331 w02.1spring 05 announcements hw1 is due on this friday appendix a (on cd) is very helpful to...
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331 W02.1 Spring 05
Announcements
HW1 is due on this Friday
Appendix A (on CD) is very helpful to HW1.
331 W02.2 Spring 05
Review I: Execute Cycle
Q1: How does control know which instruction to fetch?
Q2: who does decode? What happens in decode phase?
Q3: How do control and datapath interact to finish exec phase?
Q4: What does datapath have?
Fetch
DecodeExec
331 W02.4 Spring 05
Assembly Language
Language of the machine
More primitive than higher level languagese.g., no sophisticated control flow
Very restrictivee.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, … 32-bit architecture
- 32 bit data line and address line- data and addresses are 32-bit
331 W02.5 Spring 05
MIPS R3000 Instruction Set Architecture
Instruction Categories Load/Store Computational Jump and Branch Floating Point
- coprocessor
Memory Management Special
R0 - R31
PCHI
LO
OP
OP
OP
rs rt rd sa funct
rs rt immediate
jump target
3 Instruction Formats: all 32 bits wide
Registers
Q: How many already familiar with MIPS ISA?
331 W02.6 Spring 05
MIPS Arithmetic Instruction
MIPS assembly language arithmetic statement
add $t0, $s1, $s2
sub $t0, $s1, $s2
Each arithmetic instruction performs only one operation
Each arithmetic instruction specifies exactly three operands
destination source1 op source2
Those operands are contained in the datapath’s register file ($t0, $s1,$s2)
Operand order is fixed (destination first)
331 W02.7 Spring 05
Compiling More Complex Statements
Assuming variable b is stored in register $s1, c is stored in $s2, d is stored in $s3 and the result is to be left in $s0, and $t0 is a temporary register, what is the assembler equivalent to the C statement
h = (b - c) + d
331 W02.8 Spring 05
Registers
Registers are Faster than main memory Can hold variables so that
- code density improves (since registers are named with fewer bits than a memory location) – why is that?
Register addresses are indicated by using $
331 W02.9 Spring 05
MIPS Register File
Operands of arithmetic instructions must be from a limited number of special locations contained in the datapath’s register file
Holds thirty-two 32-bit registers- With two read ports and
- One write portRegister File
src1 addr
src2 addr
dst addr
write data
32 bits
src1data
src2data
32locations
325
32
5
5
32
331 W02.10 Spring 05
0 $zero constant 0
1 $at reserved for assembler
2 $v0 expression evaluation &
3 $v1 function results
4 $a0 arguments
5 $a1
6 $a2
7 $a3
8 $t0 temporary: caller saves
. . . (callee can clobber)
15 $t7
Naming 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
331 W02.11 Spring 05
Registers vs. Memory
Arithmetic instructions operands must be registers,
— only thirty-two registers provided
What about programs with lots of variables? Store variables in the memory Load variables from memory to registers before use;
store them back to memory after use.
Processor
Control
Datapath
Memory
Devices
Input
Output
331 W02.12 Spring 05
MIPS has two basic data transfer instructions for accessing memorylw $t0, 4($s3) #load word from memory
sw $t0, 8($s3) #store word to memory
The data transfer instruction must specify where in memory to read from (load) or write to (store) –
memory address where in the register file to write to (load) or read from
(store) – register destination (source)
The memory address is formed by summing the constant portion of the instruction and the contents of the second register
Accessing Memory
331 W02.13 Spring 05
Memory is viewed as a large, single-dimension array, with an address
A memory address is an index into the array
Processor – Memory Interconnections
ProcessorMemory Addressable
locations
read addr/write addr
read data
write data
32
32
32
232
Q: what should be the smallest addressable unit?
