al vs. ah

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cryo1970

Joined: 21 Oct 2014
Posts: 2

cryo1970 21 Oct 2014, 07:31

I am new to assembly. I have searched the Internet but cannot find an answer to this question:

Why does

Code:

format ELF64 executable
entry start

start:

  mov  r13, test_byte
  mov  al, [r13]

  mov  rax, 60
  xor  rdi, rdi
  syscall

test_byte  db  'a'

assemble (with fasm test.asm test) and run fine, while

Code:

format ELF64 executable
entry start

start:

  mov  r13, test_byte
  mov  ah, [r13]

  mov  rax, 60
  xor  rdi, rdi
  syscall

test_byte  db  'a'

fails with following error:
disallowed combination of registers.

???

If I use for example rbx instead of r13 there is no problem moving the byte pointed to into both al and ah. What is the difference? Please enlighten me on this....

21 Oct 2014, 07:31

tthsqe

Joined: 20 May 2009
Posts: 767

tthsqe 21 Oct 2014, 08:25

the error reported by fasm is accurate. that combinary of regiseter and mod/rm is not allowed. see the intel manuals, the one I am looking at has a table called

Table 3-1. Register Codes Associated With +rb, +rw, +rd, +ro

21 Oct 2014, 08:25

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20761
Location: In your JS exploiting you and your system

revolution 21 Oct 2014, 08:55

The registers AH, BH CH and DH cannot be used in all places in 64-bit mode. There are encoding restrictions in the instruction set. It is mostly a legacy thing concerning the history and evolution of the x86 instructions over time.

For the same reason SIL, DIL, BPL and SPL cannot be used in 32-bit mode.

21 Oct 2014, 08:55

cryo1970

Joined: 21 Oct 2014
Posts: 2

cryo1970 21 Oct 2014, 09:46

Thank you for both answers. I still have a LOT to learn. I shall do my homework and check with the Intel manual, when I get back home.

Have a nice day.

21 Oct 2014, 09:46

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