Unable to set the lowest byte in a register x86-64?

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I'm writing a function in x86-64 to convert a 1-byte value into a hexadecimal string representing the ASCII code for that byte. At the start of my function, I try to use

movb %dil, %r11b

to store the 1-byte value in the lowest byte of register %r11. However, when I examine this in gdb, %r11b is never set. Instead, the higher bytes of %r11 are getting set. This is what I get when using gdb:

Breakpoint 1, 0x00000000004011f0 in byte_as_hex ()
(gdb) print /x $r11b
$1 = 0x0
(gdb) print /x $r11
$2 = 0x246
(gdb) print /x $rdi
$3 = 0x48
(gdb) print /x $dil
$4 = 0x48
(gdb) stepi  /* subq $8, %rsp */
0x00000000004011f4 in byte_as_hex ()
(gdb) print /x $r11b
$5 = 0x0
(gdb) print /x $r11
$6 = 0x246
(gdb) print /x $rdi
$7 = 0x48
(gdb) print /x $dil
$8 = 0x48
(gdb) stepi /* movb %dil, %r11b */
0x00000000004011f7 in byte_as_hex ()
(gdb) print /x $r11b
$9 = 0x0
(gdb) print /x $r11
$10 = 0x248
(gdb) print /x $rdi
$11 = 0x48
(gdb) print /x $dil
$12 = 0x48
(gdb) print /x $r11d
$13 = 0x248
(gdb) print /x $r11w
$14 = 0x248
(gdb) print /x $r11b
$15 = 0x0

I'm very confused because I specifically tried to movb from %dil into %r11b, but I still can't set the byte. Could anyone explain to me why this is this happening? Thanks!

x86-64 cpu-registers gdb
1 Answers

Multiple problems are at play here:

  1. (Reported as GDB bug.) An undefined convenience variable (a GDB-local variable that starts with $), when printed with an explicit format specifier, is shown as 0 instead of the default void, which is displayed when format is not specified:
$ gdb /bin/true
Reading symbols from /bin/true...
(gdb) p $asdf
$1 = void          <------ undefined, OK
(gdb) p/x $asdf
$2 = 0x0           <------ the problem
(gdb) set $asdf=4345
(gdb) p $asdf
$3 = 4345
(gdb) p/x $asdf
$4 = 0x10f9
(gdb)
  1. Register values have the same syntax as the values of convenience variables. Thus, when you mistake the name of a register, e.g. use r11b instead of GDB's r11l, you refer to a(n undefined) convenience variable. Moreover, even if you simply use the correct name in incorrect case, like R11L, you bump into this too.
  2. GDB uses its own set of names for x86(_64) registers. Sometimes they differ from the names given e.g. in Intel manuals (e.g. ftag instead of Intel's FTW). In any case, the lowest bytes of the general purpose registers have the following names in GDB:
al
cl
dl
bl
spl
bpl
sil
dil
r8l
...
r15l

There are no aliases for them like e.g. r11b for r11l, so one must use the correct names.

user_6738520
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