331 W02.14 Spring 05
MIPS Data Types
Integer: (signed or unsigned) 32 bits
Character: 8 bits
Floating point numbers: 32 bits
Memory addresses (pointers): 32 bits
Instructions: 32 bits
Bit String: sequence of bits of a particular length 8 bits is a byte 16 bits is a half-word 32 bits (4 bytes) is a word 64 bits is a double-word
331 W02.15 Spring 05
Byte Addresses
Since 8-bit bytes are so useful, most architectures address individual bytes in memory
memory: 232 bytes = 230 words
Therefore, the memory address of a word must be a multiple of 4 (alignment restriction)
Alignment restriction: requires that objects fall on address that is multiple of their size.
0 1 2 3
Aligned
NotAligned
331 W02.16 Spring 05
Addressing Objects: Endianess and Alignment Big Endian: leftmost byte is word address
IBM 360/370, Motorola 68k, MIPS, Sparc, HP PA
Little Endian: rightmost byte is word addressIntel 80x86, DEC Vax, DEC Alpha (Windows NT)
msb lsb
3 2 1 0
little endian byte 0
0 1 2 3big endian byte 0
331 W02.17 Spring 05
MIPS Memory Addressing The memory address is formed by summing the
constant portion of the instruction and the contents of the second (base) register
lw $t0, 4($s3) #what? is loaded into $t0
sw $t0, 8($s3) #$t0 is stored where?
Memory
. . . 0 1 0 0Data Word Address
0
4
8
12
16
20
24
. . . 1 0 0 0
. . . 0 0 1 0
. . . 0 0 0 1
. . . 1 1 0 0
. . . 0 1 0 1
. . . 0 1 1 0$s3 holds 8
331 W02.18 Spring 05
Compiling with Loads and Stores
Assuming variable b is stored in $s2 and that the base address of integer array A is in $s3, what is the MIPS assembly code for the C statement
A[8] = A[2] - b
$s3+4
$s3+8
$s3+12
$s3
. . .
A[2]
A[3]
. . .
A[1]
A[0]
331 W02.19 Spring 05
Compiling with a Variable Array Index
Assuming A is an integer array whose base is in register $s4, and variables b, c, and i are in $s1, $s2, and $s3, respectively, what is the MIPS assembly code for the C statement
c = A[i] - b
331 W02.20 Spring 05
MIPS Instructions, so far
Category Instr Op Code Example Meaning
Arithmetic
(R format)
add 0 and 32 add $s1, $s2, $s3 $s1 = $s2 + $s3
subtract 0 and 34 sub $s1, $s2, $s3 $s1 = $s2 - $s3
Data
transfer
(I format)
load word 35 lw $s1, 100($s2) $s1 = Memory($s2+100)
store word 43 sw $s1, 100($s2) Memory($s2+100) = $s1
331 W02.21 Spring 05
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:
Can you guess what the field names stand for?
Machine Language - Arithmetic Instruction
op rs rt rd shamt funct
000000 10001 10010 01000 00000 100000
331 W02.22 Spring 05
MIPS Instruction Fields
op
rs
rt
rd
shamt
funct
op rs rt rd shamt funct6 bits 5 bits 5 bits 5
bits5 bits
6 bits
= 32 bits
331 W02.23 Spring 05
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 previous format was R-type for register
Example: lw $t0, 24($s2)
Where's the compromise?
