i + j

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Chapter 2
Instructions: language of the
Machine
授課教師: 張傳育 博士 (Chuan-Yu Chang Ph.D.)
E-mail: [email protected]
Tel: (05)5342601 ext. 4337
Instructions:
1400
1300
SPARC
1200
Hitachi SH
1100
PowerPC
1000
Language of the Machine
Other
Motorola 68K
MIPS
900
IA-32
800
ARM
More primitive than higher level languages
e.g., no complex control flow
700
600
500
400
Very restrictive
e.g., MIPS Arithmetic Instructions
300
200
100
0
1998
1999
2000
2001
2002
We’ll be working with the MIPS instruction set
architecture
similar to other architectures developed since the 1980's
Almost 100 million MIPS processors manufactured in 2002
used by NEC, Nintendo, Cisco, Silicon Graphics, Sony, …
Design goals: To find a language that makes it easy to build the hardware
and compiler while maximizing performance and minimize cost,
reduce design time
2
MIPS arithmetic
All instructions have 3 operands
Operand order is fixed (destination first)
$s0, $s1,… for registers that correspond to variables in
C programs
$t0, $t1,… for temporary registers needed to compile
the program into MIPS instruction.
Example:
C code:
A = B + C
MIPS code:
add $s0, $s1, $s2
(associated with variables by compiler)
3
Examples
Compiling Two C Assignment Statements into MIPS
a = b + c;
d = a – e;
Solution
add a, b, c
sub d, a, e
Compiling a Complex C Assignment into MIPS
f = (g + h) – (i + j);
Solution
add t0, g, h
add t1, i, j
sub f, t0, t1
4
MIPS arithmetic
Design Principle: simplicity favors regularity(簡單明瞭有
助於一致性).
The variables a, b, c, d, e and f are
Example:
a =
e =
MIPS code: add
add
sub
C code:
assigned to the registers $s0, $s1, $s2,
$s3, $s4, $s5.
b + c + d;
f - a;
$t0, $s1, $s2
$s0, $t0, $s3
$s4, $s5, $s0
The size of a register
in the MIPS
architecture is 32 bits
Operands must be registers, only 32 registers provided
Design Principle: smaller is faster.
5
Example
Compiling a C assignment Using Registers
f = (g + h) – (i + j);
Solution
The variables f, g, h, i, and j are assigned to the registers
$s0, $s1, $s2, $s3, $s4.
The compiled MIPS code :
add t0, $s1, $s2
add t1, $s3, $s4
sub $s0, $t0, $t1
6
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 CPU can keep only a small amount of data in registers.
Computer memory contains millions of data elements.
MIPS must include instructions (data transfer instruction) that
transfer data between memory and registers..
Control
Input
Memory
Datapath
Processor
Output
I/O
7
Memory Organization
How can a computer represent and access large memory?
Viewed as a large, single-dimension array, with an
address.
A memory address is an index into the array
"Byte addressing" means that the index points to a byte of
memory.
0
1
2
3
4
5
6
...
8 bits of data
8 bits of data
3
100
2
10
1
101
0
1
Address
Data
8 bits of data
8 bits of data
8 bits of data
8 bits of data
8 bits of data
Processor
Memory
8
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
9
Instructions
Load instructions
lw: (load word): moves data from memory to a register.
Store instructions
sw: (store word): transfers data from a register to memory
Store word has destination last
Example:
Let’s assume that A is an array of 100 words and that the compiler
has associated the variables g and h with the registers $s1 and $s2
as before. The starting address of the array is in $s3. Translate this C
assignment statement:
C code:
g = h + A[8]
Base register
MIPS code:
lw
$t0, 32($s3)
add
$s1, $s2, $t0
A[8] = h + A[8];
lw
$t0, 32($s3)
add
$t0, $s2, $t0
sw
$t0, 32($s3)
Remember arithmetic operands are registers, not memory!
