MIPS pseudo instructions

Pseudo-instructions

In MIPS, some operations can be performed with help of other instructions. The most common operations are unified in pseudo-instructions — they can be coded in assembly language, and assembler will expand them to real instructions according to following rules:

Data moves

Name	Assembly syntax	Expansion	Operation in C
move	move $t, $s	addiu $t, $s, 0	t = s
clear	clear $t	addu $t, $zero, $zero	t = 0
load 32-bit immediate	li $t, C	lui $t, C_hi ori $t, $t, C_lo	t = C
load label address	la $t, A	lui $t, A_hi ori $t, $t, A_lo	t = A

Branches

Name	Assembly syntax	Expansion
branch unconditionally	b C	beq $zero, $zero, C
branch unconditionally and link	bal C	bgezal $zero, C
branch if greater than	bgt $s, $t, C	slt $at, $t, $s bne $at, $zero, C
branch if less than	blt $s, $t, C	slt $at, $s, $t bne $at, $zero, C
branch if greater than or equal	bge $s, $t, C	slt $at, $s, $t beq $at, $zero, C
branch if less than or equal	ble $s, $t, C	slt $at, $t, $s beq $at, $zero, C
branch if greater than unsigned	bgtu $s, $t, C	sltu $at, $t, $s bne $at, $zero, C
branch if zero	beqz $s, C	beq $s, $zero, C

Multiplication/Division

Name	Assembly syntax	Expansion	Operation in C
multiplicate and return 32 bits	mul $d, $s, $t	mult $s, $t mflo $d	d = (s * t) & 0xFFFFFFFF
quotient	div $d, $s, $t	div $s, $t mflo $d	d = s / t
remainder	rem $d, $s, $t	div $s, $t mfhi $d	d = s % t

Jumps

Name	Assembly syntax	Expansion	Operation in C
jump register and link to ra	jalr $s	jalr $s, $ra	ra = PC + 4; goto s;

Logical operations

Name	Assembly syntax	Expansion	Operation in C
not	not $t, $s	nor $t, $s, $zero	t = ~s

No-operations

Name	Assembly syntax	Expansion	Operation in C
nop	nop	sll $zero, $zero, 0	{}

In fact, every MIPS instruction that has $zero as its destination and doesn't touch memory, access I/O system, and/or call a trap, can be treated as a nop; but using sll $zero, $zero, 0 is the most convinient because it's byte code is all-zeroes 0x00000000.

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Should we implement pseudo-instructions

Since 2014/2015, all instructions are printed as real MIPS instructions in MIPT-MIPS. Unexpanded pseudo-instructions are left only in testing traces source files (.s files).