Segfault running cmp 'A', %al‽

Question

For my own sick pleasure, I'm writing a small program in x86_64 assembly for Linux. However, I've encountered a segfault that makes absolutely no sense to me, in an instruction comparing an immediate operand to a register. What gives?

Here's the code leading up to the crash:

_start:
    sub $8, %rsp
    mov %rsp, %rbx
    lea le_string(%rip), %rsi
    mov %rsi, %rdi
    add $8, %rdi
    mov $26, %cl
    mov (%rsi), %al
    cmp 'A', %al /* This line segfaults */
    /* snip code that never runs */

le_string:
.ascii "YrFgevat"

I'm assembling with gcc -nostdlib, which is calling the GNU assembler.

Dumping the registers after the crash reveals:

• %rsi contains the expected pointer to the string
• %al contains the expected first character in the string
• %rip points to an instruction that doesn't touch memory

Please ignore the lack of normal calling conventions—I'm not calling out to anything besides the syscall interface, and this crashes before it's even gotten that far!

Answer 1

'A' is being interpreted as an address after all. If you want to use it as a constant, you need to write:

cmp $'A', %al