C code:
MIPS code:
offset
10
Compiling using a variable index
Example: g = h + A [i]
Assume A is an array of 100 elements whose base is in register $s3
and that the compiler associates the variables g, h and i with the
registers $s1, $s2, and $s4. What is the MIPS assembly code
corresponding to this C segment?
Solution:
add
add
add
lw
add
$t1,
$t1,
$t1,
$t0,
$s1,
$s4, $s4
$t1, $t1
$t1, $s3
0($t1)
$s2, $t0
# Temp reg $t1 = 2 * i
# Temp reg $t1 = 4 *i
# $t1 = address of A[i] (4*i+$s3)
# Temporary reg $t0 = A[i]
# g = h + A[i]
11
Constant and Immediate Operands
The constants would have been placed in
memory when the program was loaded.
lw $t0, AddrConstant4 ($s1) # $t0=constant 4
add $s3, $s3, $t0
# $s3=$s3+$t0 ($t0==4)
Assuming that AddrConstant4 is the memory
address of the constant 4.
Add immediate (addi)
Addi
$s3, $s3, 4
#$s3=$s3+4
To add 4 to register $s3
12
So far we’ve learned:
MIPS
— loading words but addressing bytes
— arithmetic on registers only
Instruction
add
sub
lw
sw
$s1,
$s1,
$s1,
$s1,
Meaning
$s2, $s3
$s2, $s3
100($s2)
100($s2)
$s1 = $s2 + $s3
$s1 = $s2 – $s3
$s1 = Memory[$s2+100]
Memory[$s2+100] = $s1
13
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
$s0~$s7:16~23, $t0~$t7:8-15
Instruction Format:
R-Type
0000001000110010 01000 00000 100000
op
rs
rt
rd shamt funct
6 bits
5 bits
5 bits
5bits
5 bits
6 bits
14
Machine Language
MIPS Fields
op: opcode, basic operation of the instruction
rs: the first source operand register
rt: the second source operand register
rd: the destination operand register
shamt: shift amount
funct: function, selects the specific variant of the op
15
Machine Language
Consider the load-word and store-word instructions,
What would the regularity principle have us do?
New principle: Good design demands a compromise
Introduce a new type of instruction format
I-type for data transfer instructions
other format was R-type for register
Example:lw $t0, 32($s3)#Temporary reg $t0 gets A[8]
35
op
6 bits
19
rs
5 bits
8
rt
32
address
5 bits
16 bits
Where's the compromise?
Specifies the destination register
To keep all instructions the same length, thereby requiring different
kinds of instruction formats for different kinds of instructions.
16
MIPS Instruction Encoding
Instruction Format op
rs
rt
rd
shamt
funct
address
add
R
0
reg reg reg
0
32
n.a.
sub
R
0
reg reg reg
0
34
n.a.
lw
I
35
reg reg n.a.
n.a.
n.a.
address
sw
I
43
reg reg n.a.
n.a.
n.a.
address
17
Translating MIPS Assembly Language into
Machine Language
Example: Assuming that $t1 has the base of the array A and that $s2
corresponds to h, the C assignment statement
A[300] = h + A[300]
is compiled into
lw $t0, 1200($t1)
add $t0, $s2, $t0
sw $t0, 1200($t1)
What is the MIPS machine language code for these three instructions?
Solution:
op
rs
rt
35
9
8
0
18
8
43
9
8
rd
Address/
shamt
funct
1200
8
0
32
1200
18
Stored Program Concept
Today’s computers are built on two key principles:
Instructions are represented as numbers (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
Bits in the register "control" the subsequent actions
Fetch the “next” instruction and continue
19
Instructions for Making Decisions:
Control
Decision making instructions
alter the control flow,
i.e., change the "next" instruction to be executed
MIPS conditional branch instructions:
Branch if equal: beq
beq register1, register2, L1
if the value of register1 equals the value of register2 then go to
the statement labeled L1.
