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MIPS basic content.

This is based on MIPS32 as an introduction.

0x01 Register

(1) General purpose registers

MIPS has 32 general purpose registers, represented by the dollar sign ($). Can be expressed as $0~$31, and can also be represented by register names such as $sp, $t9, $fp, and so on.

| Register Number | Conventional Name | Usage | Usage |

| --------------- | ----------------- | ------------------------------------------------------ | --------------------------------------------------------- |

| $0 | $zero | Hard-wired to 0 | |

| $1 | $at | Reserved for pseudo-instructions | |

$2 - $3 | $v0, $v1 | Return values from functions | Save expression or function return value | $4 - $7 | $a0 - $a3 | Arguments to functions - not preserved by subprograms | | $8 - $15 | $t0 - $t7 | Temporary data, not preserved by subprograms | |

| $16 - $23 | $s0 - $s7 | Saved registers, preserved by subprograms | |

| $24 - $25 | $t8 - $t9 | More temporary registers, not preserved by subprograms | Temporary registers, as a complement to $t0 - $t7, $t9 is usually related to calling functions | | $26 - $27 | $k0 - $k1 | Reserved for kernel. Do not use. | |

| $28 | $gp | Global Area Pointer (base of global data segment) | |

MIPS as a load-store architecture means that when we want to access memory we must access it through load and store instructions. All other instructions (add, sub, mul, div, and so on) must fetch their operands from the register and store their results in registers. For example, the following example:

sum = x + y

We assume that sum and x , y are variables in the program, and their MIPS assembly is expressed as:

# sum = x + y
    lw      $t0, x          # Load x from memory into a CPU register

    lw      $t1, y          # Load y from memory into a CPU register

    add     $t0, $t0, $t1   # Add x and y

    sw      $t0, sum        # Store the result from the CPU register to memory

( 2 ) Special registers

The MIPS32 architecture also defines three special registers, PC (program counter), HI (multiply and divide result high register), and LO (multiply and divide result low register). When multiplying, HI and LO hold the result of the multiplication, where HI holds the upper 32 bits and LO holds the lower 32 bits. In the division operation, HI saves the remainder and the LO saves the quotient.

0x2 Instruction Instruction

ADD – Add (with overflow)

| Description: | Adds two registers and stores the result in a register |

| ------------ | ------------------------------------------------------ |

| Operation: | $d = $s + $t; advance_pc (4); |

| Syntax: | add $d, $s, $t |

| Encoding: | 0000 00ss ssh tttt dddd d000 0010 0000 |

ADDI -- Add immediate (with overflow)

| Description: | Adds a register and a sign-extended immediate value and stores the result in a register |

| ------------ | ------------------------------------------------------------ |

| Operation: | $t = $s + imm; advance_pc (4); |

| Syntax: | addi $t, $s, imm |

| Encoding: | 0010 00ss ssst tttt iiii iiii iiii iiii |

ADDIU -- Add immediate unsigned (no overflow)

| Description: | Adds a register and a sign-extended immediate value and stores the result in a register |

| ------------ | ------------------------------------------------------------ |

| Operation: | $t = $s + imm; advance_pc (4); |

| Syntax: | add $ t, $ s, imm | | Encoding: | `0010 01ss sis tttt iiii iiii iiii iiii | | ADDU -- Add unsigned (no overflow)

| Description: | Adds two registers and stores the result in a register |

| ------------ | ------------------------------------------------------ |

| Operation: | $d = $s + $t; advance_pc (4); |

| Syntax: | addu $d, $s, $t |

| Encoding: | 0000 00ss shh tttt dddd d000 0010 0001 |

AND -- Bitwise and

| Description: | Bitwise ands two registers and stores the result in a register |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $s & $t; advance_pc (4); |

| Syntax: | and $d, $s, $t |

| Encoding: | 0000 00ss thanks tttt dddd d000 0010 0100 |

ANDI -- Bitwise and immediate

| Description: | Bitwise ands a register and an immediate value and stores the result in a register |

