MIPS Instruction Set

An overview of the instruction set of the MIPS32 architecture as implemented by the mipsy and SPIM emulators. Adapted from reference documents from the University of Stuttgart and Drexel University, from material in the appendix of Patterson and Hennessey's Computer Organization and Design, and from the MIPS32 (r5.04) Instruction Set reference.

Registers

As implemented by mipsy, MIPS has 32× 32-bit general purpose registers as well as two special registers Hi and Lo for manipulating 64-bit integer quantities.

The 32 general purpose registers can be referenced $0 through $31, or by symbolic names, and are used as follows:

Regs	Names	Description
`$0`	`$zero`	the value 0; writes are discarded
`$1`	`$at`	assembler temporary; reserved for assembler use
`$2` `$3`	`$v0` `$v1`	value from expression evaluation or function return
`$4` `$5` `$6` `$7`	`$a0` `$a1` `$a2` `$a3`	first four arguments to a function/subroutine
`$8` `$9` `$10` `$11` `$12` `$13` `$14` `$15`	`$t0` `$t1` `$t2` `$t3` `$t4` `$t5` `$t6` `$t7`	temporary; callers relying on their values must save them before calling subroutines as they may be overwritten
`$16` `$17` `$18` `$19` `$20` `$21` `$22` `$23`	`$s0` `$s1` `$s2` `$s3` `$s4` `$s5` `$s6` `$s7`	saved; subroutines must guarantee their values are unchanged (by, for example, restoring them)
`$24` `$25`	`$t8` `$t9`	temporary; callers relying on their values must save them before calling subroutines as they may be overwritten
`$26` `$27`	`$k0` `$k1`	for kernel use; may change unexpectedly — avoid using in user programs
`$28`	`$gp`	global pointer (address of global area)
`$29`	`$sp`	stack pointer (top of stack)
`$30`	`$fp`	frame pointer (bottom of current stack frame); if not using a frame pointer, becomes a save register
`$31`	`$ra`	return address of most recent caller

Memory

mipsy's memory is partitioned as follows:

Segment	Base	Description
`text`	`0x00400000`	where user program code resides; In mipsy, it is the only area of memory where instructions are executable; its initial size is 256 kiB. This is the only area of memory where instructions are executable. In mipsy,this area of memory is also writeable. On a real system, this area of memory would generally be read-only.
`data`	`0x10000000`	where user data resides; its initial size is 256 kiB, but its size is not fixed, and can be changed with the sbrk syscall up to a maximum of 1 MiB. This area of memory is not executable.
`stack`	`0x7ffffeff`	the function call stack; grows towards negative addresses. its initial size is 64 kiB, but it will grow as needed up to a maximum of 256 kiB. This area of memory is not executable.
`k_text`	`0x80000000`	protected executable code, not accessible in user mode; in a real system, the operating system kernel's code would be mapped here. In mipsy, the entry point is loaded here; its initial size is 64 kiB
`k_data`	`0x90000000`	protected data, not accessible in user mode; in a real system, the operating system's data would be mapped here. In mipsy, the entry point's data is loaded here; its initial size is 64 kiB; but it will grow as needed up to a maximum of 1 MiB.

Syntax

Each instruction is written on a single line, and has the general format

[label:]   opcode   [operand₁[, operand₂[, operand₃]]]  [# comment]

The number of operands for each instruction varies, but could be between zero and three. In the descriptions below, the following notation is used to describe instruction operands.

Operand	Description
`R_n`	a register — commonly, `R_s` and `R_t` are sources, and `R_d` is a destination; registers may be specified either by a numeric name (`$0` to `$31`), or by a symbolic name (`$sN`, `$tN`, etc.)
`Imm`	a literal constant value, or immediate: may be specified as an octal, decimal, hexadecimal, or character literal; if followed by a number (e.g., `Imm₁₆`) that specifies the width in bits and implies the range of the value.
`Label`	a symbolic name which is associated with a memory address
`Addr`	a memory address, in one of the formats described below

Many instructions have an address operand; these may be written in a number of formats:

Useful for accessing arrays.

Format	Address
`Label`	a symbolic name which is associated with a memory address
`(R_n)`	the value stored in register `R_n` (indirect address)
`Imm(R_n)`	the sum of `Imm` and the value stored in register `R_n` Useful for accessing the stack.
`Label(R_n)`	the sum of `Label`'s address and the value stored in register `R_n`
`Label ± Imm`	the sum of `Label`'s address and `Imm` Useful for accessing structs.
`Label ± Imm(R_n)`	the sum of `Label`'s address and `Imm` and the value stored in register `R_n` Useful for accessing arrays of structs.

