Computer Architecture Patterson chapter 3.ppt

Computer Organization and
Computer Organization and
Architecture (AT70.01)
Architecture (AT70.01)
Comp. Sc. and Inf. Mgmt.
Comp. Sc. and Inf. Mgmt.
Asian Institute of Technology
Asian Institute of Technology
Instructor: Dr. Sumanta Guha
Slide Sources: Patterson &
Hennessy COD book website
(copyright Morgan Kaufmann)
adapted and supplemented

COD Ch. 3
Instructions: Language of
the Machine

Instructions: Overview
 Language of the machine
 More primitive than higher level languages, e.g., no
sophisticated control flow such as while or for loops
 Very restrictive
 e.g., MIPS arithmetic instructions
 We’ll be working with the MIPS instruction set architecture
 inspired most architectures developed since the 80's
 used by NEC, Nintendo, Silicon Graphics, Sony
 the name is not related to millions of instructions per second !
 it stands for microcomputer without interlocked pipeline stages !
 Design goals: maximize performance and minimize cost and
reduce design time

MIPS Arithmetic
 All MIPS arithmetic instructions have 3 operands
 Operand order is fixed (e.g., destination first)
 Example:
C code: A = B + C
MIPS code: add $s0, $s1, $s2
compiler’s job to associate
variables with registers

MIPS Arithmetic
 Design Principle 1: simplicity favors regularity.
Translation: Regular instructions make for simple hardware!
 Simpler hardware reduces design time and manufacturing cost.
 Of course this complicates some things...
C code: A = B + C + D;
E = F - A;
MIPS code add $t0, $s1, $s2
(arithmetic): add $s0, $t0, $s3
sub $s4, $s5, $s0
 Performance penalty: high-level code translates to denser machine code.
Allowing variable number
of operands would
simplify the assembly
code but complicate the
hardware.

MIPS Arithmetic
 Operands must be in registers – only 32 registers provided
(which require 5 bits to select one register). Reason for
small number of registers:
 Design Principle 2: smaller is faster. Why?
 Electronic signals have to travel further on a physically larger
chip increasing clock cycle time.
 Smaller is also cheaper!

Registers vs. Memory
Processor I/O
Control
Datapath
Memory
Input
Output
 Arithmetic instructions operands must be in registers
 MIPS has 32 registers
 Compiler associates variables with registers
 What about programs with lots of variables (arrays, etc.)?
Use memory, load/store operations to transfer data from
memory to register – if not enough registers spill registers
to memory
 MIPS is a load/store architecture

Memory Organization
 Viewed as a large single-dimension array with access by
address
 A memory address is an index into the memory array
 Byte addressing means that the index points to a byte of
memory, and that the unit of memory accessed by a
load/store is a byte
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

Memory Organization
 Bytes are load/store units, but most data items use larger words
 For MIPS, a word is 32 bits or 4 bytes.
 232
bytes with byte addresses from 0 to 232
-1
 230
words with byte addresses 0, 4, 8, ... 232
-4
 i.e., words are aligned
 what are the least 2 significant bits of a word address?
0
4
8
12
...
32 bits of data
32 bits of data
32 bits of data
32 bits of data
Registers correspondingly hold 32 bits of data

Load/Store Instructions
 Load and store instructions
 Example:
C code: A[8] = h + A[8];
MIPS code (load): lw $t0, 32($s3)
(arithmetic): add $t0, $s2, $t0
(store): sw $t0, 32($s3)
 Load word has destination first, store has destination last
 Remember MIPS arithmetic operands are registers, not memory
locations
 therefore, words must first be moved from memory to registers using
loads before they can be operated on; then result can be stored back to
memory
offset address
value

A MIPS Example

Can we figure out the assembly code?
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

So far we’ve learned:
 MIPS
 loading words but addressing bytes
 arithmetic on registers only
 Instruction Meaning
add $s1, $s2, $s3 $s1 = $s2 + $s3
sub $s1, $s2, $s3 $s1 = $s2 – $s3
lw $s1, 100($s2) $s1 = Memory[$s2+100]
sw $s1, 100($s2) Memory[$s2+100]= $s1