| ------------ | ------------------------------------------------------------ |

| Operation: | $t = $s & imm; advance_pc (4); |

| Syntax: | andi $t, $s, imm |

| Encoding: | 0011 00ss ssst tttt iiii iiii iiii iiii |

BEQ -- Branch on equal

| Description: | Branches if the two registers are equal |

| ------------ | ----------------------------------------------------------- |

| Operation: | if $s == $t advance_pc (offset << 2)); else advance_pc (4); |

| Syntax: | beq $s, $t, offset |

| Encoding: | 0001 00ss ssst tttt iiii iiii iiii iiii |

BGEZ -- Branch on greater than or equal to zero

| Description: | Branches if the register is greater than or equal to zero |

| ------------ | ---------------------------------------------------------- |

| Operation: | if $s >= 0 advance_pc (offset << 2)); else advance_pc (4); |

| Syntax: | bgez $s, offset |

| Encoding: | 01ss sss0 0000 0001 IIII fourth iiii |

BGEZAL -- Branch on greater than or equal to zero and link

| Description: | Branches if the register is greater than or equal to zero and saves the return address in $31 |

| ------------ | ------------------------------------------------------------ |

| Operation: | if $s >= 0 $31 = PC + 8 (or nPC + 4); advance_pc (offset << 2)); else advance_pc (4); |

| Syntax: | bgezal $s, offset |

| Encoding: | 0000 01ss sss1 0001 iiii iiii iiii iiii` |

BGTZ -- Branch on greater than zero

| Description: | Branches if the register is greater than zero |

| ------------ | --------------------------------------------------------- |

| Operation: | if $s > 0 advance_pc (offset << 2)); else advance_pc (4); |

| Syntax: | bgtz $s, offset |

| Encoding: | 11ss sss0 0001 0000 IIII fourth iiii |

BLEZ -- Branch on less than or equal to zero

| Description: | Branches if the register is less than or equal to zero |

| ------------ | ---------------------------------------------------------- |

| Operation: | if $s <= 0 advance_pc (offset << 2)); else advance_pc (4); |

| Syntax: | blez $s, offset |

| Encoding: | 0001 10ss sss0 0000 iiii iiii iiii iiii |

BLTZ -- Branch on less than zero

| Description: | Branches if the register is less than zero |

| ------------ | --------------------------------------------------------- |

| Operation: | if $s < 0 advance_pc (offset << 2)); else advance_pc (4); |

| Syntax: | bltz $s, offset |

| Encoding: | 01ss sss0 0000 0000 IIII fourth iiii |

BLTZAL -- Branch on less than zero and link

| Description: | Branches if the register is less than zero and saves the return address in $31 |

| ------------ | ------------------------------------------------------------ |

| Operation: | if $s < 0 $31 = PC + 8 (or nPC + 4); advance_pc (offset << 2)); else advance_pc (4); |

| Syntax: | bltzal $s, offset |

| Encoding: | 0000 01ss sss1 0000 iiii iiii iiii iiii |

BNE -- Branch on not equal

| Description: | Branches if the two registers are not equal |

| ------------ | ----------------------------------------------------------- |

| Operation: | if $s != $t advance_pc (offset << 2)); else advance_pc (4); |

| Syntax: | bne $s, $t, offset |

| Encoding: | `0001 01ss sis tttt iiii iiii iiii iiii | |

DIV -- Divide

| Description: | Divides $s by $t and stores the quotient in $LO and the remainder in $HI |

| ------------ | ------------------------------------------------------------ |

| Operation: | $LO = $s / $t; $HI = $s % $t; advance_pc (4); |

| Syntax: | div $s, $t |

| Encoding: | 0000 00ss shhh ttt 0000 0000 0001 1010 |

DIVU - Divide unsigned

| Description: | Divides $s by $t and stores the quotient in $LO and the remainder in $HI |

| ------------ | ------------------------------------------------------------ |

| Operation: | $LO = $s / $t; $HI = $s % $t; advance_pc (4); |

| Syntax: | divu $s, $t |

| Encoding: | 0000 00ss shhh ttt 0000 0000 0001 1011 |

J -- Jump

| Description: | Jumps to the calculated address |

| ------------ | --------------------------------------------------- |

| Operation: | PC = nPC; nPC = (PC & 0xf0000000) | (target << 2); |

| Syntax: | j target |

| Encoding: | 0000 10ii iiii iiii iiii iiii iiii iiii |

JAL -- Jump and link

| Description: | Jumps to the calculated address and stores the return address in $31 |

| ------------ | ------------------------------------------------------------ |

| Operation: | $31 = PC + 8 (or nPC + 4); PC = nPC; nPC = (PC & 0xf0000000) | (target << 2); |

