?
 Can a word be placed on any byte boundary?
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Endianess and Alignment
Big Endian : 68k, SPARC, MIPS, HP PA
 Little Endian : 80x86,DEC(Vax, Alpha)

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Generic of Instruction format width
Variable :
...
Fixed:
Hybrid :
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Summary ISA
Variable length instructions, if code
size is very important.
 Fixed length instructions, if
performance is most important.
 Embedded Machine (ARM, MIPS) have
optional mode to execute 16-bit wide.
(decide performance or density)

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To be Continuous
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Part II : Lecture 2
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MIPS ISA Target
Embedded System
 used by NEC, Nintendo, Silicon
Graphics, Sony

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MIPS ISA
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MIPS Addressing Modes

All instructions have 32-bit wide.
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MIPS Arithmetic Instruction
Instruction
1.add
2.subtract
3.add imme
4.add unsign
5.
6.
7.multiply
8.
Example
add $1,$2,$3
sub $1,$2,$3
addi $1,$2,100
addu $1,$2,$3
subu $1,$2,$3
addiu $1,$2,100
mult $2,$3
multu $2,$3
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Meaning
$1 = $2 + $3
------------------------------------------------------------------------------------Hi,Lo = $2x$3
------------------
MIPS Arithmetic Instruction
Instruction
9.divide
Example
div $2,$3
Meaning
Lo=$2/$3,
Hi=$2mod$3,
10.
11.mov
12.
Divu $2,$3
mfhi $1
mflo $1
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MIPS Logical Instruction
Instruction
Example
Meaning
13.AND
and
14.OR
or
15.XOR
xor
16.NOR
nor
17.
andi
18.
ori
19.
xori
20.shift left logical
sll $1,$2,10
21.
Srl $1,$2,10
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MIPS Logical Instruction
Instruction
Example
22.shift right arithm sra $,$2,10
23.
sllv
24.
srlv
25.
srav
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Meaning
(sign extend)
MIPS data transfer instructions
26.
27.
28.
29.
30.
31.
32
sw 500(r4),r3
sh 502(r4),r3
sb 41(r4,r3
lw r1,30(r2)
Lh r1,40(r2)
Lb r1,40(r2)
lui r1,40
load
Store word
store half word
store byte
load word
load half word
load byte
upper
immediate (16 bits shifted left by 16)
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Testing Condition
Condition Code
add
r1,r2,r3
bz
label
 Condition Register
cmp
r1,r2,r3
bgt
r1,label
 Compare and Branch
bgt
r1,r2,label

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MIPS Compare and Branch
Compare and Branch
BEQ
rs,rt,offset
BNE
rs,rt,offset
 compare to zero and Branch

BLEZ
BGTZ
BLT
BGEZ
BLTZAL
rs, offset
rs, offset
<
>=
if R[rs] < 0 then branch and link(to R31)
BGEZAL
>=
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MIPS Jump, Branch Compare
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Software conventions for Register
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Note for MIPS Instruction Set






R0 always = “0” (even if u try to write)
Branch/jump and link PC+4 ->R31
Imme arith and logical are extended
- logical imme op are zero extend to 32 bits
- arith imme op are sign extend to 32 bits
data loaded by lb, lh extended
- lbu, lhu are zero extended
- lb, lh are sign extedned
Overflow occur in ADD, SUB, ADDI
Don’t occur in ADDU, SUBU, ADDIU, AND, OR,
XOR, NOR, SHIFT, MULT, MULTU, DIV, DIVU
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MIPS arithmetic
Instruction has 3 operands
 Operand order is fixed

Pascal Code : a := b + c;
MIPS Code : add $s0, $s1, $s2
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MIPS Arithmetic
Pascal Code :
MIPS Code
:
a := b + c + d;
e := f - a;
add $t0, $s1, $s2
add $s0, $t0, $s3
sub $s4, $s5, $s0
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Register & Memory

Registers were used in Arithmetic
Instructions - 32 registers
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Memory Organization
Memory is an index into the array
 Byte Addressing = points to a byte of
memory

1
2
3
4
5
6
7
8 bits of Data
8 bits of Data
8 bits of Data
8 bits of Data
8 bits of Data
8 bits of Data
8 bits of Data
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Memory Organization
For MIPS, a word is 32-bit or 4 bytes
 232 bytes with byte addresses from 0 to
232-1
 230 words with byte address from 0, 4,
32 bits of Data
6, …,232-4 0
32 bits of Data
4
32 bits of Data
8
12 32 bits of Data
16 32 bits of Data
20 32 bits of Data
24 32 bits of Data

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MIPS Load/Store Instruction
Code
:
A[8] = h + A[8];
A[8] ==> 8 x 4 = 32 (word alignment)
MIPS Code :
lw $t0, 32($s3);
add $t0,$s2,$t0;
sw $t0, 32($s3);
Arithmetic Operand is Register, not
Memory!!
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Example :

Swap (int v[], int k);
{ Int temp;
temp = v[k];
v[k] = v[k+1];
v[k+1]=temp;
}
swap: muli $2, $5, 4
add $2,$4,$2
lw $15, 0[$2]
lw $16, 4[$2]
sw $16, 0[$2]
sw $15, 4[$2]
jr $31
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Meaning
add $2, $4, $2
lw $16, 0[$2]
sw $15, 4[$2]
$2 = $4 + $2;
$16 = Memory[0 + $2]
Memory[4+$2] = $15
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Machine Language
Instructions, like register & words of
data are 32 bits long.
- add $t0, $s1, $s2
- register : $t0 = 9, $s1 = 17, $s2 = 18
 Instruction Format

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Machine Language
I-type for Data transfer instruction
 Example : lw $t0, 32($s2)

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Control
Decision Making instructions
 MIPS conditional branch instructions:
- bne $t0, $t1, label
- beq $t0, $t1, label
 Example : if (i=j) h= i +j;
bne $s0, $s1, Label
add $s3, $s0, $s1
Label : ...

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Control
MIPS unconditional Branch
 Example :
if ( i != j)
beq $s4, $s5, label;
h=i+j;
add $s3, $s4, $s5;
else
j
lab2
h=i-j;
lab1: sub $s3, $s4, $s5
lab2: …

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Summarize :
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Homework
Use MIPS Assembly to write program.
1. “Factorial Program”
n is input, Example : if n=3,
result = 3! = 3 x 2 x 1 = 6
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Homework
Use MIPS Assembly to write program.
2. Write program that use Most of the
MIPS instructions
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MIPS Instruction Encoding
Please see more detail in the
Figure 3.18
Page 153,
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