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CHAPTER 3
INSTRUCTIONS:
LANGUAGE OF THE
MACHINE
1
Instruction Set Architecture
• Computer architecture instruction set is the interface
between hardware and software.
• The attributes of an instruction set include
– Instruction Set (what operations can be performed?)
– Instruction Format (how are instructions specified?)
– Data storage (where is data located?)
– Addressing Modes (how is data accessed?)
– Exceptional Conditions (what happens if something goes wrong?)
• A good understanding of computer architecture is
important for compiler writers, operating system designers,
and general computer programmers.
2
MIPS R3000 Instruction Set Architecture (Summary)
• Instruction Categories
–
–
–
–
–
–
R0 - R31
Load/Store
Computational
Jump and Branch
Floating Point
Memory Management
Special
PC
HI
LO
3 Instruction Formats: all 32 bits wide (fixed Size)
shamt funct
OP
rs
rd
rt
OP
OP
rs
rt
immediate
jump target
3
Instruction Execution
Instruction
Fetch
Instruction
Decode
Obtain instruction from program storage
Determine required actions and instruction size
Locate and obtain operand data:
Operand
Fetch
Execute
Result
Store
Next
Instruction
From where: memory, instruction, etc.
How many operands?
How are the operands located?
Compute result value or status:
What data type is the result?
Deposit results in storage for later use
Where to deposit the result?
Determine successor instruction
4
MIPS Instructions
• MIPS is an Assembly Language
• Assembly languages are more primitive than higher level
languages e.g., no sophisticated control flow
• Assembly languages are very restrictive
e.g., MIPS Arithmetic Instructions
• We’ll be working with the MIPS instruction set architecture
– similar to other architectures developed since the 1980's
– used by NEC, Nintendo, Silicon Graphics, Sony
Design goals: maximize performance and
minimize cost, reduce design time
5
Registers
• Registers are special locations on the processor
– “Bricks” of computer construction
– Used in hardware design and visible to the programmer
– Very fast access
6
MIPS Addressing Modes (Location of operands)
• Addressing modes specify where the data used by an
instruction is located. Data can be in registers, memory or
within the instruction itself (available immediately).
Mode
Example
register direct add $s1, $s2, $s3
Action
$s1 = $s2 + $s3
immediate
addi $s1, $s2, 200 $s1 = $s2 + 200
base + index
lw $s1, 200($s2)
$s1 = mem[200 + $s2]
PC-relative
beq $s1, $s2, 200
if ($s1 == $s2)
PC = PC+4+200*4
Pseudo-direct
j 4000
PC = (PC[31:28],
4000*4)
7
MIPS Addressing Modes
• All MIPS instructions are 32 bits wide - fixed length
• The instruction format depends on the addressing mode
Register (direct)
op
rs
rt
add $s1, $s2, $s3
rd
register
Immediate
Base+index
op
rs
rt
immed
addi $s1, $s2, 200
op
rs
rt
immed
What happens if immed. Increases?
register
Memory
+
lw $s1, 200($s2)
PC-relative
op
rs
PC
rt
immed
Memory
+
beq $s1, $s2, 200 9
MIPS Addressing Modes/Instruction Formats
Pseudo-direct
op
address
Memory
x$4
j 4000
Addressing
Mode
Register
(direct)
Instruction Format/Type
R-type
op
rs
rt
I-Type
op
rs
rt
rd
Immediate
Base + index
immed
PC-relative
Pseudo-direct J-Type
op
address
10
MIPS Instruction Fields : R-Type and I-Type
• Register type (R-type) and immediate type (I-Type) instructions
have the following formats:
e.g. add, sub, and, or
R-type
I-type
Field
op
rs
rt
rd
shamt
funct
6 bits
5 bits
5 bits
5 bits
5 bits
6 bits
op
rs
rt
immed
sw, lw, beq
op
Meaning
Basic operation of the instruction (opcode)
rs
First register source operand
rt
rd
Second register source operand
Register destination operand (gets result)
shamt
Shift amount
funct
Function field - selects the variant of the operation in the op field
(function code)
immed
Immediate value
11
Representing Instructions in the Computer
• Computer represents numbers in base 2 (binary)
– Series of high and low electronic signals in hardware (on and off)
• Instructions are stored in hardware in the same way
– Can be represented as numbers
• Assembly to machine code
add $t0, $s0, $s1
op
rs
rt
17
18
8
6 bits
5 bits
5 bits
5 bits
000000
10001
10010
01000
0
rd
shamt
0
5 bits
00000
funct
32
6 bits
100000
12
Register Names in MIPS Assembly Language
• There is a convention for mapping register names into general
purpose register numbers. Only 32 registers provided.