 Instructions, like registers and words of data, are also 32 bits
long
 Example: add $t0, $s1, $s2
 registers are numbered, e.g., $t0 is 8, $s1 is 17, $s2 is
18
 Instruction Format R-type (“R” for aRithmetic):
Machine Language
1000110010 01000 00000 100000
000000
6 bits 5 bits 5 bits 5 bits 5 bits 6 bits
op rs rt rd shamt funct
opcode –
operation
first
register
source
operand
second
register
source
operand
register
destin-
ation
operand
shift
amount
function field -
selects variant
of operation

 Consider the load-word and store-word instructions,
 what would the regularity principle have us do?

we would have only 5 or 6 bits to determine the offset from a base register - too
little…
 Design Principle 3: Good design demands a compromise
 Introduce a new type of instruction format
 I-type (“I” for Immediate) for data transfer instructions
 Example: lw $t0, 1002($s2)
100011 10010 01000 0000001111101010
op rs rt 16 bit offset
Machine Language
6 bits 5 bits 5 bits 16 bits

 Instructions are bit sequences, just like data
 Programs are stored in memory
 to be read or written just like data
 Fetch & Execute Cycle
 instructions are fetched and put into a special register
 bits in the register control the subsequent actions (= execution)
 fetch the next instruction and repeat
Processor Memory
memory for data, programs,
compilers, editors, etc.
Stored Program Concept

SPIM – the MIPS simulator
 SPIM (MIPS spelt backwards!) is a MIPS simulator that
 reads MIPS assembly language files and translates to machine
language
 executes the machine language instructions
 shows contents of registers and memory
 works as a debugger (supports break-points and single-stepping)
 provides basic OS-like services, like simple I/O
 SPIM is freely available on-line
 An important part of our course is to actually write MIPS
assembly code and run using SPIM – the only way to learn
assembly (or any programming language) is to write lots and
lots of code!!!
 Refer to our material, including slides, on SPIM

Memory Organization:
Big/Little Endian Byte Order
 Bytes in a word can be numbered in two ways:
 byte 0 at the leftmost (most significant) to byte 3 at the rightmost
(least significant), called big-endian
 byte 3 at the leftmost (most significant) to byte 0 at the rightmost
(least significant), called little-endian
0 1 2 3
3 2 1 0
Word 0
Word 1
Bit
31
Bit
0
Byte 0Byte 1Byte 2Byte 3
Word 0
Word 1
Bit
31
Bit
0
Big-endian
Memory
Little-endian
Memory

 SPIM’s memory storage depends on that of the underlying
machine
 Intel 80x86 processors are little-endian
 because SPIM always shows words from left to right a “mental
adjustment” has to be made for little-endian memory as in Intel
PCs in our labs: start at right of first word go left, start at right of
next word go left, …!
 Word placement in memory (from .data area of code) or
word access (lw, sw) is the same in big or little endian
 Byte placement and byte access (lb, lbu, sb) depend on big or
little endian because of the different numbering of bytes
within a word
 Character placement in memory (from .data area of code)
depend on big or little endian because it is equivalent to byte
placement after ASCII encoding
 Run storeWords.asm from SPIM examples!!
Memory Organization:
Big/Little Endian Byte Order

 Decision making instructions
 alter the control flow,

i.e., change the next instruction to be executed
 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:....
Control: Conditional Branch
I-type instructions
000100 01000 01001 0000000000011001
beq $t0, $t1, Label
(= addr.100)
word-relative addressing:
25 words = 100 bytes;
also PC-relative (more…)

 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
 Format:
 16 bits is too small a reach in a 232
address space
 Solution: specify a register (as for lw and sw) and add it to offset
 use PC (= program counter), called PC-relative addressing, based on
 principle of locality: most branches are to instructions near current
instruction (e.g., loops and if statements)
I
Addresses in Branch
op rs rt 16 bit offset

 Further extend reach of branch by observing all MIPS
instructions are a word (= 4 bytes), therefore word-relative
addressing:
 MIPS branch destination address = (PC + 4) + (4 * offset)
 so offset = (branch destination address – PC – 4)/4
 but SPIM does offset = (branch destination address – PC)/4
Addresses in Branch
Because hardware typically increments PC
early
in execute cycle to point to next instruction