Branch if not equal: bne
bne register1, register2, L1
if the value of register1 does not equals the value of register2 then
go to the statement labeled L1.
Example:
if (i==j) h = i + j; Assume that i, j and h mapping to
$s0,$s1, and $s3
bne $s0, $s1, Label
add $s3, $s0, $s1
Label:
....
20
Example: Compiling an If Statement into a
Conditional Branch.
Example: C code segment
if (i == j) go to L1;
f = g + h;
L1: f = f-i;
Assuming that the five variables f through j correspond to the five
registers $s0 through $s4, what is the compiled MIPS code?
Solution:
L1:
beq
add
sub
$s3, $s4, L1
$s0, $s1, $s2
$s0, $s0, $s3
21
Control: if-then-else
MIPS unconditional branch instructions:
j label
Example:
if (i!=j)
h=i+j;
else
h=i-j;
beq $s3, $s4, Lab1
add $s2, $s3, $s4
j Lab2
Lab1: sub $s2, $s3, $s4
Lab2: ...
Example:
ij
i =j
i == j?
if ( i == j ) f = g + h; else f = g – h;
Else:
Solution:
Else:
Exit:
bne
add
j
sub
$s3, $s4, Else
$s0, $s1, $s2
Exit
$s0, $s1, $s2
f =g– h
f= g+h
Exit:
22
Control: Loops
Example: Here is a loop in C:
Loop:
g = g + A[i];
i = i + j;
if ( i != h ) goto Loop;
Assume that A is in $s5, g, h, i,j are in $s1 through $s4. What is the
MIPS assembly code corresponding to this C loop?
Solution:
Loop:
add
add
add
lw
add
add
bne
$t1,
$t1,
$t1,
$t0,
$s1,
$s3,
$s3,
$s3, $s3
$t1, $t1
$t1, $s5
0($t1)
$s1, $t0
$s3, $s4
$s2, Loop
#Temp reg $t1 = 2 * i
#Temp reg $t1 = 4 * i
#$t1 = address of A[i]
# g = g + A[i]
# i = i + j
# if ( i != h ) goto Loop
23
Control: While Loops
Example: Here is a traditional loop in C
while ( save [ i ] == k)
i = i + j;
Assume that i, j, and k correspond to registers $s3, $s4, and $s5,
and the base of the array save is in $s6. What is the MIPS assembly
code corresponding to this C segment?
Solution:
Loop: add
add
add
lw
bne
add
j
Exit:
$t1,
$t1,
$t1,
$t0,
$t0,
$s3,
Loop
$s3, $s3
$t1, $t1
$t1, $s6
0($t1)
$s5, Exit
$s3, $s4
# go to Exit if save[i] != k
# i = i +j
# go to Loop
24
So far:
Instruction
Meaning
add $s1,$s2,$s3
sub $s1,$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
$s1 = Memory[$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
I
op
rs
rt
16 bit address
J
op
shamt
funct
26 bit address
25
Control Flow
We have: beq, bne, what about Branch-if-less-than?
New instruction:
if
slt $t0, $s1, $s2
$s1 < $s2 then
$t0 = 1
else
$t0 = 0
slt (set on less than)
Note that the assembler needs a register to do this,
— there are policy of use conventions for registers
26
Control: Branch on Less Than
Example:
What is the code to test if register $s0 is less than register $s1 and
than branch to label Less if the condition holds?
Solution:
slt $t0, $s0, $s1
bne $t0, $zero, Less
Register $zero always contains 0.
The pair of instructions, slt and bne, implements
branch on less than.
Jump register (jr)
An unconditional jump to the address specified in a register.
27
Control: Case/Switch Statement
The simplest way to implement switch is via a
sequence of conditional tests, turning the switch
statement into a chain of if-then-else statements.
Encoded as a table of addresses of alternative instruction
sequences, called jump address table.
The program needs only to index into the table and then
jump to the appropriate sequence.
The jump table is an array of words containing addresses
that correspond to labels in the code.
