arm64 程序集：向前分支 n 个字节

Question

Hey so I'm trying to get into lower level systems programming in 64 bit arm, and I'm trying to branch forward 64 bytes, specifically to skip the Image header . I noticed that under arm64, PC is no longer an accessible register. How would I branch forward 64 bytes relative to the current position in arm64 assembly? I need it to fit in two or less instructions (code0 and code1). Thanks

Answer 1

something like this (give or take a word/byte to fine tune it)

b hello
.word 0,0,0,0,0,0,0,0
.word 0,0,0,0,0,0,0,0
.word 0,0,0,0,0,0,0,0
.word 0,0,0,0,0,0,0,0
hello:

Disassembly of section .text:

0000000000000000 <hello-0x84>:
   0:   14000021    b   84 <hello>
    ...

which then leads to

b hello
.word 0,0,0,0,0,0,0,0
.word 0,0,0,0,0,0,0,0
.word 0,0,0,0,0,0,0,0
.word 0,0,0,0,0,0,0,0
hello:
.inst 0x14000021

Disassembly of section .text:

0000000000000000 <hello-0x84>:
   0:   14000021    b   84 <hello>
    ...

0000000000000084 <hello>:
  84:   14000021    b   108 <hello+0x84>

okay so I cant count to 64 correctly...But you get the idea, I think I meant to use.byte not.word...

You either use labels or if you want to have a fixed offset use the machine code.

Some assemblers (assembly is defined by the assembler not the target) might support something like this:

hello:
b .+64

Disassembly of section .text:

0000000000000000 <hello>:
   0:   14000010    b   40 <hello+0x40>

But I would expect that to be extremely assembler (assembly language) specific and not port across aarch64 assemblers. (where some flavor of.word 0x14000010 would port)