 MIPS unconditional branch instructions:
j Label
 Example:
if (i!=j) beq $s4, $s5, Lab1
h=i+j; add $s3, $s4, $s5
else j Lab2
h=i-j; Lab1: sub $s3, $s4, $s5
Lab2: ...
 J-type (“J” for Jump) instruction format

Example: j Label # addr. Label = 100
Control: Unconditional
Branch (Jump)
6 bits 26 bits
op 26 bit number
000010 00000000000000000000011001
word-relative
addressing:
25 words = 100 bytes

Addresses in Jump
 Word-relative addressing also for jump instructions
 MIPS jump j instruction replaces lower 28 bits of the PC with A00
where A is the 26 bit address; it never changes upper 4 bits
 Example: if PC = 1011X (where X = 28 bits), it is replaced with 1011A00
 there are 16(=24
) partitions of the 232
size address space, each partition
of size 256 MB (=228
), such that, in each partition the upper 4 bits of the
address is same.
 if a program crosses an address partition, then a j that reaches a
different partition has to be replaced by jr with a full 32-bit address
first loaded into the jump register
 therefore, OS should always try to load a program inside a single
partition
op 26 bit address
J

 Small constants are used quite frequently (50% of operands)
e.g., A = A + 5;
B = B + 1;
C = C - 18;
 Solutions? Will these work?
 create hard-wired registers (like $zero) for constants like 1
 put program constants in memory and load them as required
 MIPS Instructions:
addi $29, $29, 4
slti $8, $18, 10
andi $29, $29, 6
ori $29, $29, 4
 How to make this work?
Constants

Immediate Operands
 Make operand part of instruction itself!
 Design Principle 4: Make the common case fast
 Example: addi $sp, $sp, 4 # $sp = $sp + 4
001000 11101 0000000000000100
op r
s
rt 16 bit number
6 bits 5 bits 5 bits 16 bits
11101

 First we need to load a 32 bit constant into a register
 Must use two instructions for this: first new load upper
immediate instruction for upper 16 bits
lui $t0, 1010101010101010
 Then get lower 16 bits in place:
ori $t0, $t0, 1010101010101010
 Now the constant is in place, use register-register arithmetic
1010101010101010 0000000000000000
0000000000000000 1010101010101010
1010101010101010 1010101010101010
ori
1010101010101010 0000000000000000
filled with zeros
How about larger constants?

So far
 Instruction Format Meaning
add $s1,$s2,$s3R $s1 = $s2 + $s3
sub $s1,$s2,$s3R $s1 = $s2 – $s3
lw $s1,100($s2)I $s1 = Memory[$s2+100]
sw $s1,100($s2)I Memory[$s2+100] = $s1
bne $s4,$s5,Lab1 I Next instr. is at Lab1 if $s4 != $s5
beq $s4,$s5,Lab2 I Next instr. is at Lab2 if $s4 = $s5
j Lab3 J Next instr. is at Lab3
 Formats:
op rs rt 16 bit address
op 26 bit address
R
I
J

 We have: beq, bne. What about branch-if-less-than?
 New instruction:
if $s1 < $s2 then
$t0 = 1
slt $t0, $s1, $s2 else
$t0 = 0
 Can use this instruction to build blt $s1, $s2, Label
 how? We generate more than one instruction – pseudo-instruction
 can now build general control structures
 The assembler needs a register to manufacture instructions from pseudo-
instructions
 There is a convention (not mandatory) for use of registers
Control Flow

Policy-of-Use Convention for
Registers
Name Register number Usage
$zero 0 the constant value 0
$v0-$v1 2-3 values for results and expression evaluation
$a0-$a3 4-7 arguments
$t0-$t7 8-15 temporaries
$s0-$s7 16-23 saved
$t8-$t9 24-25 more temporaries
$gp 28 global pointer
$sp 29 stack pointer
$fp 30 frame pointer
$ra 31 return address
Register 1, called $at, is reserved for the assembler; registers 26-27,
called $k0 and $k1 are reserved for the operating system.