Instructions

The mipsy emulator implements instructions from the MIPS32 instruction set, as well as pseudo-instructions (which look like MIPS instructions, but which aren't provided on real hardware). Real MIPS instructions are marked with a ✓. All other instructions are pseudo-instructions. Operators in expressions have the same meaning as their C counterparts.

	Instruction		Description	Encoding
back to top CPU Arithmetic Instructions
✓	`ADD`	`R_d, R_s, R_t`	INTEGER OVERFLOW `R_d = R_s + R_t`	`000000ssssstttttddddd00000100000`
✓	`ADDI`	`R_t, R_s, Imm₁₆`	INTEGER OVERFLOW `R_t = R_s + Imm₁₆`	`001000ssssstttttIIIIIIIIIIIIIIII`
✓	`ADDU`	`R_d, R_s, R_t`	`R_d = R_s + R_t`	`000000ssssstttttddddd00000100001`
✓	`ADDIU`	`R_t, R_s, Imm₁₆`	`R_t = R_s + Imm₁₆`	`001001ssssstttttIIIIIIIIIIIIIIII`
✓	`SUB`	`R_d, R_s, R_t`	INTEGER OVERFLOW `R_d = R_s - R_t`	`000000ssssstttttddddd00000100010`
✓	`SUBU`	`R_d, R_s, R_t`	`R_d = R_s - R_t`	`000000ssssstttttddddd00000100011`
✓	`MUL`	`R_d, R_s, R_t`	`R_d = R_s * R_t`	`011100ssssstttttddddd00000000010`
✓	`MULT`	`R_s, R_t`	`(Hi,Lo) = R_s * R_t`	`000000sssssttttt0000000000011000`
✓	`MULTU`	`R_s, R_t`	`(Hi,Lo) = R_s * R_t`	`000000sssssttttt0000000000011001`
✓	`MADD`	`R_s, R_t`	`(Hi,Lo) += R_s * R_t`	`011100sssssttttt0000000000000000`
✓	`MADDU`	`R_s, R_t`	`(Hi,Lo) += R_s * R_t`	`011100sssssttttt0000000000000001`
✓	`MSUB`	`R_s, R_t`	`(Hi,Lo) -= R_s * R_t`	`011100sssssttttt0000000000000100`
✓	`MSUBU`	`R_s, R_t`	`(Hi,Lo) -= R_s * R_t`	`011100sssssttttt0000000000000101`
✓	`DIV`	`R_s, R_t`	`Lo = R_s / R_t; Hi = R_s % R_t`	`000000sssssttttt0000000000011010`
✓	`DIVU`	`R_s, R_t`	`Lo = R_s / R_t; Hi = R_s % R_t`	`000000sssssttttt0000000000011011`
	`DIV`	`R_d, R_s, R_t`	`R_d = R_s / R_t`	pseudo-instruction
	`DIVU`	`R_d, R_s, R_t`	`R_d = R_s / R_t`	pseudo-instruction
	`REM`	`R_d, R_s, R_t`	`R_d = R_s % R_t`	pseudo-instruction
	`REMU`	`R_d, R_s, R_t`	`R_d = R_s % R_t`	pseudo-instruction
✓	`CLO`	`R_d, R_s`	`R_d = count_leading_ones(R_s)`	`000000ssssstttttddddd00001010001`
✓	`CLZ`	`R_d, R_s`	`R_d = count_leading_zeroes(R_s)`	`000000ssssstttttddddd00001010000`
✓	`SEB`	`R_d, R_s`	`R_d = sign_extend (R_s & 0x000000ff)`	`01111100000tttttddddd10000100000`
✓	`SEH`	`R_d, R_s`	`R_d = sign_extend (R_s & 0x0000ffff)`	`01111100000tttttddddd11000100000`
	`SEQ`	`R_d, R_s, R_t`	`R_d = R_s == R_t`	pseudo-instruction
	`SNE`	`R_d, R_s, R_t`	`R_d = R_s != R_t`	pseudo-instruction
	`SLE`	`R_d, R_s, R_t`	`R_d = R_s <= R_t`	pseudo-instruction