• Design Principle: smaller is faster. Why?
A large number of registers increase the instruction
execution time because electronic signals longer
travel further.
With 32 registers, each can be represented using
just 5 bits. If you increase the number of registers,
more bits will be required.
13
Register Names in MIPS Assembly Language
Register name
Register number
Register usage
$zero
0
constant 0
$at
1
assembler: large constants
$v0-$v1
2-3
results (func. ret. values)
$a0-$a3
4-7
arguments
$t0-$t7
8-15
temporaries
$s0-$s7
16-23
saved
$t8-$t9
24-25
more temps
HI, LO
26-27
multiplication and division
$gp
28
global pointer
$sp
29
stack pointer
$fp
30
frame pointer
$ra
31
return address
14
MIPS arithmetic
• A compiler translates high-level code to assembly language e.g
MIPS. All MIPS arithmetic instructions have 3 operands
– Design Principle: Simplicity favors regularity
– Same number of operands and in the same order
• Variables are typically stored in registers - why ?
• Operand order is fixed (destination first)
• Example 1:
C code:
a = b + c;
MIPS code: add $s0, $s1, $s2
The registers $s0, $s1, $s2 are associated with variables by compiler, say $s0
with a, $s1 with b and $s2 with c.
•
Example 2:
C code:
MIPS code:
a = b - c;
sub $s0, $s1, $s2
15
MIPS arithmetic: Using temporary registers
• Example 3:
C Code:
a = (b + c) - (d + c);
Assume the variables a, b, c, and d are in registers $s3, $s4,
$s5, and $s6, respectively.
Instruction
add $t2, $s4, $s5
add $t3, $s6, $s5
sub $s3, $t2, $t3
Comment
$t2 = b + c
$t3 = d + c
a = $t2 - $t3
16
Registers vs. Memory
• Arithmetic instructions operands must be registers,
— only 32 registers provided
• Compiler associates variables with registers
• What about programs with lots of variables? The compiler
keeps the most frequently used variables in registers and the
rest in memory. This is called spilling of registers.
Control
Input
Memory
Datapath
Processor
Output
I/O
17
Memory Organization
• Viewed as a large, single-dimension array, with an address.
• A memory address is an index into the array
• "Byte addressing" means that each index points to a byte of
memory.
0
1
2
3
4
5
6
...
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
• Bytes are nice, but most data items use larger "words"
• For MIPS, a word is 32 bits or 4 bytes.
0
4
8
12
...
32 bits of data
32 bits of data
32 bits of data
Registers hold 32 bits of data
32 bits of data
• 232 bytes with byte addresses from 0 to 232-1
• 230 words with byte addresses 0, 4, 8, ... 232-4
• Words are aligned. That is, their addresses are multiples of 4.
19
Stored Program Concept
• Instructions are bits
• Programs are stored in memory
— to be read or written just like data
Processor
Memory
memory for data, programs,
compilers, editors, etc.
Fetch & Execute Cycle
– Instructions are fetched and put into a special register
(PC)
– Bits in the register "control" the subsequent actions
– Fetch the “next” instruction and continue
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Example of Using MIPS Instructions - Arrays
• Arrays are often stored in memory - why?
