bits.s

#!/usr/bin/env -S sh -c 'f="$1";shift;sed 1d "$f" | cc -no-pie -o "$f.elf" -xassembler - && ./"$f.elf" "$@"' -
.intel_syntax noprefix

__USER32_CS = 35
__USER_DS = 43
__USER_CS = 51

.global main
main:
    push rbp
    mov rbp, rsp

    mov ax, __USER_DS
    mov ds, ax

    lea rdi, [rip + s_main]
    lea rsi, [rip + main]
    call printf

    mov eax, 15  # ah = 00, al = 0fh

    mov [rip + save_rsp], rsp
    ljmp [rip + 1f]
1:
    .long offset 1f
    .word __USER32_CS
1:
.code32
    aaa          # ah = 01, al = 05 (ax = 105h, 261)
    ljmp [1f]
1:
    .long offset 2f
    .word __USER_CS
.code64
2:
    mov rsp, [rip + save_rsp]

    lea rdi, [rip + s_aaa]
    mov esi, eax
    call printf

    xor eax, eax
    pop rbp
    ret

s_main:
    .asciz "main: 0x%016lx\n"

s_aaa:
    .asciz "aaa: %d\n"

.bss
save_rsp: .zero 8

.section .note.GNU-stack,"",@progbits