	`SLEU`	`R_d, R_s, R_t`	`R_d = R_s <= R_t`	pseudo-instruction
✓	`SLT`	`R_d, R_s, R_t`	`R_d = R_s < R_t`	`000000ssssstttttddddd00000101010`
✓	`SLTU`	`R_d, R_s, R_t`	`R_d = R_s < R_t`	`000000ssssstttttddddd00000101011`
	`SGT`	`R_d, R_s, R_t`	`R_d = R_s > R_t`	pseudo-instruction
	`SGTU`	`R_d, R_s, R_t`	`R_d = R_s > R_t`	pseudo-instruction
	`SGE`	`R_d, R_s, R_t`	`R_d = R_s >= R_t`	pseudo-instruction
	`SGEU`	`R_d, R_s, R_t`	`R_d = R_s >= R_t`	pseudo-instruction
✓	`SLTI`	`R_t, R_s, Imm₁₆`	`R_t = R_s < Imm₁₆`	`001010ssssstttttIIIIIIIIIIIIIIII`
✓	`SLTIU`	`R_t, R_s, Imm₁₆`	`R_t = R_s < Imm₁₆`	`001011ssssstttttIIIIIIIIIIIIIIII`
	`ABS`	`R_t, R_s`	`R_t = \|R_s\|`	pseudo-instruction
	`NEG`	`R_t, R_s`	`INTEGER OVERFLOW R_t = -R_s`	`SUB R_t, $0, R_s`
	`NEGU`	`R_t, R_s`	`R_t = -R_s`	`SUBU R_t, $0, R_s`
back to top CPU Logical Instructions
✓	`AND`	`R_d, R_s, R_t`	`R_d = R_s & R_t`	`000000ssssstttttddddd00000100100`
✓	`ANDI`	`R_t, R_s, Imm₁₆`	`R_t = R_s & Imm₁₆`	`001100ssssstttttIIIIIIIIIIIIIIII`
✓	`OR`	`R_d, R_s, R_t`	`R_d = R_s \| R_t`	`000000ssssstttttddddd00000100101`
✓	`ORI`	`R_t, R_s, Imm₁₆`	`R_t = R_s \| Imm₁₆`	`001101ssssstttttIIIIIIIIIIIIIIII`
✓	`NOR`	`R_d, R_s, R_t`	`R_d = ~(R_s \| R_t)`	`000000ssssstttttddddd00000100111`
✓	`XOR`	`R_d, R_s, R_t`	`R_d = R_s ^ R_t`	`000000ssssstttttddddd00000100110`
✓	`XORI`	`R_t, R_s, Imm₁₆`	`R_t = R_s ^ Imm₁₆`	`001110ssssstttttIIIIIIIIIIIIIIII`
	`NOT`	`R_t, R_s`	`R_t = ~R_s`	`NOR R_t, R_s, $0`
back to top CPU Shift Instructions
	`ROL`	`R_d, R_t, R_s`	`R_d = R_t rot < R_s`	pseudo-instruction
	`ROR`	`R_d, R_t, R_s`	`R_d = R_t rot > R_s`	pseudo-instruction
✓	`ROTR`	`R_d, R_t, _a`	`R_d = R_t rot > _a`	`00000000001tttttdddddaaaaa000010`
✓	`ROTRV`	`R_d, R_t, R_s`	`R_d = R_t rot > R_s`	`000000ssssstttttddddd00001000110`
✓	`SLL`	`R_d, R_t, _a`	`R_d = R_t << _a`	`00000000000tttttdddddaaaaa000000`
✓	`SLLV`	`R_d, R_t, R_s`	`R_d = R_t << R_s`	`000000ssssstttttddddd00000000100`
✓	`SRA`	`R_d, R_t, _a`	`SIGN EXTENDED R_d = R_t >> _a`	`00000000000tttttdddddaaaaa000011`
✓	`SRAV`	`R_d, R_t, R_s`	`SIGN EXTENDED R_d = R_t >> R_s`	`000000ssssstttttddddd00000000111`
✓	`SRL`	`R_d, R_t, _a`	`R_d = R_t >> _a`	`00000000000tttttdddddaaaaa000010`
✓	`SRLV`	`R_d, R_t, R_s`	`R_d = R_t >> R_s`	`000000ssssstttttddddd00000000110`
back to top CPU Load, Store, and Memory Control Instructions
	`LI`	`R_t, Imm`	`R_t = Imm`	pseudo-instruction
	`LA`	`R_t, Label`	`R_t = Label`	pseudo-instruction
