bnez MIPS 指令的编码是什么? [英] what would be the encoding of a bnez MIPS instruction?

问题描述

如果 Loop 的地址是 0x00012344，最后一条指令的编码是什么?

循环:添加 $a0, $0, $t0原 $v0, $0, 4系统调用添加 $t0, $t0, -1bnez $t0, 循环

这最后一条指令相当于:

bne $t0, $0, 循环

但是，我将如何用机器代码编写它?

这就是我得到的:

000101 01000 00000 iiii iiii iiiiii

我不知道该怎么办，但因为我在这里.Loop 的地址是 0x00012344，但是它是一个 17 位的二进制数.我只有 16 位

解决方案

MIPS 中的分支使用偏移量(左移 2 个位置)，而不是绝对地址.

<块引用>

BNE -- 不等于分支

说明:

如果两个寄存器不相等则分支

操作:

if $s != $t advance_pc (offset <<2));else Advance_pc (4);

语法:

bne $s, $t, 抵消

编码:

0001 01ss ssst tttt iiii iiii iiii iiii

来源

所以 BNE 指令的 Loop 偏移量应该放在 i 部分，而不是它的绝对地址.

<小时>

EDIT:编辑问题代码:

循环:addu $a0, $0, $t0原 $v0, $0, 4系统调用添加 $t0, $t0, -1bnez $t0, 循环

由于所有 MIPS 指令都是 32 位宽(4 个字节)，我们可以轻松计算偏移量:

• Loop 地址与 BNEZ 指令相距 -4 条指令，因此相距 -4 x 4 = -16 字节.
• 但是我们需要再添加一个字，因为在 MIPS PC 已经增加了 4 并且当 BNEZ 执行时，PC 已经指向下一条指令(参见 MIPS 中的延迟槽).所以最终的偏移量是 -20 = -(0x14) = 0xFFFEC.
• 我们右移 2 个位置(除以 4)以获得字(而不是字节)的偏移量:0xFFFEC >> 2 = 0xFFFB

也可以直接按字数计算:

• Loop 地址是来自 BNEZ 指令的 -4 条指令
• 额外的 -1，因为 PC 已经递增:-5
• -5 = -(0x5) = 0xFFFB

所以我们得到了 i 部分的 0xFFFB；所以整个指令是0x1500FFFB.

顺便说一下，您可以使用任何 MIPS 汇编器或 模拟器 来检查这一点.>

(火星模拟器截图)

What would be the encoding of this last instruction if the address of the Loop is 0x00012344?

Loop:
    addu $a0, $0, $t0
    ori $v0, $0, 4
    syscall
    addi $t0, $t0, -1
    bnez $t0, Loop

this last instruction is equivalent to:

bne $t0, $0, Loop

however, How would I write this in machine code?

this is how for I got:

000101 01000 00000 iiiii iiiii iiiiii

I dont know what to but for the i's here. The address of Loop is 0x00012344 however that is a 17 bit binary number. I only have 16 bits

解决方案

Branch in MIPS works with offsets (shifted left 2 positions), not absolute addresses.

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

Source

So it's Loop offset from the BNE instruction that should go on the i part, not its absolute address.

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EDIT: with edited question code:

Loop:
    addu $a0, $0, $t0
    ori $v0, $0, 4
    syscall
    addi $t0, $t0, -1
    bnez $t0, Loop

Since all MIPS instructions are 32-bit wide (4 bytes), we can calculate offset easily:

• Loop address is -4 instructions from BNEZ instruction, so it's -4 x 4 = -16 bytes away.
• But we need to add one more word because in MIPS PC was already incremented by 4 and when BNEZ is executing, PC is already pointing to next instruction (see delay slot in MIPS). So final offset is -20 = -(0x14) = 0xFFFEC.
• We shift right 2 positions (divide by 4) to get this offset in words (instead of bytes): 0xFFFEC >> 2 = 0xFFFB

You can also do it by counting by words directly:

• Loop address is -4 instructions from BNEZ instruction
• Additional -1 because PC already incremented: -5
• -5 = -(0x5) = 0xFFFB

So we got 0xFFFB for i part; so whole instruction is 0x1500FFFB.

By the way you can check this with any MIPS assembler or simulator.

(Screenshot from MARS simulator)

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