MIPS includes a jump register (jr) instruction, meaning an
unconditional jump to the address specified in a register.
The program loads the appropriate entry from the jump
table into a register, and then it jumps to the proper address
using a jump register.
28
Control: Case/Switch Statement
Example:
switch (k) {
case 0: f = i + j; break;
case 1: f = g + h; break;
case 2: f = g - h; break;
case 3: f = i - j; break;
}
Assume that variables f through k correspond to six registers $s0
through $s5 and the register $t2 contains 4. What’s the MIPS code?
Solution:
slt
bne
slt
beq
add
add
add
lw
jr
檢查k是否小於0 檢查k是否小於4
$t3, $s5, $zero
L0:
add
$s0, $s3, $s4
$t3, $zero, Exit
j
Exit
$t3, $s5, $t2
L1:
add
$s0, $s1, $s2
$t3, $zero, Exit
j
Exit
$t1, $s5, $s5
L2:
sub
$s0, $s1, $s2
$t1, $t1, $t1
j
Exit
$t1, $t1, $t4
L3:
sub
$s0, $s3, $s4
$t0, 0($t1)
Exit:
$t0
假設JumpTable的起始位址為$t4
29
Control: Case/Switch Statement
Jump address table
Jump address table 的起始位址為
$t4,每個location為4 bytes。
L0
L1
L2
L3
Jump Address Table
30
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
$ra
31
Usage
the constant value 0
values for results and expression evaluation
arguments
temporaries
saved
more temporaries
global pointer
stack pointer
frame pointer
return address
Register 1 ($at) reserved for assembler, 26-27 for operating system
31
Supporting Procedures in Computer Hardware
Execution a procedure, the program must follow these six steps:
Place parameters in a place where the procedure can access them
Transfer control to the procedure
Acquire the storage resources need for the procedure.
Perform the desired task.
Place the result value in a place
Return control to the point of origin
MIPS allocates 7 registers for procedure call
$a0-$a3: to pass parameters
$v0-$v1: to return values
$ra: to return the point of origin.
Jump-and-link instruction (jal)
jal Procedure Address
The jal instruction saves PC+4 in register $ra
32
What happens when a procedure is called
Before calling a procedure, the caller must:
1. Pass the arguments to the callee procedure;
The 4 arguments are passed in registers $a0-$a3 ($4 -$7). The remaining arguments
are placed on the stack.
2. Save any caller-saved registers that the caller expects to use after the call.
This includes the argument registers and the temporary registers $t0-$t9. (The
callee may use these registers, altering the contents.)
3. Execute a jal to the called procedure (callee). This saves the return address
in $ra.
At this point, the callee must set up its stack frame:
1. Allocate memory on the stack by subtracting the frame size from the $sp.
2. Save any registers the caller expects to have left unchanged.
These include $ra, $fp, and the registers $s0 - $s7.
3. Set the value of the frame pointer by adding the stack frame size to $fp and
subtracting 4.
The procedure can then execute its function.
Note that the argument list on the stack belongs to the stack frame of the
caller.
33
Returning from a procedure
When the callee returns to the caller, the following steps are
required:
1. If the procedure is a function returning a value, the value is placed in
register $v0 and, if two words are required, $v1 (registers $2 and $3).
2. All callee-saved registers are restored by popping the values from the
stack, in the reverse order from which they were pushed.
3. The stack frame is popped by adding the frame size to $sp.
4. The callee returns control to the caller by executing jr $ra
Note that some of the operations may not be required for every
procedure call, and modern compilers would only generate the steps
required for each particular procedure.
For example, the lowest level subprograms to be called (\leaf nodes")
would not have to save $ra.
If a programming language does not allow a subprogram to call itself
(recursion) then implementing a stack frame may not be required,
but a stack is still required for nested procedure calls.
34
Supporting Procedures in Computer Hardware (cont.)