✓	`LUI`	`R_t, Imm₁₆`	`R_t = Imm₁₆ << 16`	`00111100000tttttIIIIIIIIIIIIIIII`
✓	`LB`	`R_t, Offset₁₆(R_b)`	`R_t = RAM[R_b + Offset₁₆]` ADDRESS ERROR SIGN EXTENDED	`100000bbbbbtttttOOOOOOOOOOOOOOOO`
✓	`LBU`	`R_t, Offset₁₆(R_b)`	`R_t = RAM[R_b + Offset₁₆]` ADDRESS ERROR	`100100bbbbbtttttOOOOOOOOOOOOOOOO`
✓	`LH`	`R_t, Offset₁₆(R_b)`	`R_t = RAM[R_b + Offset₁₆]` ADDRESS ERROR SIGN EXTENDED	`100001bbbbbtttttOOOOOOOOOOOOOOOO`
✓	`LHU`	`R_t, Offset₁₆(R_b)`	`R_t = RAM[R_b + Offset₁₆]` ADDRESS ERROR	`100101bbbbbtttttOOOOOOOOOOOOOOOO`
✓	`LW`	`R_t, Offset₁₆(R_b)`	`R_t = RAM[R_b + Offset₁₆]` ADDRESS ERROR	`100011bbbbbtttttOOOOOOOOOOOOOOOO`
✓	`SB`	`R_t, Offset₁₆(R_b)`	`RAM[R_b + Offset₁₆] = R_t` ADDRESS ERROR	`101000bbbbbtttttOOOOOOOOOOOOOOOO`
✓	`SH`	`R_t, Offset₁₆(R_b)`	`RAM[R_b + Offset₁₆] = R_t` ADDRESS ERROR	`101001bbbbbtttttOOOOOOOOOOOOOOOO`
✓	`SW`	`R_t, Offset₁₆(R_b)`	`RAM[R_b + Offset₁₆] = R_t` ADDRESS ERROR	`101011bbbbbtttttOOOOOOOOOOOOOOOO`
	`PUSH`	`R_s`	`$sp -= 4 RAM[$sp] = R_s`	pseudo-instruction (mipsy only)
	`POP`	`R_s`	`R_s = RAM[$sp] $sp += 4`	pseudo-instruction (mipsy only)
	`BEGIN`		`$sp -= 4` `RAM[$sp] = $fp` `$fp = $sp`	pseudo-instruction (mipsy only)
	`END`		`$sp = $fp + 4` `$fp = RAM[$fp]`	pseudo-instruction (mipsy only)
back to top CPU Move Instructions
✓	`MFHI`	`R_d`	`R_d = HI`	`0000000000000000ddddd00000010000`
✓	`MFLO`	`R_d`	`R_d = LO`	`0000000000000000ddddd00000010010`
✓	`MTHI`	`R_d`	`HI = R_d`	`000000sssss000000000000000010001`
✓	`MTLO`	`R_d`	`LO = R_d`	`000000sssss000000000000000010011`
	`MOVE`	`R_t, R_s`	`R_t = R_s`	`ADDU R_t, $0, R_s`
✓	`MOVZ`	`R_d, R_s, R_t`	`IF R_t == 0 THEN R_d = R_s`	`000000ssssstttttddddd00000001010`
✓	`MOVN`	`R_d, R_s, R_t`	`IF R_t != 0 THEN R_d = R_s`	`000000ssssstttttddddd00000001011`
back to top CPU Branch and Jump Instructions
Note: In assembly you write a `Label`. mipsy automatically converts it to the correct `Offset₁₆` for branches and `Address₂₆` for jumps.
	`B`	`Label`	`PC += Offset₁₆ << 2`	`0001000000000000OOOOOOOOOOOOOOOO` `BEQ $t0, $t0, Label`
✓	`BEQ`	`R_s, R_t, Label`	`IF R_s == R_t THEN` `PC += Offset₁₆ << 2`	`000100ssssstttttOOOOOOOOOOOOOOOO`
	`BEQ`	`R_s, Imm, Label`	`IF R_s == Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
	`BEQZ`	`R_s, Label`	`IF R_s == 0 THEN` `PC += Offset₁₆ << 2`	`BEQ $0, R_s, Label`
✓	`BNE`	`R_s, R_t, Label`	`IF R_s != R_t THEN` `PC += Offset₁₆ << 2`	`000101ssssstttttOOOOOOOOOOOOOOOO`
	`BNE`	`R_s, Imm, Label`	`IF R_s != Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
