在ARM反汇编的一段中被TBB弄糊涂了 [英] Confused by TBB in a section of ARM disassembly

问题描述

作为学习 Arm 汇编的介绍，我正在尝试用更高级的语言重新创建反汇编的函数.但是，我对以下汇编感到困惑:

As an intro to learning Arm assembly, I'm trying to recreate disassembled functions in a higher level language. However I'm confused by the following bit of assembly:

0000315e        2101    movs    r1, #1
00003160    e8dff000    tbb [pc, r0]
00003164        030e    lsls    r6, r1, #12
00003166        0907    lsrs    r7, r0, #4
00003168        050b    lsls    r3, r1, #20
0000316a        2106    movs    r1, #6
0000316c        e008    b.n 0x3180
0000316e        2102    movs    r1, #2
00003170        e006    b.n 0x3180
00003172        2103    movs    r1, #3
00003174        e004    b.n 0x3180
00003176        2104    movs    r1, #4
00003178        e002    b.n 0x3180
0000317a        2105    movs    r1, #5
0000317c        e000    b.n 0x3180
0000317e        2100    movs    r1, #0
00003180        4608    mov r0, r1
00003182        4770    bx  lr

我相信这可能是某种 switch 语句，但我不确定它到底在做什么

I believe it may be some kind of switch statement but I'm unsure to what exactly it's doing

推荐答案

是的，那是一个开关.tbb代表Table Branch Byte，它需要一个字节偏移表，以pc为基数，r0，并使用它来做一个分支.

Yes, that is a switch. tbb stands for Table Branch Byte, it takes a table of byte-offsets, with base at pc, and index at r0, and uses that to do a branch.

所以:

0000315e        2101    movs    r1, #1           ; ret = default value
00003160    e8dff000    tbb [pc, r0]             ; switch (r0)

; jump table, byte-sized offsets
00003164        03 0e 09 07 05 0b

; case 1: (0x3164 + 0x3 * 2)
0000316a        2106    movs    r1, #6           ; ret = 6
0000316c        e008    b.n 0x3180               ; break

; case 5: (0x3164 + 0x5 * 2)
0000316e        2102    movs    r1, #2           ; ret = 2
00003170        e006    b.n 0x3180               ; break

; case 2: (0x3164 + 0x7 * 2)
00003172        2103    movs    r1, #3
00003174        e004    b.n 0x3180

; case 3: (0x3164 + 0x9 * 2)
00003176        2104    movs    r1, #4
00003178        e002    b.n 0x3180

; case 4: (0x3164 + 0xb * 2)
0000317a        2105    movs    r1, #5
0000317c        e000    b.n 0x3180

; default:
0000317e        2100    movs    r1, #0

; case 0: (0x3164 + 0xe * 2)
: end switch
00003180        4608    mov r0, r1        ; mov ret to r0 (return value)
00003182        4770    bx  lr            ; return

基本思路要清晰.

这篇关于在ARM反汇编的一段中被TBB弄糊涂了的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！