x86 64 - How do you access low-byte registers for r8-r15 from gdb in x86-64?

Question

In gdb, I can't seem to access any of the pseudo-registers: r8b, r9b, r10b, r11b, r12b, r13b, r14b, nor r15b (however, r15d and r15w seem to work, and same for sil).

See an example:

section .text
global main

main:
  xor esi, esi
  mov sil, 0x1f
  xor r13d, r13d
  mov r13b, sil
  ret

Running with gdb:

(gdb) p $sil
$1 = -15
(gdb) p $r13 
$2 = 241
(gdb) p $r13b
$3 = void
(gdb) p /x $r13b
$4 = 0x0

I couldn't find anything in the gdb manual, and they aren't printed with the info all-registers command. I'm using GDB 10.

Answer 1

You can reference to the low byte of those registers using the l suffix:

r8l, r9l, r10l, r11l, r12l, r13l, r14l, r15l.

(gdb) p $r13l
$1 = -15
(gdb) p /x $r13l
$2 = 0xf1

I couldn't find these aliases documented anywhere, except in their definition in the gdb codebase:

/* Register names for byte pseudo-registers.  */

static const char * const amd64_byte_names[] =
{
  "al", "bl", "cl", "dl", "sil", "dil", "bpl", "spl",
  "r8l", "r9l", "r10l", "r11l", "r12l", "r13l", "r14l", "r15l",
  "ah", "bh", "ch", "dh"
};

Furthermore, there was a bug in the gdb codebase that would print 0 instead of void if a format is specified to the print command, as it happens in the last example. Future versions of gdb should display void if the value that you are printing is not defined.