 Assembly provides convenient symbolic representation
 much easier than writing down numbers
 regular rules: 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 actual
number of machine instructions that will execute
Assembly Language vs.
Machine Language

Procedures
 Example C code:
// procedure adds 10 to input parameter
int main()
{ int i, j;
i = 5;
j = add10(i);
i = j;
return 0;}
int add10(int i)
{ return (i + 10);}

Procedures
 Translated MIPS assembly
 Note more efficient use of registers possible!
.text
.globl main
main:
addi $s0, $0, 5
add $a0, $s0, $0
jal add10
add $s1, $v0, $0
add $s0, $s1, $0
li $v0, 10
syscall
add10:
addi $sp, $sp, -4
sw $s0, 0($sp)
addi $s0, $a0, 10
add $v0, $s0, $0
lw $s0, 0($sp)
addi $sp, $sp, 4
jr $ra
$sp
Content of $s0
High address
Low address
Run this code with PCSpim: procCallsProg1.asm
MEMOR
Y
argument
to callee result
to caller
jump and link
control returns here
save register
in stack, see
figure below
restore
values
system code
& call to
exit
return

0 zero constant 0
1 at reserved for assembler
2 v0 results from callee
3 v1 returned to caller
4 a0 arguments to callee
5 a1 from caller: caller saves
6 a2
7 a3
8 t0 temporary: caller saves
. . . (callee can clobber)
15 t7
MIPS: Software
Conventions
for Registers 16 s0 callee saves
. . . (caller can clobber)
23 s7
24 t8 temporary (cont’d)
25 t9
26 k0 reserved for OS kernel
27 k1
28 gp pointer to global area
29 sp stack pointer
30 fp frame pointer
31 ra return Address (HW):
caller saves

Procedures (recursive)
 Example C code – recursive factorial subroutine:
int main()
{ int i;
i = 4;
j = fact(i);
return 0;}
int fact(int n)
{ if (n < 1) return (1);
else return ( n*fact(n-1) );}

Procedures (recursive)
 Translated MIPS assembly:
.text
.globl main
main:
addi $a0, $0, 4
jal fact
nop
move $a0, $v0
li $v0, 1
syscall
li $v0, 10
syscall
fact:
addi $sp, $sp, -8
sw $ra, 4($sp)
sw $a0, 0($sp)
slti $t0, $a0, 1
beq $t0, $0, L1
nop
addi $v0, $0, 1
addi $sp, $sp, 8
jr $ra
L1:
addi $a0, $a0, -1
jal fact
nop
lw $a0, 0($sp)
lw $ra, 4($sp)
addi $sp, $sp, 8
mul $v0, $a0, $v0
jr $ra
save return
address and
argument in
stack
exit
print value
returned by
fact
branch to
L1 if n>=1
return 1
if n < 1
if n>=1 call
fact recursively
with argument
n-1
restore return
address, argument,
and stack pointer
return
n*fact(n-1)
return control
Run this code with PCSpim: factorialRecursive.asm
control
returns
from fact

Using a Frame Pointer
Saved argument
registers (if any)
Local arrays and
structures (if any)
Saved saved
registers (if any)
Saved return address
b.
$sp
$sp
$sp
c.
$fp
$fp
$fp
a.
High address
Low address
Variables that are local to a procedure but do not fit into registers (e.g., local arrays, struc-
tures, etc.) are also stored in the stack. This area of the stack is the frame. The frame pointer
$fp points to the top of the frame and the stack pointer to the bottom. The frame pointer does
not change during procedure execution, unlike the stack pointer, so it is a stable base
register from which to compute offsets to local variables.
Use of the frame pointer is optional. If there are no local variables to store in the stack it is
not efficient to use a frame pointer.