• Replace the C code for
A[11] = A[10] + b
•
•
•
•
by equivalent MIPS instructions.
Assume b is in register $s5, the starting address for array A is in
$s6, using and 32-bit integer data.
Instruction
Comment
lw $t3, 40 ($s6)
$t3 = A[10]
add $t4, $t3, $s5
$t4 = A[10] + b
sw $t4, 44($s6)
A[11] = $t4
Why are array indices multiplied by 4? Store word has
destination last
Write assembly instructions to:
b = A[10] + c;
A[11] = b + c;
21
MIPS – Conditional / Unconditional Instructions
• Conditional statements allow us to make decisions. These
decision making instructions
– alter the control flow,
– i.e., change the "next" instruction to be executed
• MIPS unconditional branch instructions:
j
label
• MIPS conditional branch instructions:
bne $t0, $t1, Label
beq $t0, $t1, Label
• Example:
MIPS:
if (i==j) h = i + j;
bne $s0, $s1, Label
add $s3, $s0, $s1
Label:
....
22
MIPS – Conditional / Unconditional Instructions
• Replace the C code for
if (i = = j) f = g + h; else f = g - h;
by equivalent MIPS instructions.
Assume variables f through j correspond to registers $s0 through $s4.
Instruction
bne $s3, $s4, Else
add $s0, $s1, $s2
j Exit
Else: sub $s0, $s1, $s2
Exit:
Comment
if (i != j) goto Else
f=g+h
go to Exit
f=g-h
• How would you implement the loop
while (k < j) k = k + j;
23
MIPS function assembling
• Assume v = $a0, k = $a1, temp = $s1.
• Temporaries used are $t1 (storing into array) and a
temporary holder $t2 (for address of array element).
swap(int v[], int k)
{
int temp;
temp = v[k];
v[k] = v[k+1];
v[k+1] = temp;
}
swap:
# save s/t registers on stack here
muli $t2, $a1, 4
#t2 = k*4
add $t2, $a0, $t2 #t2 has address of v[k]
lw $s1, 0($t2)
#temp = v[k]
lw $t1, 4($t2)
#t1 gets v[k+1]
sw $t1, 0($t2)
#store t1 into v[k]
sw $s1, 4($t2)
#store temp into v[k+1]
# restore registers from stack here
jr $ra
#jump back after call
24
Machine Language
• Instructions, like registers and words of data, are also 32 bits
long
– Example: add $t0, $s1, $s2
– registers have numbers, $t0=8, $s1=17, $s2=18
• Instruction Format:
000000 10001
op
rs
10010
rt
01000
00000
100000
rd shamt funct
• Can you guess what the field names
stand for?
op = Basic operation of the instruction: opcode
rs = The first register source operand
rt = The second register source operand
rd = The register destination operand
shamt = shift amount
funct = function code
25
Machine Language
• Consider the load-word and store-word instructions,
– Design Principle: Good design demands a compromise (maintaining
same format)
• Introduce a new type of instruction format
– I-type for data transfer instructions
– Other format was R-type for register computations
• Example: lw $t0, 32($s2)
35
op
$s2
$t0
18
rs
8
rt
32
16 bit number
• Where's the compromise? (To maintain same length, we settled
with introducing another format RATHER that having the same
format but working with varying lengths)
26
Machine Language
27
So far:
• Instruction
Meaning
add $s1,$s2,$s3
sub $s1,$s2,$s3
and $s1,$s2,$s3
lw $s1,100($s2)
sw $s1,100($s2)
bne $s4,$s5,L
beq $s4,$s5,L
j Label
$s1 = $s2 + $s3
$s1 = $s2 – $s3
$s1 = $s2 and $s3
$s1 = Memory[$s2+100]
Memory[$s2+100] = $s1
Next instr. is at Label if $s4 ° $s5
Next instr. is at Label if $s4 = $s5
Next instr. is at Label
• Formats:
R
op
rs
rt
rd
shamt
funct
I
op
rs
rt
16 bit address
J
op
26 bit address
28
Constants
• Small constants are used quite frequently (50% of operands)
e.g.,
A = A + 5;
B = B + 1;
C = C - 18;
• Design Principle: Make the common fast
– put 'typical constants' in memory and load them.