	`BNEZ`	`R_s, Label`	`IF R_s != 0 THEN` `PC += Offset₁₆ << 2`	`BNE $0, R_s, Label`
	`BGE`	`R_s, R_t, Label`	`IF R_s >= R_t THEN` `PC += Offset₁₆ << 2`	`SLT $at, R_s, R_t` `BEQ $0, $at, Label`
	`BGE`	`R_s, Imm, Label`	`IF R_s >= Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
	`BGEU`	`R_s, R_t, Label`	UNSIGNED COMPARISON `IF R_s >= R_t THEN` `PC += Offset₁₆ << 2`	`SLTU $at, R_s, R_t` `BEQ $0, $at, Label`
	`BGEU`	`R_s, Imm, Label`	UNSIGNED COMPARISON `IF R_s >= Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
✓	`BGEZ`	`R_s, Label`	`IF R_s >= 0 THEN` `PC += Offset₁₆ << 2`	`000001sssss00001OOOOOOOOOOOOOOOO`
	`BGT`	`R_s, R_t, Label`	`IF R_s > R_t THEN` `PC += Offset₁₆ << 2`	`SLT $at, R_t, R_s` `BNE $0, $at, Label`
	`BGT`	`R_s, Imm, Label`	`IF R_s > Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
	`BGTU`	`R_s, R_t, Label`	UNSIGNED COMPARISON `IF R_s > R_t THEN` `PC += Offset₁₆ << 2`	`SLTU $at, R_t, R_s` `BNE $0, $at, Label`
	`BGTU`	`R_s, Imm, Label`	UNSIGNED COMPARISON `IF R_s > Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
✓	`BGTZ`	`R_s, Label`	`IF R_s > 0 THEN` `PC += Offset₁₆ << 2`	`000111sssss00000OOOOOOOOOOOOOOOO`
	`BLT`	`R_s, R_t, Label`	`IF R_s < R_t THEN` `PC += Offset₁₆ << 2`	`SLT $at, R_s, R_t` `BNE $0, $at, Label`
	`BLT`	`R_s, Imm, Label`	`IF R_s < Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
	`BLTU`	`R_s, R_t, Label`	UNSIGNED COMPARISON `IF R_s < R_t THEN` `PC += Offset₁₆ << 2`	`SLTU $at, R_s, R_t` `BNE $0, $at, Label`
	`BLTU`	`R_s, Imm, Label`	UNSIGNED COMPARISON `IF R_s < Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
✓	`BLTZ`	`R_s, Label`	`IF R_s < 0 THEN` `PC += Offset₁₆ << 2`	`000001sssss00000OOOOOOOOOOOOOOOO`
	`BLE`	`R_s, R_t, Label`	`IF R_s <= R_t THEN` `PC += Offset₁₆ << 2`	`SLT $at, R_t, R_s` `BEQ $0, $at, Label`
	`BLE`	`R_s, Imm, Label`	`IF R_s <= Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
	`BLEU`	`R_s, R_t, Label`	UNSIGNED COMPARISON `IF R_s <= R_t THEN` `PC += Offset₁₆ << 2`	`SLTU $at, R_t, R_s` `BEQ $0, $at, Label`
	`BLEU`	`R_s, Imm, Label`	UNSIGNED COMPARISON `IF R_s <= Imm THEN` `PC += Offset₁₆ << 2`	pseudo-instruction
✓	`BLEZ`	`R_s, Label`	`IF R_s <= 0 THEN` `PC += Offset₁₆ << 2`	`000110sssss00000OOOOOOOOOOOOOOOO`
✓	`J`	`Label`	`PC = PC[31-28] && Address₂₆ << 2`	`000010AAAAAAAAAAAAAAAAAAAAAAAAAA`
✓	`JAL`	`Label`	`$ra = PC + 4` `PC = PC[31-28] && Address₂₆ << 2`	`000011AAAAAAAAAAAAAAAAAAAAAAAAAA`
✓	`JR`	`R_s`	`PC = R_s`	`000000sssss0000000000hhhhh001000`
