path: root/linux/ex5.subx

# Read a character from stdin, save it to a local on the stack, write it to stdout.
#
# To run:
#   $ bootstrap/bootstrap translate ex5.subx -o ex5
#   $ bootstrap/bootstrap run ex5

== code
#   instruction                     effective address                                                   register    displacement    immediate
# . op          subop               mod             rm32          base        index         scale       r32
# . 1-3 bytes   3 bits              2 bits          3 bits        3 bits      3 bits        2 bits      2 bits      0/1/2/4 bytes   0/1/2/4 bytes

Entry:

    # allocate x on the stack
    81          5/subop/subtract    3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # subtract from esp

    # read(stdin, x, 1)
    # . fd = 0 (stdin)
    bb/copy-to-ebx  0/imm32
    # . data = x (location to write result to)
    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none              1/r32/ecx   4/disp8         .                 # copy esp+4 to ecx
    # . size = 1 character
    ba/copy-to-edx  1/imm32
    # . syscall
    e8/call  syscall_read/disp32

    # syscall(write, stdout, x, 1)
    # . fd = 1 (stdout)
    bb/copy-to-ebx  1/imm32
    # . data = x (location to read from)
    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none              1/r32/ecx   4/disp8         .                 # copy esp+4 to ecx
    # . size = 1 character
    ba/copy-to-edx  1/imm32
    # . syscall
    e8/call  syscall_write/disp32

    # exit(ebx)
    e8/call  syscall_exit/disp32

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