| Syntax: | jal target |

| Encoding: | 0000 11ii iiii iiii iiii iiii iiii iiii |

JR -- Jump register

| Description: | Jump to the address contained in register $s |

| ------------ | -------------------------------------------- |

| Operation: | PC = nPC; nPC = $s; |

| Syntax: | jr $s |

| Encoding: | 0000 00ss sss0 0000 0000 0000 0000 1000 |

LB -- Load byte

| Description: | A byte is loaded into a register from the specified address. |

| ------------ | ------------------------------------------------------------ |

| Operation: | t = MEM[s + offset]; advance_pc (4); |

| Syntax: | lb t, offset(s) |

| Encoding: | 1000 00ss ssst tttt iiii iiii iiii iiii |

LUI -- Load upper immediate

| Description: | The immediate value is shifted left 16 bits and stored in the register. The lower 16 bits are zeroes. |

| ------------ | ------------------------------------------------------------ |

| Operation: | $t = (imm << 16); advance_pc (4); |

| Syntax: to $ t, imm | | Encoding: | 0011 11 - --- tttt iiii iiii iiii iiii |

LW -- Load word

| Description: | A word is loaded into a register from the specified address. |

| ------------ | ------------------------------------------------------------ |

| Operation: | t = MEM[s + offset]; advance_pc (4); |

| Syntax: | lw t, offset(s) |

| Encoding: | 1000 11ss ssst tttt iiii iiii iiii iiii |

MFHI -- Move from HI

| Description: | The contents of register HI are moved to the specified register. |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $HI; advance_pc (4); |

| Syntax: | mfhi $d |

| Encoding: | 0000 0000 0000 0000 dddd d000 0001 0000 |

MFLO -- Move from LO

| Description: | The contents of register LO are moved to the specified register. |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $LO; advance_pc (4); |

| Syntax: | mflo $d |

| Encoding: | 0000 0000 0000 0000 dddd d000 0001 0010 |

MULT - Multiply

| Description: | Multiplies $s by $t and stores the result in $LO. |

| ------------ | ------------------------------------------------- |

| Operation: | $LO = $s * $t; advance_pc (4); |

| Syntax: | mult $s, $t | | Encoding: | 0000 00ss shhh 0000 0000 0001 1000 |

MULTU - Multiply unsigned

| Description: | Multiplies $s by $t and stores the result in $LO. |

| ------------ | ------------------------------------------------- |

| Operation: | $LO = $s * $t; advance_pc (4); |

| Syntax: | multu $s, $t |

| Encoding: | 0000 00ss shhh ttt 0000 0000 0001 1001 |

NOOP -- no operation

| Description: | Performs no operation. |

| ------------ | ----------------------------------------- |

| Operation: | advance_pc (4); |

| Syntax: | noop |

| Encoding: | 0000 0000 0000 0000 0000 0000 0000 0000 |

Note: The encoding for a NOOP represents the instruction SLL $0, $0, 0 which has no side effects. In fact, nearly every instruction that has $0 as its destination register will have no side effect and can thus be considered a NOOP instruction.

OR -- Bitwise or

| Description: | Bitwise logical ors two registers and stores the result in a register |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $s | $t; advance_pc (4); |

| Syntax: | or $d, $s, $t |

| Encoding: | 0000 00ss ssh tttt dddd d000 0010 0101 |

ORI -- Bitwise or immediate

| Description: | Bitwise ors a register and an immediate value and stores the result in a register |

| ------------ | ------------------------------------------------------------ |

| Operation: | $t = $s | imm; advance_pc (4); |

| Syntax: | ori $t, $s, imm |

| Encoding: | `0011 01ss sis tttt iiii iiii iiii iiii | |

SB -- Store byte

| Description: | The least significant byte of $t is stored at the specified address. |

| ------------ | ------------------------------------------------------------ |

| Operation: | MEM[$s + offset] = (0xff & $t); advance_pc (4); |

| Syntax: | sb t, offset(s) |

| Encoding: | 1010 00ss ssst tttt iiii iiii iiii iiii |

SLL -- Shift left logical

| Description: | Shifts a register value left by the shift amount listed in the instruction and places the result in a third register. Zeroes are shifted in. |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $t << h; advance_pc (4); |

| Syntax: | sll $d, $t, h |

| Encoding: | 0000 00ss, thanks for dddd dhhh hh00 0000 |

SLLV -- Shift left logical variable

| Description: | Shifts a register value left by the value in a second register and places the result in a third register. Zeroes are shifted in. |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $t << $s; advance_pc (4); |

| Syntax: | sllv $d, $t, $s |

| Encoding: | 0000 00ss but tttt dddd d --- --00 0100 |

SLT -- Set on less than (signed)

| Description: | If $s is less than $t, $d is set to one. It gets zero otherwise. |