✓	`JALR`	`R_s`	`$ra = PC + 4` `PC = R_s`	`000000sssss0000011111hhhhh001001`
✓	`JALR`	`R_d, R_s`	`R_d = PC + 4` `PC = R_s`	`000000sssss00000dddddhhhhh001001`
back to top CPU Trap Instructions
✓	`SYSCALL`		perform a system call	`000000cccccccccccccccccccc001100`
✓	`BREAK`		trigger a breakpoint	`000000cccccccccccccccccccc001101`
✓	`TEQ`	`R_s, R_t`	`IF R_s == R_t THEN` trigger a breakpoint	`000000ssssstttttcccccccccc110100`
✓	`TEQI`	`R_s, Imm₁₆`	`IF R_s == Imm₁₆ THEN` trigger a breakpoint	`000001sssss01100IIIIIIIIIIIIIIII`
✓	`TNE`	`R_s, R_t`	`IF R_s != R_t THEN` trigger a breakpoint	`000000ssssstttttcccccccccc110110`
✓	`TNEI`	`R_s, Imm₁₆`	`IF R_s != Imm₁₆ THEN` trigger a breakpoint	`000001sssss01110IIIIIIIIIIIIIIII`
✓	`TGE`	`R_s, R_t`	`IF R_s >= R_t THEN` trigger a breakpoint	`000000ssssstttttcccccccccc110000`
✓	`TGEU`	`R_s, R_t`	UNSIGNED COMPARISON `IF R_s >= R_t THEN` trigger a breakpoint	`000000ssssstttttcccccccccc110001`
✓	`TGEI`	`R_s, Imm₁₆`	`IF R_s >= Imm₁₆ THEN` trigger a breakpoint	`000001sssss01000IIIIIIIIIIIIIIII`
✓	`TGEIU`	`R_s, Imm₁₆`	UNSIGNED COMPARISON `IF R_s >= Imm₁₆ THEN` trigger a breakpoint	`000001sssss01001IIIIIIIIIIIIIIII`
	`TGT`	`R_s, R_t`	`IF R_s > R_t THEN` trigger a breakpoint	pseudo-instruction
	`TGTU`	`R_s, R_t`	UNSIGNED COMPARISON `IF R_s > R_t THEN` trigger a breakpoint	pseudo-instruction
	`TGTI`	`R_s, Imm₁₆`	`IF R_s > Imm₁₆ THEN` trigger a breakpoint	pseudo-instruction
	`TGTIU`	`R_s, Imm₁₆`	UNSIGNED COMPARISON `IF R_s > Imm₁₆ THEN` trigger a breakpoint	pseudo-instruction
✓	`TLT`	`R_s, R_t`	`IF R_s < R_t THEN` trigger a breakpoint	`000000ssssstttttcccccccccc110010`
✓	`TLTU`	`R_s, R_t`	UNSIGNED COMPARISON `IF R_s < R_t THEN` trigger a breakpoint	`000000ssssstttttcccccccccc110011`
✓	`TLTI`	`R_s, Imm₁₆`	`IF R_s < Imm₁₆ THEN` trigger a breakpoint	`000001sssss01010IIIIIIIIIIIIIIII`
✓	`TLTIU`	`R_s, Imm₁₆`	UNSIGNED COMPARISON `IF R_s < Imm₁₆ THEN` trigger a breakpoint	`000001sssss01011IIIIIIIIIIIIIIII`
	`TLE`	`R_s, R_t`	`IF R_s <= R_t THEN` trigger a breakpoint	pseudo-instruction
	`TLEU`	`R_s, R_t`	UNSIGNED COMPARISON `IF R_s <= R_t THEN` trigger a breakpoint	pseudo-instruction
	`TLEI`	`R_s, Imm₁₆`	`IF R_s <= Imm₁₆ THEN` trigger a breakpoint	pseudo-instruction
	`TLEIU`	`R_s, Imm₁₆`	UNSIGNED COMPARISON `IF R_s <= Imm₁₆ THEN` trigger a breakpoint	pseudo-instruction
back to top CPU Control Instructions
	`NOP`		do nothing	`00000000000000000000000000000000` `SLL $0, $0, 0`