Return jump
jr $ra
Stack (last in first out)
Push: placing data onto the stack
Pop: removing data from the stack
Stack grow from higher address to lower address.
Stack pointer
$sp : used to save the registers needed by the callee
caller
callee X
($a0~$a3)
jal X
($v0~$v1)
jr $ra
35
Example: Compiling a procedure
that does not call another procedure
What is the compiled MIPS assembly code?
int leaf_example(int g, int h, int i, int j)
{
int f;
f = ( g + h ) – ( i + j ); 在leaf_example的procedure中,
會使用到 3個暫時性的暫存器,
return
f;
因此需預留3*4=12byte
}
Solution
The parameter variables g, h, i, and j correspond to the argument registers
$a0, $a1, $a2, and $a3, and f corresponds to $s0.
sub
sw
sw
sw
add
add
sub
add
lw
lw
lw
add
jr
High address
$sp, $sp, 12
$t1, 8($sp)
$t0, 4($sp)
$s0, 0($sp)
$sp
$sp
Contents of register $t1
$t0, $a0, $a1
Contents of register $t0
$t1, $a2, $a3
$sp
Contents of register $s0
$s0, $t0, $t1
$v0, $s0, $zero
$s0, 0($sp)
Low address
a.
b.
c.
$t0, 4($sp)
$t1, 8($sp)
將暫存器$t1, $t2, $s0的值,儲存在堆疊中。
$sp, $sp, 12
$ra
36
將堆疊中的值,回存暫存器$t1, $t2, $s0。
Nest Procedures
Push all the other registers that must be preserved onto
the stack.
The stack pointer $sp is adjusted to account for the
number of registers placed on the stack.
37
Example: Compiling a recursive procedure,
showing nested procedure linking
int fact (int n)
{
if
( n <1 ) return (1);
else
return (n * fact (n-1) );
}
Solution
fact:
因為會用到$a0和返回位址$ra,
sub
$sp, $sp, 8
所以需要2個位址,並且先將$a0和
sw
$ra, 4($sp)
$ra儲存在堆疊中。
sw
$a0, 0($sp)
slt
$t0, $a0, 1
#test for n<1
beq
$t0, $zero, L1 #if n>=1, goto L1
add
$v0, $zero, 1
#return 1
add
$sp, $sp, 8
#pop 2 items off stack
jr
$ra
L1:
sub
$a0, $a0, 1
#n>=1: argument gets (n-1)
jal
fact
#call fact with (n-1)
lw
$a0, 0($sp) #return from jal: restore argument n
lw
$ra, 4 ($sp)
#restore the return address
addi
$sp, $sp, 8
mul
$v0, $a0, $v0
jr
$ra
38
The MIPS memory allocation for program and data
The data segment is divided
into 2 parts, the lower part for
static data (with size known at
compile time) and the upper
part, which can grow, upward,
for dynamic data structures.
$sp
7fff ffff
hex
The stack segment varies in
size during the execution of a
program, as functions are
called and returned from.
It starts at the top of memory
and grows down.
Stack
Dynamic data
$gp
1000 8000
hex
1000 0000
Static data
hex
Text
pc
0040 0000
hex
Reserved
0
39
What is preserved across a procedure call
Preserved
Not Preserved
Saved registers: $s0~$s7
temporary registers: $t0~$t9
Stack pointer register : $sp
Argument register : $a0~$a3
Return address register: $ra
Return value register: $v0~$v1
Stack above the stack pointer
Stack below the stack pointer
40
Beyond Numbers
Load byte (lb)
Loads a byte from memory, placing it in the rightmost 8 bits of a register.
Store byte (sb)
Takes a byte from the rightmost 8 bits of a register and writes it to
memory.
lb
sb
$t0, 0($sp) # read byte from source
$t0, 0($sp) # write byte to destination
There are three choices for representing a string:
The first position of the string is reserved to give the length of a string.
An accompanying variable has the length of the string
The last position of a string is indicated by a character used to mark the
end of a string.
