MIPS 基础内容?
这里以 MIPS32 为基础作为介绍内容。
0x01 寄存器?
（1）通用寄存器?
MIPS 有 32 个通用寄存器 （General purpose registers），以美元符号 （$） 表示。可以表示为 $0~$31，也可以用寄存器名称表示如，$sp 、 $t9 、$fp 等等。
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 | 保存表达式或者函数返回值 |
$4 - $7 | $a0 - $a3 | Arguments to functions - not preserved by subprograms | 函数的参数 |
$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 | 临时寄存器，作为 $t0 - t7的补充，t7的补充，t9 通常与调用函数有关 |
$26 - $27 | $k0 - $k1 | Reserved for kernel. Do not use. | |
$28 | $gp | Global Area Pointer (base of global data segment) |
MIPS 作为一种 load-store 架构，意味着当我们要访问内存必须要通过加载和存储指令才能访问。所有其他指令（add，sub，mul，div 等等）必须从寄存器获取其操作数并将其结果存储在寄存器中。例如下面的例子：
sum = x + y
我们假设 sum 和 x , y 是程序中的变量，其 MIPS 汇编表示为：
# 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）特殊寄存器?
MIPS32 架构还定义了 3 个特殊寄存器，分别是 PC（程序计数器）、HI （乘除结果高位寄存器）和 LO （乘除结果低位寄存器）。在进行乘运算的时候，HI 和 LO 保存乘法的结果，其中 HI 保存高 32 位，LO 保存低 32 位。而在除法运算中，HI 保存余数，LO 保存商。
0x2 指令 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 ssst 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: | addiu $t, $s, imm |
Encoding: | 0010 01ss ssst 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 ssst 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 ssst 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: | 0000 01ss sss0 0001 iiii iiii iiii 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: | 0001 11ss sss0 0000 iiii iiii iiii 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: | 0000 01ss sss0 0000 iiii iiii iiii 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 ssst 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 ssst tttt 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 ssst tttt 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[t=MEM[s + offset]; advance_pc (4); |
Syntax: | lb t,offset(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: | lui $t, imm |
Encoding: | 0011 11-- ---t tttt iiii iiii iiii iiii |
LW -- Load word
Description: | A word is loaded into a register from the specified address. |
Operation: | t=MEM[t=MEM[s + offset]; advance_pc (4); |
Syntax: | lw t,offset(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 ssst tttt 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 ssst tttt 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 ssst 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 ssst 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(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 < |
Syntax: | sll $d, $t, h |
Encoding: | 0000 00ss ssst tttt 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 ssst 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 ssst 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 |
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 |
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 ssst 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 ssst 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 ssst 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 ssst 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(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 ssst 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 |