– create hard-wired registers (like $zero) for constants like zero.
addi $t0, $zero, 100
add $t0, $t1, $zero
29
Assembly Language vs. Machine Language
• Assembly provides convenient symbolic representation
– much easier than writing down numbers
– e.g., destination first
• Machine language is the underlying reality
– e.g., destination is no longer first
• Assembly can provide 'pseudo-instructions'
– e.g., “move $t0, $t1” exists only in Assembly
– would be implemented using “add $t0,$t1,$zero”
• When considering performance you should count real
instructions
30
Pseudo-instructions
• The MIPS assembler supports several pseudo-instructions:
– not directly supported in hardware
– implemented using one or more supported instructions
– simplify assembly language programming and translation
• For example, the pseudo-instruction
move $t0, $t1
is implemented as
add $t0, $zero, $t1
• The pseudo-instruction
blt $s0, $s1, Else
is implemented as
slt $at, $s0, $s1
bne $at, $zero, Else
It is safer to use labels, rather than constants, when implementing
branches. Why?
31
Other Issues
• Things we are not going to cover
sign and zero-extension
handling larger constants
support for procedures
linkers, loaders, memory layout
stacks, frames, recursion
manipulating strings and pointers
interrupts and exceptions
system calls and conventions
• We've focused on architectural issues
– basics of MIPS assembly language and machine code
32
Miscellaneous MIPS Instructions
• break
– A breakpoint trap occurs, transfers control to exception handler
• syscall
– A system trap occurs, transfers control to exception handler
• coprocessor instructions
– Provide support for floating point
• TLB instructions
– Provide support for virtual memory
• return from exception
– Used after an exception is generated to restore control to user
• load word left/right
– Supports misaligned word loads
• store word left/right
– Supports misaligned word stores
• All MIPS R2000 Instructions are given in Appendix A.10
33
Alternative Architectures
• Design alternative:
– provide more powerful operations
– goal is to reduce number of instructions executed
– danger is a slower cycle time and/or a higher CPI
• Sometimes referred to as “RISC vs. CISC” Reduced vs.
Complex Instruction Set Computer.
– virtually all new instruction sets since 1982 have been RISC
– VAX: minimize code size, make assembly language easy
instructions from 1 to 54 bytes long!
34
PowerPC
• Indexed addressing
– example:
lw $t1,$a0+$s3
– What do we have to do in MIPS?
#$t1=Memory[$a0+$s3]
• Update addressing
– update a register as part of load (for marching through arrays)
– example: lwu $t0,4($s3)
#$t0=Memory[$s3+4];$s3=$s3+4
– What do we have to do in MIPS?
• Others:
– load multiple/store multiple
– a special counter register “bc Loop”
decrement counter, if not 0 goto loop
35
80x86
•
•
•
•
•
1978: The Intel 8086 is announced (16 bit architecture)
1980: The 8087 floating point coprocessor is added
1982: The 80286 increases address space to 24 bits, + instructions
1985: The 80386 extends to 32 bits, new addressing modes
1989-1995: The 80486, Pentium, Pentium Pro add a few
instructions
(mostly designed for higher performance)
• 1997: MMX is added
“This history illustrates the impact of the “golden handcuffs” of compatibility
“adding new features as someone might add clothing to a packed bag”
“an architecture that is difficult to explain and impossible to love”
36
A dominant architecture: 80x86
• See your textbook for a more detailed description
• Complexity:
–
–
–
–
Instructions from 1 to 17 bytes long
one operand must act as both a source and destination
one operand can come from memory
complex addressing modes
e.g., “base or scaled index with 8 or 32 bit displacement”
• Saving grace:
– the most frequently used instructions are not too difficult to build
– compilers avoid the portions of the architecture that are slow
“what the 80x86 lacks in style is made up in quantity,
making it beautiful from the right perspective”
37
Summary
• Instruction complexity is only one variable
– lower instruction count vs. higher CPI / lower clock rate
• Design Principles:
–
–
–
–
simplicity favors regularity
smaller is faster
good design demands compromise
make the common case fast
• Instruction set architecture
– a very important abstraction indeed!