41
Example: Compiling a string copy
procedure, showing how to use C strings
void
strcpy (char x[], char y[])
{
int
i;
i = 0;
while ( ( x[i] = y[i] ) != 0)
i = i + 1 ;
}
array x and y are in $a0 and $a1
Solution
i is in $s0
strcpy:
sub
sw
add
L1: add
lb
add
sb
beq
add
j
L2: lw
add
jr
$sp, $sp, 4
$s0, 0($sp)
$s0, $zero, $zero
$t1, $a1, $s0
$t2, 0($t1)
$t3, $a0, $s0
$t2, 0($t3)
$t2, $zero, L2
$s0, $s0, 1
L1
$s0, 0($sp)
$sp, $s0, 4
$ra
#adjust stack for 1 more item
# save $s0
#i=0
#address of y[i] in $t1
$t2 = y[i]
#address of x[i] in $t3
#x[i] = y[i]
# if y[i]==0, go to L2
#i = i+1
# go to L1
# y[i] ==0; end of string
#restore old $s0, pop 1 word off stack
#return
42
Constants
Small constants are used quite frequently (50% of operands)
e.g.,
A = A + 5;
B = B + 1;
C = C - 18;
Solutions? Why not?
put 'typical constants' in memory and load them.
Ex: to add the constant 4 to register $sp
Lw
$t0, AddrConstant4
add
$sp, $sp, $t0
create hard-wired registers (like $zero) for constants like one.
Example: Translating Assembly Constants into Machine Language
Solution: addi $sp, $sp, 4
op (6 bit) rs (5 bit)
rt (5 bit)
Immediate(16 bit)
8
29
29
4
001000
11101
11101
0000 0000 0000 0100
43
Immediate Operands
Immediate version of the set on less than instruction:
$t0, $s2, 10
slti
#$t0 = 1 if $s2 <10
Load upper immediate instruction:
To set the upper 16 bits of a constant in a register.
$t0, 255
lui
# $t0 is register 8
The machine language version of lui $t0, 255
op
rs
rt
immediate
001111
00000
01000
0000 0000 1111 1111
Content of register $t0 after executing lui $t0, 255
0000 0000 1111 1111
0000 0000 0000 0000
44
How about larger constants?
We'd like to be able to load a 32 bit constant into a register
Must use two instructions, new "load upper immediate" instruction
lui $t0, 1010101010101010
filled with zeros
1010101010101010
0000000000000000
Then must get the lower order bits right, i.e.,
ori $t0, $t0, 1010101010101010
1010101010101010
0000000000000000
0000000000000000
1010101010101010
1010101010101010
1010101010101010
ori
45
Example: Loading a 32-bit constant
What is the MIPS assembly code to load this 32-bit
constant into register $s0?
0000 0000 0011 1101 0000 1001 0000 0000
Solution (1):
lui
$s0, 61
0000 0000 0011 1101 0000 0000 0000 0000
addi
$s0, $s0, 2304
0000 0000 0011 1101 0000 1001 0000 0000
Solution (2): (discuss in chapter 4)
lui
ori
$s0, 61
$s0, $s0, 2304
46
Addresses in Branches and Jumps
Instructions:
bne $t4,$t5,Label
Next instruction is at Label if $t4 <> $t5
beq $t4,$t5,Label
Next instruction is at Label if $t4 = $t5
j Label
Next instruction is at Label
Formats:
I
J
op
op(6bit)
rs
rt
16 bit address
26 bit address
Conditional branch
unconditional branch
Addresses are not 32 bits
How do we handle this with load and store instructions?
Program counter = register + branch address
PC-relative addressing
47
Showing Branch Offset in Machine Language
If we assume that the loop is placed starting at location 80000.
