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Computer Architecture CPSC 321

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Computer ArchitectureCPSC 321

E. J. Kim

- Logical Instructions
- Shifts

- Up until now, we’ve done arithmetic (add, sub,addi ), memory access (lw and sw), and branches and jumps.
- All of these instructions view contents of register as a single quantity (such as a signed or unsigned integer)
- New Perspective: View contents of register as 32 bits rather than as a single 32-bit number
- Since registers are composed of 32 bits, we may want to access individual bits (or groups of bits) rather than the whole.
- Introduce two new classes of instructions:
- Logical Operators
- Shift Instructions

- Two basic logical operators:
- AND: outputs 1 only if both inputs are 1
- OR: outputs 1 if at least one input is 1

- Two basic logical operators:
- AND: outputs 1 only if both inputs are 1
- OR: outputs 1 if at least one input is 1

- Truth Table: standard table listing all possible combinations of inputs and resultant output for each
- Truth Table for AND and OR
A B AND OR

00

01

10

11

- Instruction Names:
- and, or: Both of these expect the third argument to be a register
- andi, ori: Both of these expect the third argument to be an immediate

- MIPS Logical Operators are all bitwise, meaning that bit 0 of the output is produced by the respective bit 0’s of the inputs, bit 1 by the bit 1’s, etc.

- Note that ANDing a bit with 0 produces a 0 at the output while ANDing a bit with 1 produces the original bit.
- This can be used to create a mask.
- Example:
1011 0110 1010 0100 0011 1101 1001 1010

0000 0000 0000 0000 0000 1111 1111 1111

- The result of ANDing these two is:
0000 0000 0000 0000 0000 1101 1001 1010

- Example:
- The second bit string in the example is called a mask. It is used to isolate the rightmost 12 bits of the first bit string by masking out the rest of the string (e.g. setting it to all 0s).

- Thus, the and operator can be used to set certain portions of a bit string to 0s, while leaving the rest alone.
- In particular, if the first bitstring in the above example were in $t0, then the following instruction would mask it:
andi $t0,$t0,0xFFF

- In particular, if the first bitstring in the above example were in $t0, then the following instruction would mask it:
- Similarly, note that ORing a bit with 1 produces a 1 at the output while ORing a bit with 0 produces the original bit.
- This can be used to force certain bits of a string to 1s.
- For example, if $t0 contains 0x12345678, then after this instruction:
ori$t0, $t0, 0xFFFF

- $t0 contains 0x1234FFFF (e.g. the high-order 16 bits are untouched, while the low-order 16 bits are forced to 1s).

- For example, if $t0 contains 0x12345678, then after this instruction:

- Move (shift) all the bits in a word to the left or right by a number of bits.
- Example: shift right by 8 bits
0001 0010 0011 0100 0101 0110 0111 1000

- Example: shift right by 8 bits

0000 0000 0001 0010 0011 0100 0101 0110

- Example: shift left by 8 bits
0001 0010 0011 0100 0101 0110 0111 1000

0011 0100 0101 0110 0111 1000 0000 0000

MIPS shift instructions:

1. sll (shift left logical): shifts left and fills emptied bits with 0s

2. srl (shift right logical): shifts right and fills emptied bits with 0s

3. sra (shift right arithmetic): shifts right and fills emptied bits by sign extending

- Shift Instruction Syntax:
1 2,3,4

- where
1) operation name

2) register that will receive value

3) first operand (register)

4) shift amount (constant <= 32)

- where

- Example: shift right arith (sra) by 8 bits
0001 0010 0011 0100 0101 0110 0111 1000

0000 0000 0001 0010 0011 0100 0101 0110

- Example: shift right arith (sra) by 8 bits
1001 0010 0011 0100 0101 0110 0111 1000

1111 1111 1001 0010 0011 0100 0101 0110

- Suppose we want to isolate byte 0 (rightmost 8 bits) of a word in $t0. Simply use:
andi $t0,$t0,0xFF

- Suppose we want to isolate byte 1 (bit 15 to bit 8) of a word in $t0. We can use:
andi $t0,$t0,0xFF00

but then we still need to shift to the right by 8 bits...