38
To summarize: MIPS Instructions
MIPS operands
Name
32 registers
Example
Comments
$s0-$s7, $t0-$t9, $zero, Fast locations for data. In MIPS, data must be in registers to perform
$a0-$a3, $v0-$v1, $gp,
arithmetic. MIPS register $zero always equals 0. Register $at is
$fp, $sp, $ra, $at
reserved for the assembler to handle large constants.
Memory[0],
Accessed only by data transfer instructions. MIPS uses byte addresses, so
30
2 memory Memory[4], ...,
sequential words differ by 4. Memory holds data structures, such as arrays,
words
and spilled registers, such as those saved on procedure calls.
Memory[4294967292]
add
MIPS assembly language
Example
Meaning
add $s1, $s2, $s3
$s1 = $s2 + $s3
Three operands; data in registers
subtract
sub $s1, $s2, $s3
$s1 = $s2 - $s3
Three operands; data in registers
$s1 = $s2 + 100
$s1 = Memory[$s2 + 100]
Memory[$s2 + 100] = $s1
$s1 = Memory[$s2 + 100]
Memory[$s2 + 100] = $s1
Used to add constants
Category
Arithmetic
Instruction
addi $s1, $s2, 100
lw $s1, 100($s2)
load word
sw $s1, 100($s2)
store word
lb $s1, 100($s2)
load byte
sb $s1, 100($s2)
store byte
load upper immediate lui $s1, 100
add immediate
Data transfer
Conditional
branch
Unconditional jump
$s1 = 100 * 2
16
Comments
Word from memory to register
Word from register to memory
Byte from memory to register
Byte from register to memory
Loads constant in upper 16 bits
branch on equal
beq
$s1, $s2, 25
if ($s1 == $s2) go to
PC + 4 + 100
Equal test; PC-relative branch
branch on not equal
bne
$s1, $s2, 25
if ($s1 != $s2) go to
PC + 4 + 100
Not equal test; PC-relative
set on less than
slt
$s1, $s2, $s3
if ($s2 < $s3) $s1 = 1;
else $s1 = 0
Compare less than; for beq, bne
set less than
immediate
slti
jump
j
jr
jal
jump register
jump and link
$s1, $s2, 100 if ($s2 < 100) $s1 = 1;
Compare less than constant
else $s1 = 0
2500
$ra
2500
Jump to target address
go to 10000
For switch, procedure return
go to $ra
$ra = PC + 4; go to 10000 For procedure call
39
To summarize: Policy of Use Conventions
Name Register number
$zero
0
$v0-$v1
2-3
$a0-$a3
4-7
$t0-$t7
8-15
$s0-$s7
16-23
$t8-$t9
24-25
$gp
28
$sp
29
$fp
30
$ra
31
Usage
the constant value 0
values for results and expression evaluation
arguments
temporaries
saved
more temporaries
global pointer
stack pointer
frame pointer
return address
40
To summarize: MIPS Addressing Modes
1. Immediate addressing
op
rs
rt
Immediate
2. Register addressing
op
rs
rt
rd
...
funct
Registers
Register
3. Base addressing
op
rs
rt
Memory
Address
+
Register
Byte
Halfword
Word
4. PC-relative addressing
op
rs
rt
Memory
Address
PC
+
Word
5. Pseudodirect addressing
op
Address
PC
Memory
Word
41