Loop: add
$t1, $s3, $s3
add
$t1, $t1, $t1
add
$t1, $t1, $s6
lw
$t0, 0($t1)
bne
$t0, $s5, Exit
add
$s3, $s3, $s4
j
Loop
Exit:80000
0
19
19
9
0
Solution: 80004
0
9
9
9
0
80008
0
9
22
9
0
80012
35
9
8
0
80016
5
8
21
8 (2)
80020
80024
0
19
20
19
0
80028
2
80000 (20000)
32
32
32
32
48
Showing Branch Offset in Machine Language (cont.)
The while loop on page 74 was compiled into this MIPS assembler code:
Loop:
sll
$t1, $s3, 2
add
$t1, $t1, $s6
lw
$t0, 0($t1)
bne
$t0, $s5, Exit
addi
$s3, $s3, 1
j
Loop
Exit:
If we assume we place the loop starting at location 80000 in memory, what is
the MIPS machine code for this loop?
80000
80004
80008
80012
80016
80020
0
0
19
9
2
0
0
9
22
9
0
32
35
9
8
0
5
8
21
2
8
19
19
1
2
20000
49
Branching Far Away
How about the conditional branch instruction to jump far
away?
Insert an unconditional jump to the branch target
Inverts the condition so that the branch decides whether to skip
the jump.
Example:
Given a branch on register $s0 being equal to register $s1
beq
$s0, $s1, L1
replace it by a pair of instructions that offers a much greater
branching distance.
Solution
bne
j
$s0, $s1, L2
L1
L2;
50
MIPS Addressing Mode Summary
Addressing modes:
Register addressing
The operand is a register
Base addressing
The operand is at the memory location whose address is the
sum of a register and a constant in the instruction.
Immediate addressing
The operand is a constant within the instruction itself.
PC-relative addressing
The address is the sum of the PC and a constant in the
instruction.
Pseudo-direct addressing
The jump address is the 26 bits of the instruction concatenated
with the upper bits of the PC.
51
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
Memor y
Address
+
Register
Byte
Halfword
Word
4. PC-relative addressing
op
rs
rt
Memor y
Address
PC
+
Word
5. Pseudodirect addressing
op
Address
PC
Memor y
Word
52
Example: Decoding machine code
What is the assembly language corresponding to this
machine instruction? 0000 0000 1010 1111 1000 0000
0010 0000
Solution:
op
rs
rt
rd
shamt
funct
000000
00101
01111
10000
0000
100000
查表(Fig. 2.25)
可得
add $s0, $a1, $t7
53
To summarize:
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
54
MIPS instruction encoding
55
56
Translating and Staring a Program
A translation hierarchy
為了加速編譯的過程,有些步驟會被合併
或省略,例如:
1. 有些compiler直接產生object code
2. 採用linking loader整合linker和loader
C program
Compiler
Assembly language program
Assembler
Object: Machine language module
Object: Library routine (machine language)
Linker
Executable: Machine language program
Loader
Memory
57
Translating and Staring a Program
Compiler
The compiler transforms the C program into an assembly
language program.
A symbolic form of what the machine understands
Assembler
The assembler convert the assembly language instruction
into the machine language.
Pseudoinstruction
Sometimes, an assembler will accept a statement that does
not correspond exactly to a machine instruction. For example,
it may correspond to a small set of machine instructions.
These are called pseudoinstructions.
Assemblers keep track of labels used in branches and data
transfer instructions in symbol table.
58
Translating and Staring a Program
The object file for UNIX systems typically contains
Object file header
Describes the size and position of the other pieces of the object file.
Text segment
Contains the machine code.
Static data segment
Contains data allocated for the life of the program.
Relocation information
Identifies instructions and data words that depend on absolute address
when the program is loaded into memory.
Symbol table
Contains the remaining labels that are not defined, such as external
reference.
Debugging information
Contains a concise description of how the modules were compiled .
59
Translating and Staring a Program
Linker (link editor)
Place code and data modules symbolically in memory
Determine the addresses of data and instruction labels.
Patch both the internal and external references.
The linker uses the relocation information and symbol table in each object
module to resolve all undefined labels.
