linux/100.txt



c09c91e1 ^


16f2bd11 ^


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# Example showing file syscalls.
#
# Create a file, open it for writing, write a character to it, close it, open
# it for reading, read a character from it, close it, delete it, and return
# the character read.
#
# To run:
#   $ bootstrap/bootstrap translate ex7.subx -o ex7
#   $ bootstrap/bootstrap run ex7
# Expected result:
#   $ echo $?
#   97

== 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:
    # creat(Filename)
    bb/copy-to-ebx  Filename/imm32
    b9/copy-to-ecx  0x180/imm32/fixed-perms
    e8/call  syscall_creat/disp32

    # stream = open(Filename, O_WRONLY, 0)  # we can't use 'fd' because it looks like a hex byte
    bb/copy-to-ebx  Filename/imm32
    b9/copy-to-ecx  1/imm32/wronly
    ba/copy-to-edx  0x180/imm32/fixed-perms
    e8/call  syscall_open/disp32
    # save stream
    bb/copy-to-ebx  Stream/imm32
    89/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to *ebx

    # write(Stream, "a", 1)
    # . load stream
    bb/copy-to-ebx  Stream/imm32
    8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    # .
    b9/copy-to-ecx  A/imm32
    ba/copy-to-edx  1/imm32/size
    e8/call  syscall_write/disp32

    # close(Stream)
    # . load stream
    bb/copy-to-ebx  Stream/imm32
    8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    # .
    e8/call  syscall_close/disp32

    # stream = open(Filename, O_RDONLY, 0)
    bb/copy-to-ebx  Filename/imm32
    b9/copy-to-ecx  0/imm32/rdonly
    ba/copy-to-edx  0x180/imm32/fixed-perms
    e8/call  syscall_open/disp32
    # . save Stream
    bb/copy-to-ebx  Stream/imm32
    89/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to *ebx

    # read(Stream, B, 1)
    # . load stream
    bb/copy-to-ebx  Stream/imm32
    8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    # .
    b9/copy-to-ecx  B/imm32
    ba/copy-to-edx  1/imm32/size
    e8/call  syscall_read/disp32

    # close(Stream)
    # . load stream
    bb/copy-to-ebx  Stream/imm32
    8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    #
    e8/call  syscall_close/disp32

    # unlink(filename)
    bb/copy-to-ebx  Filename/imm32
    e8/call  syscall_unlink/disp32

    # exit(b)
    # . load b
    bb/copy-to-ebx  B/imm32
    8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    #
    e8/call  syscall_exit/disp32

== data

Stream:
    0/imm32
A:
    61/imm32/A
B:
    0/imm32
Filename:
    2e 66 6f 6f 00 00 00 00
#   .  f  o  o  null

# . . vim:nowrap:textwidth=0