CODEWITHSUNDEEP
assemblyarmarm64bare-metal
Sreehari
Sreehari

Reputation: 111

arm64 assembly: branch forward n bytes

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

Upvotes: 1

Views: 1299

Answers (1)

old_timer
old_timer

Reputation: 71536

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)

Upvotes: 4

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