The linker produces an executable file that can be run on a computer
Loader
Reads the executable file header to determine size of the text and data
segment.
Creates an address space large enough for the text and data
Copies the instructions and data from the executable file into memory.
Initializes the machine registers and sets the stack pointer to the first free
location.
Jumps to a start-up routine that copies the parameters into the argument
registers and calls the main routine of the program.
60
Dynamic Linked Libraries
Although the traditional static linking
libraries is the fastest way to call library
routines, it has a few disadvantages:
The library routines become part of the
executable code.
It loads the whole library even if all of the library
is not used when the program is run.
Dynamic Linked Libraries
The library routines are not linked and loaded
until the program is run.
61
62
Starting a Java Program
Java is compiled first to instructions that are easy to interpret:
the Java bytecode instruction set.
Java programs are distributed in the binary version of these
bytecodes.
Java Virtual Machine (JVM) is an interpreter which can execute
Java bytecodes.
63
Array Version of Clear
clear1 (int array[ ], int size)
{
int
i;
for
( i =0, i <size, i = i
array[i ] = 0;
}
sll
Solution:
move $t0, $zero
loop1: add
$t1, $t0, $t0
add
$t1, $t1, $t1
add
$t2, $a0, $t1
sw
$zero, 0($t2)
addi
$t0, $t0, 1
slt
$t3, $t0, $a
bne
$t3, $zero, loop1
+1)
array: $a0
size: $a1
i: $t0
$t1, $t0, 2
# i=0
# $t1=i*4
# $t2=address of array[i]
# array[i]=0
# i=i+1
# $t3= (i<size)
# if (i<size) goto loop1
64
Pointer Version of Clear
clear2 (int *array, int size)
array: $a0
{
size: $a1
p: $t0
int
*p;
for
( p =&array[0], p < &array[size], p = p +1)
*p = 0;
}
Solution:
move
loop2: sw
addi
add
add
add
slt
bne
$t0, $a0
$zero, 0($t0)
$t0, $t0, 4
$t1, $a1, $a1
$t1, $t1, $t1
$t2, $a0, $t1
$t3, $t0, $t2
$t3, $zero, loop2
# p=address of array[0]
# Memory[p]=0
# p=p+4
# $t1=i*4
#(sll $t1, $a1, 2)
# $t2=address of array[size]
# $t3=(p<&array[size])
# if (p<&array[size]) goto loop2
65
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”
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!
We’ll look at PowerPC and 80x86
66
PowerPC
Indexed addressing
example:
lw $t1,$a0+$s3
#$t1=Memory[$a0+$s3]
What do we have to do in MIPS?
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
67
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”
68
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”
69
Some typical 80x86 Instruction & their
functions
Function
Instruction
JE name
If equal (CC) EIP = name};
EIP – 128  name < EIP + 128
JMP name
{EIP = NAME};
CALL name
SP = SP – 4; M[SP] = EIP + 5; EIP = name;
MOVW EBX,[EDI + 45]
EBX = M [EDI + 45]
PUSH ESI
SP = SP – 4; M[SP] = ESI
POP EDI
EDI = M[SP]; SP = SP + 4
ADD EAX,#6765
EAX = EAX + 6765
TEST EDX,#42
Set condition codea (flags) with EDX & 42
MOVSL
M[EDI] = M[ESI];
EDI = EDI + 4; ESI = ESI + 4
70
Typical 80x86 instruction formats
a. JE EIP + displacement
4
4
8
JE
Condition
Displacement
b. CALL
8
32
CALL
Offset
c. MOV EBX, [EDI + 45]
6
1 1
MOV
d w
8
r-m
postbyte
8
Displacement
d. PUSH ESI
5
3
PUSH
Reg
e. ADD EAX, #6765
4
3
1
32
ADD
Reg
w
Immediate
f. TEST EDX, #42
7
1
8
32
TEST
w
Postbyte
Immediate
71
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!
72
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