Machine Language - Load Instruction
op rs rt 16 bit number
35 18 8 24
100011 10010 01000 0000000000011000
331 W02.24 Spring 05
Memory Address Location
Example: lw $t0, 24($s2)Memory
data word address (hex)0x000000000x000000040x000000080x0000000c
0xf f f f f f f f
$s2 0x12004094
0x00000002
2410 + $s2 =
Note that the offset can be positive or negative
331 W02.25 Spring 05
Example: sw $t0, 24($s2)
A 16-bit address means access is limited to memory locations within a region of 213 or 8,192 words (215 or 32,768 bytes) of the address in the base register $s2
Machine Language - Store Instruction
op rs rt 16 bit number
43 18 8 24
101011 10010 01000 0000000000011000
331 W02.26 Spring 05
Assembling Code Remember the assembler code we compiled for the C
statement
A[8] = A[2] - b
lw $t0, 8($s3) #load A[2] into $t0sub $t0, $t0, $s2 #subtract b from A[2]sw $t0, 32($s3) #store result in A[8]
Assemble the MIPS object code for these three instructions
331 W02.27 Spring 05
Review: MIPS Data Types
Integer: (signed or unsigned) 32 bits
Character: 8 bits
Floating point numbers: 32 bits
Memory addresses (pointers): 32 bits
Instructions: 32 bits
Bit String: sequence of bits of a particular length 8 bits is a byte 16 bits is a half-word 32 bits (4 bytes) is a word 64 bits is a double-word
331 W02.28 Spring 05
Beyond Numbers Most computers use 8-bit bytes to represent characters with the
American Std Code for Info Interchange (ASCII)
So, we need instructions to move bytes around
ASCII Char ASCII Char ASCII Char ASCII Char ASCII Char ASCII Char
0 Null 32 space
48 0 64 @ 96 ` 112 p
1 33 ! 49 1 65 A 97 a 113 q
2 34 “ 50 2 66 B 98 b 114 r
3 35 # 51 3 67 C 99 c 115 s
4 EOT 36 $ 52 4 68 D 100 d 116 t
5 37 % 53 5 69 E 101 e 117 u
6 ACK 38 & 54 6 70 F 102 f 118 v
7 39 ‘ 55 7 71 G 103 g 119 w
8 bksp 40 ( 56 8 72 H 104 h 120 x
9 tab 41 ) 57 9 73 I 105 i 121 y
10 LF 42 * 58 : 74 J 106 j 122 z
11 43 + 59 ; 75 K 107 k 123 {
12 FF 44 , 60 < 76 L 108 l 124 |
15 47 / 63 ? 79 O 111 o 127 DEL
331 W02.29 Spring 05
Loading and Storing Bytes
MIPS provides special instructions to move bytes
lb $t0, 1($s3) #load byte from memory
sb $t0, 6($s3) #store byte to memory
What 8 bits get loaded and stored? load byte places the byte from memory in the rightmost 8
bits of the destination register
- what happens to the other bits in the register?
store byte takes the byte from the rightmost 8 bits of a register and writes it to a byte in memory
op rs rt 16 bit number
331 W02.30 Spring 05
Example of Loading and Storing Bytes Given following code sequence and memory state
(contents are given in hexidecimal), what is the state of the memory after executing the code?
add $s3, $zero, $zerolb $t0, 1($s3)sb $t0, 6($s3)
Memory
0 0 9 0 1 2 A 0Data Word
Address (Decimal)
0
4
8
12
16
20
24
F F F F F F F F
0 1 0 0 0 4 0 2
1 0 0 0 0 0 1 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 What value is left in $t0?
What if the machine was little Endian?
331 W02.31 Spring 05
Review: MIPS Instructions, so far
Category Instr Op Code Example Meaning
Arithmetic
(R format)
add 0 and 32 add $s1, $s2, $s3 $s1 = $s2 + $s3
subtract 0 and 34 sub $s1, $s2, $s3 $s1 = $s2 - $s3
Data
transfer
(I format)
load word 35 lw $s1, 100($s2) $s1 = Memory($s2+100)
store word 43 sw $s1, 100($s2) Memory($s2+100) = $s1
load byte 32 lb $s1, 101($s2) $s1 = Memory($s2+101)
store byte 40 sb $s1, 101($s2) Memory($s2+101) = $s1
331 W02.32 Spring 05
Review: MIPS R3000 ISA Instruction Categories
Load/Store Computational Jump and Branch Floating Point
- coprocessor
Memory Management Special
3 Instruction Formats: all 32 bits wide
R0 - R31
PCHI
LO
OP rs rt rd shamt funct
OP rs rt 16 bit number
OP 26 bit jump target
Registers
R format
I format
6 bits 5 bits 5 bits 5 bits 5 bits 6 bits