- Could use instead:
sll $t0,$t0,16

srl $t0,$t0,24

t0

0001 0010 0011 0100 0101 0110 0111 1000

After sll

0101 0110 0111 1000 0000 0000 0000 0000

After srl

0000 0000 0000 0000 0000 0000 0101 0110

- In binary:
Multiplying by 2 is same as shifting left by 1:

112 x 102 = 1102

10102 x 102 = 101002

Multiplying by 4 is same as shifting left by 2:

112 x 1002 = 11002

10102 x 1002 = 1010002

Multiplying by 2n is same as shifting left by n

- Since shifting maybe faster than multiplication, a good compiler usually notices when C code multiplies by a power of 2 and compiles it to a shift instruction:
a *= 8; (in C)

would compile to:

sll $s0,$s0,3 (in MIPS)

- Likewise, shift right to divide by powers of 2
- remember to use sra

- We introduced numerous MIPS assembly language instructions.
- We are now familiar with registers and register usage conventions.
- We know how to use system calls, basic I/O
- We have learned how the stack works
- What is missing?

Practice! Practice! Practice!

- We need a more detailed knowledge about the instruction formats to fully appreciate certain restrictions.
- The functional interface is easy to understand, since it is basically familiar procedural programming
- We need to understand how the computer interprets the instruction, so that we can transition to the discussion of the MIPS hardware architecture

What does that mean?

- Machine language level programming means that we have to provide the bit encodings for the instructions
- For example, add $t0, $s1, $s2 represents the 32bit string
- 00000010001100100100000000100000
- Assembly language mnemonics usually translate into one instruction
- We also have pseudo-instructions that translate into several instructions

Register format

Immediate format

Jump format

op-code rs rt rd shamt funct

6 555 5 6

op-code rs rt immediate value

6 55 16

op-code 26 bit current segment address

6 26

Register format

op: basic operation of instruction

funct: variant of instruction

rs: first register source operand

rt: second register source operand

rd: register destination operand

shamt: shift amount

op-code rs rt rd shamt funct

6 555 5 6

- add $t0, $s1, $s2
- 00000010001100100100000000100000
- 000000 10001 10010 01000 00000 100000
- Operation and function field tell the computer to perform an addition

- 000000 10001 10010 01000 00000 100000
- registers $17, $18 and $8

op-code rs rt rd shamt funct

6 555 5 6

Number

Value

Name

Registers

return values

from functions

pass parameters

to functions

$t0-$t7 are caller saved

registers –

use these registers

in functions

$s0-$s7 are callee-saved

registers – use these

registers for values

that must be maintained

across function calls.

- add $t0, $s1, $s2
- 00000010001100100100000000100000
- 000000 10001 10010 01000 00000 100000
- source registers $s1=$17 and $s2=$18 and target register $t0=$8

op-code rs rt rd shamt funct

6 555 5 6

Register format

(op, funct)=(0,32): add

rs=17: first source operand is $s1

rt=18: second source operand is $s2

Rd=8: register destination is $t0

add $t0, $s1, $s2

0 17 18 8 0 32

6 555 5 6

Immediate format

op determines the instruction (op <> 0)

rs is the source register

rt is the destination register

16bit immediate value

op rs rt immediate value

6 55 16

Immediate format

op=8 means addi

rs=29 means source register is $sp

rt=29 means $sp is destination register

immediate value = 4

addi $sp, $sp, 4

8 29 29 4

6 55 16

- The MIPS assembly language has the command andi, an immediate bit-wise and operation
- We can say li $s0, 0xCDEF1234 to load register $s0 with the content 0xCDEF1234
- Why is this strange?
- In the immediate format, you can only load 16 bits, but the constant is 32 bits!

- li $s0, 0xCDEF1234 is a pseudo-instruction
- It is a convenient shorthand for
lui $at, 0xCDEF

ori $s0, $at, 0x1234

- The register $at is used here by the assembler; this is the reason why you should not use this register.

- How can we swap the content of two registers, say $s0 and $s1, without accessing other registers or memory?
- Solution:
xor $s0, $s0, $s1

xor $s1, $s0, $s1

xor $s0, $s0, $s1

- Immediate addressing
- Register addressing
- Base displacement addressing
- PC-relative addressing
- address is the sum of the PC and a constant in the instruction

- Pseudo-direct addressing
- jump address is 26bits of instruction concatenated with upper bits of PC

- Register Addressing
- add $s1, $s2, $s3
- $s1 = $s2 + $s3

- Immediate Addressing
- addi $s1, $s2, 100
- $s1 = $s2 + 100

- Base addressing
- lw $s1, 100($s2)
- $s1 = Memory[$s2+100]

- PC-relative branch
- beq $s1, $s2, 25
- if ($s1 == $s2) goto PC + 4 + 100

- Pseudo-direct addressing
- j 1000
- goto 1000
- concatenate 26bit address with upper bits of the PC