COMP2611 Computer Organisation MISP

Registers:

Name	Register number	Usage	Preserver on call?
$zero	0	Constant Value 0	N/A
$at	1	Reserved for assembler	N/A
$v0 - $v1	2 - 3	Values for results and expression evaluation	No
$a0 - $a3	4 - 7	Arguments	No
$t0 - $t7	8 - 15	Temporares (Temp variable)	No
$s0 - $s7	16 - 23	Saved tempraries (Important data)	Yes
$t8 - $t9	24 - 25	More temporaries	No
$k0 - $k1	26 - 27	Reserved for operating system kernel	N/A
$gp	28	Pointer to global area	Yes
$sp	29	Stack pointer	Yes
$fp	30	Frame pointer	Yes
$ra	31	Return address	Yes

Some technics to preserve the register:

1. push the data of into the stack
2. using a temp register to store the data

Signed vs Unsigned vs Extension

	Signed	Unsigned	Extension
add $rd, $rs, $rt	V		X, both $rs, $rt 32bits
sub $rd, $rs, $rt	V		X, both $rs, $rt 32bits
addu $rd, $rs, $rt	V		X, both $rs, $rt 32bits
subu $rd, $rs, $rt	V		X, both $rs, $rt 32bits
addi $rt, $rs, immd	V		Sign-Extn
addiu $rt, $rs, immd	V		Sign-Extn
addu can ignore the overflow
slt $rd, $rs, $rt	V		X, both $rs, $rt 32bits
sltu $rd, $rs, $rt		V	X, both $rs, $rt 32bits
slti $rt, $rs, immd	V		Sign-Extn
sltiu $rt, $rs, immd		V	Sign-Extn before Unsign comparison
lb $rt, immd($rs)	V		address Sign-Extn
lbu $rt, immd($rs)	V		0-Extn
and $rd, $rs, $rt	N/A	N/A	X
andi $rt, $rs, immd	N/A	N/A	0-Extn

Caller & Callee:

Function {Procedure} Call

Caller (Who called the procedure)	Callee (Procedure being called)
Setup $a	Preserve $ra
Preserve anything else	Preserve $s
	Setup $v

The way to preserve register is shown below by using push and pop procedure

Procedure:

la:

Load Address of the

la reg, targetAddress

Can be written as:

lui reg, constant
ori reg, reg, constant 

Memory Block	Content	Address Fomate
.extern	Global Static	0x1000xxxx
.data	Local Static	0x1001xxxx
.heap	Dynamic	0x1004xxxx
$sp	Stack Pointer	0x7fffxxxx
.text	Instruction R/I/J Format	0x0040xxxx

for example:

la $s0, 0x003d0900 #(0000 0000 0011 1101 0000 1001 0000 0000)

Can be written as:

lui $s0, 0x003d #(0000 0000 0011 1101)
ori $s0, $s0, 0x0900 #(0000 1001 0000 0000)

push:

pushing the memory address to the stack

  addi $sp, $sp, -4  
  sw   $ra, 0($sp)

pop:

poping the memory address from the stack

  lw    $ra, 0($sp)
  addi  $sp, $sp, 4

How to Preserve register?

Untilizing the Stack with push and pop

  addi  $sp, $sp, -8    # the immd is 4n where n = no. of register need to be preserved
  sw    $ra, 0($sp)     # the immd here is 4x where x = order of register being pushed 
  sw    $s1, 4($sp)
  
  
  lw    $s1, 4($sp) 	 # same as above
  lw    $ra, 0($sp)  	 # Last-In-First-Out
  addi  $sp, $sp, 8    # move the pointer back to the top 

MIPS Processor Datapath:

Control Signals

Signal Name	Deasserted (0)	Asserted (1)
RegDst	Write to address contain in $rt (bits [20-16])	Write to address contain in $rd (bits [15-11])
RegWrite	Close the Data Write	Allow Data Write to Register {$rt/$rd}
ALUSrc	Value read from register	Value from bits [15-0]
PCSrc	PC = PC + 4	PC = PC + Jump
MemRead	Not Allow Memory Read	Allow Memory Read
MemWrite	Not Allow Memory Write	Allow Memory Write
MemtoReg	Write value from memory read to register	Write value from ALU result to register
Branch	Don’t Jump	Jump!

PCSrc = Branch $\and$ ALUZero

係beq 入面 branch 會 跟住 ALU Zero flag 哩睇 jump 唔 jump

ALU_Zero = 1 -> a = b, JUMP

ALU_Zero = 0 -> a ≠ b, Dont JUMP!!!!