System Services

The mipsy emulator provides a number of mechanisms for interacting with the host system, to provide input and output, file operations, and other miscellaneous services, which we refer to as system calls or syscalls. These are invoked via the syscall instruction after storing the service code in the register $v0.

`$v0`=	Arguments	Result	Description
back to top Printing Values
1	`$a0`: `int`		print_int: Print the integer in `$a0` to the console as a signed decimal.
2	`$f12`: `float`		print_float: Print the float in `$f12` to the console as a `%.8f`.
3	`$f12`/`$f13`: `double`		print_double: Print the double in `$f12`/`$f13` to the console as a `%.18g`
4	`$a0`: `char *`		print_string: Print the nul-terminated array of bytes referenced by `$a0` to the console as an ASCII string.
11	`$a0`: `char`		print_character: Print the character in `$a0`, analogous to putchar(3).
back to top Reading Values
5		`$v0`: `int`	read_int: Read an integral value from the console, with atol(3)'s semantics, into register `$v0`
6		`$f0`: `float`	read_float: Read a floating-point value from the console, with atof(3)'s semantics, into register `$f0`
7		`$f0`/`$f1`: `double`	read_double: Read a double-precision floating-point value from the console, with atof(3)'s semantics, into registers `$f0`/`$f1`
8	`$a0`: `char *`; `$a1`: `int`		read_string: Read a string into the provided buffer (referenced by `$a0`); up to `size` (given in `$a1`) bytes are read, and the result is nul-terminated.
12		`$v0`: `char`	read_character: Read the next character from the console into register `$v0`; analogous to getchar(3)
back to top File Manipulation
13	`$a0`: `char *`; `$a1`: `int`; `$a2`: `mode_t`	`$v0`: `fd`	open: Open the file specified by `name` (referenced by `$a0`) in a particular access mode as specified by `flags` (given by `$a1`), and, if it is to be created, with mode `mode` (given by `$a2`). Returns a file descriptor, a small non-negative `int`. Effectively, open(2).
14	`$a0`: `fd`; `$a1`: `void *`; `$a2`: `int`	`$v0`: `int`	read: On the file given by the file descriptor `fd` (given in `$a0`), read `len` bytes (given by `$a2`) into `buffer` (given by `$a1`). Returns the number of bytes read, or -1 if an error occurred. Effectively, read(2).
15	`$a0`: `fd`; `$a1`: `void *`; `$a2`: `int`	`$v0`: `int`	write: On the file given by the file descriptor `fd` (given in `$a0`), write `len` bytes (given by `$a2`) from `buffer` (given by `$a1`). Returns the number of bytes written, or -1 if an error occurred. Effectively, write(2).
16	`$a0`: `fd`	`$v0`: `int`	close: Close the file given by the file descriptor `fd` (given in `$a0`). Returns 0 if successful, or -1 if an error occurred. Effectively, close(2).
back to top Process Services
9	`$a0`: `int`		sbrk: Extend the `.data` segment by adding `$a0` bytes; a primitive useful for, e.g., implementing malloc(3)
10			exit: The program exits with code 0.
17	`$a0`: `int`		exit2: The program exits with `code` (given in `$a0`).

Directives

The mipsy assembler supports a number of directives, which allow things to be specified at assembly time.

Directive	Description
`.text`	the instructions following this directive are placed in the `text segment` of memory
`.data`	the data defined following this directive is placed in the `data segment` of memory
`.ktext`	the instructions following this directive are placed in the `kernel text segment` of memory
`.kdata`	the data defined following this directive is placed in the `kernel data segment` of memory
`.align N`	arrange that the next datum is stored with appropriate alignment (that the lower `N` bits of its address are set to zero) by inserting enough padding — nearly always automatically done; a half word requires `.align 1` (for two bytes), a word requires `.align 2` (for four bytes), and a double requires `.align 3` (for eight bytes).
`.ascii "string"`	store an ASCII string without a '\0'-terminator at the next location(s) in the current data segment. nearly always not what you want; use `.asciiz` instead!
`.asciiz "string"`	store a '\0'-terminated ASCII string at the next location(s) in the current data segment
`.space n`	allocate `n` uninitialised bytes of space at the next location in the current segment
`.byte val [, ...]`	store values in successive byte(s) at the next location(s) in the current segment
`.half val [, ...]`	store values in successive half word(s) at the next location(s) in the current segment
`.word val [, ...]`	store values in successive word(s) at the next location(s) in the current segment
`.float val [, ...]`	store values in successive float(s) at the next location(s) in the current segment
`.double val [, ...]`	store values in successive double(s) at the next location(s) in the current segment
`.globl Label [, ...]`	Declare the listed Label(s) as global to enable referencing from other files