| ------------ | ------------------------------------------------------------ |

| Operation: | if $s < $t $d = 1; advance_pc (4); else $d = 0; advance_pc (4); |

| Syntax: | slt $d, $s, $t |

| Encoding: | 0000 00ss, thanks tttt dddd d000 0010 1010 |

SLTI -- Set on less than immediate (signed)

| Description: | If $s is less than immediate, $t is set to one. It gets zero otherwise. |

| ------------ | ------------------------------------------------------------ |

| Operation: | if $s < imm $t = 1; advance_pc (4); else $t = 0; advance_pc (4); |

| Syntax: | slti $t, $s, imm |

| Encoding: | 0010 10ss ssst tttt iiii iiii iiii iiii |

SLTIU -- Set on less than immediate unsigned

| Description: | If $s is less than the unsigned immediate, $t is set to one. It gets zero otherwise. |

| ------------ | ------------------------------------------------------------ |

| Operation: | if $s < imm $t = 1; advance_pc (4); else $t = 0; advance_pc (4); |

| Syntax: | sltiu $t, $s, imm |

| Encoding: | 0010 11ss ssst tttt iiii iiii iiii iiii |

SLTU -- Set on less than unsigned

| Description: | If $s is less than $t, $d is set to one. It gets zero otherwise. |

| ------------ | ------------------------------------------------------------ |

| Operation: | if $s < $t $d = 1; advance_pc (4); else $d = 0; advance_pc (4); |

| Syntax: | sltu $d, $s, $t |

| Encoding: | 0000 00ss ssh tttt dddd d000 0010 1011 |

SRA -- Shift right arithmetic

| Description: | Shifts a register value right by the shift amount (shamt) and places the value in the destination register. The sign bit is shifted in. |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $t >> h; advance_pc (4); |

| Syntax: | sra $d, $t, h |

| Encoding: | 0000 00-- ---t tttt dddd dhhh hh00 0011 |

SRL -- Shift right logical

| Description: | Shifts a register value right by the shift amount (shamt) and places the value in the destination register. Zeroes are shifted in. |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $t >> h; advance_pc (4); |

| Syntax: | srl $d, $t, h |

| Encoding: | 0000 00-- ---t tttt dddd dhhh hh00 0010 |

SRLV -- Shift right logical variable

| Description: | Shifts a register value right by the amount specified in $s and places the value in the destination register. Zeroes are shifted in. |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $t >> $s; advance_pc (4); |

| Syntax: | srlv $d, $t, $s |

| Encoding: | 0000 00ss ssh tttt dddd d000 0000 0110 |

SUB -- Subtract

| Description: | Subtracts two registers and stores the result in a register |

| ------------ | ----------------------------------------------------------- |

| Operation: | $d = $s - $t; advance_pc (4); |

| Syntax: | sub $d, $s, $t |

| Encoding: | 0000 00ss ssh tttt dddd d000 0010 0010 |

SUBU -- Subtract unsigned

| Description: | Subtracts two registers and stores the result in a register |

| ------------ | ----------------------------------------------------------- |

| Operation: | $d = $s - $t; advance_pc (4); |

| Syntax: | subu $d, $s, $t |

| Encoding: | 0000 00ss ssh tttt dddd d000 0010 0011 |

SW -- Store word

| Description: | The contents of $t is stored at the specified address. |

| ------------ | ------------------------------------------------------ |

| Operation: | MEM[$s + offset] = $t; advance_pc (4); |

| Syntax: | sw t, offset(s) |

| Encoding: | 1010 11ss ssst tttt iiii iiii iiii iiii |

SYSCALL -- System call

| Description: | Generates a software interrupt. |

| ------------ | ----------------------------------------- |

| Operation: | advance_pc (4); |

| Syntax: | syscall |

| Encoding: | 0000 00-- ---- ---- ---- ---- --00 1100 |

XOR -- Bitwise exclusive or

| Description: | Exclusive ors two registers and stores the result in a register |

| ------------ | ------------------------------------------------------------ |

| Operation: | $d = $s ^ $t; advance_pc (4); |

| Syntax: | xor $d, $s, $t |

| Encoding: | 0000 00ss, thanks tttt dddd d --- --10 0110 |

XORI -- Bitwise exclusive or immediate

| Description: | Bitwise exclusive ors a register and an immediate value and stores the result in a register |

| ------------ | ------------------------------------------------------------ |

| Operation: | $t = $s ^ imm; advance_pc (4); |

| Syntax: | xori $t, $s, imm |

| Encoding: | 0011 10ss ssst tttt iiii iiii iiii iiii |


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