Using a Frame Pointer
 Example: procCallsProg1Modified.asm
This program shows code where it may be better to use $fp
 Because the stack size is changing, the offset of variables stored
in the stack w.r.t. the stack pointer $sp changes as well.
However, the offset w.r.t. $fp would remain constant.
 Why would this be better?
The compiler, when generating assembly, typically maintains a
table of program variables and their locations. If these locations
are offsets w.r.t $sp, then every entry must be updated every
time the stack size changes!
 Exercise:
Modify procCallsProg1Modified.asm to use a frame pointer
 Observe that SPIM names register 30 as s8 rather than fp. Of
course, you can use it as fp, but make sure to initialize it with the
same value as sp, i.e., 7fffeffc.

MIPS Addressing Modes
Byte Halfword Word
Registers
Memory
Memory
Word
Memory
Word
Register
Register
1. Immediate addressing
2. Register addressing
3. Base addressing
4. PC-relative addressing
5. Pseudodirect addressing
op rs rt
op rs rt
op rs rt
op
op
rs rt
Address
Address
Address
rd . . . funct
Immediate
PC
PC
+
+

 Simple instructions – all 32 bits wide
 Very structured – no unnecessary baggage
 Only three instruction formats
 Rely on compiler to achieve performance
 what are the compiler's goals?
 Help compiler where we can
op rs rt 16 bit address
op 26 bit address
Overview of MIPS
R
I
J

Summarize MIPS:
MIPS operands
Name Example Comments
$s0-$s7, $t0-$t9, $zero, Fast locations for data. In MIPS, data must be in registers to perform
32 registers $a0-$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
2
30
memory Memory[4], ..., sequential words differ by 4. Memory holds data structures, such as arrays,
words Memory[4294967292] and spilled registers, such as those saved on procedure calls.
MIPS assembly language
Category Instruction Example Meaning Comments
add add $s1, $s2, $s3 $s1 = $s2 + $s3 Three operands; data in registers
Arithmetic subtract sub $s1, $s2, $s3 $s1 = $s2 - $s3 Three operands; data in registers
add immediate addi $s1, $s2, 100 $s1 = $s2 + 100 Used to add constants
load word lw $s1, 100($s2) $s1 = Memory[$s2 + 100] Word from memory to register
store word sw $s1, 100($s2) Memory[$s2 + 100] = $s1 Word from register to memory
Data transfer load byte lb $s1, 100($s2) $s1 = Memory[$s2 + 100] Byte from memory to register
store byte sb $s1, 100($s2) Memory[$s2 + 100] = $s1 Byte from register to memory
load upper immediate lui $s1, 100
$s1 = 100 * 2
16 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
Conditional
branch on not equal bne $s1, $s2, 25 if ($s1 != $s2) go to
PC + 4 + 100
Not equal test; PC-relative
branch 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 $s1, $s2, 100 if ($s2 < 100) $s1 = 1;
else $s1 = 0
Compare less than constant
jump j 2500 go to 10000 Jump to target address
Uncondi- jump register jr $ra go to $ra For switch, procedure return
tional jump jump and link jal 2500 $ra = PC + 4; go to 10000 For procedure call

 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 R(educed)ISC vs. C(omplex)ISC

virtually all new instruction sets since 1982 have been RISC
 We’ll look at PowerPC and 80x86
Alternative Architectures

PowerPC Special
Instructions
 Indexed addressing
 Example: lw $t1,$a0+$s3 #$t1=Memory[$a0+$s3]
 what do we have to do in MIPS? add $t0, $a0, $s3
lw $t1, 0($t0)
 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? lw $t0, 4($s3)
addi $s3, $s3, 4
 Others:
 load multiple words/store multiple words
 a special counter register to improve loop performance:
bc Loop, ctrl != 0 # decrement counter, if not 0 goto loop
 MIPS: addi $t0, $t0, -1
bne $t0, $zero, Loop

A dominant architecture:
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”

A dominant architecture:
80x86
 Complexity
 instructions from 1 to 17 bytes long
 one operand must act as both a source and destination
 one operand may come from memory
 several complex addressing modes
 Saving grace:
 the most frequently used instructions are not too difficult to build
 compilers avoid the portions of the architecture that are slow
“an architecture that is difficult to explain and impossible to love”
“ what the 80x86 lacks in style is made up in quantity, making it beautiful from
the right perspective”

 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!
Summary

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