path: root/052kernel-string-equal.subx

# Checking null-terminated ascii strings.
#
# By default we create strings as arrays of bytes, and all arrays have a 4-byte
# length prefix.
#
# However, we sometimes need to deal with null-prefixed strings when interacting
# with the Linux kernel. This layer implements a function for comparing a
# null-terminated 'kernel string' with a length-prefixed 'SubX string'.
#
# To run (from the subx directory):
#   $ ./bootstrap translate 05[0-2]*.subx -o /tmp/tmp52
#   $ ./bootstrap run /tmp/tmp52  # runs a series of tests
#   ......  # all tests pass
#
# (We can't yet run the tests when given a "test" commandline argument,
# because checking for it would require the function being tested! Breakage
# would cause tests to not run, rather than to fail as we'd like.)

== 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:  # run all tests
    e8/call  run-tests/disp32  # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
    # syscall(exit, Num-test-failures)
    8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           3/r32/ebx   Num-test-failures/disp32          # copy *Num-test-failures to ebx
    b8/copy-to-eax  1/imm32/exit
    cd/syscall  0x80/imm8

# compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
# reason for the name: the only place we should have null-terminated ascii strings is from commandline args
kernel-string-equal?:  # s: (addr kernel-string), benchmark: (addr array byte) -> eax: boolean
    # pseudocode:
    #   n = benchmark->length
    #   s1 = s
    #   s2 = benchmark->data
    #   i = 0
    #   while (i < n)
    #     c1 = *s1
    #     c2 = *s2
    #     if (c1 == 0) return false
    #     if (c1 != c2) return false
    #     ++s1, ++s2, ++i
    #   return *s1 == 0
    #
    # registers:
    #   i: ecx
    #   n: edx
    #   s1: edi
    #   s2: esi
    #   c1: eax
    #   c2: ebx
    #
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    51/push-ecx
    52/push-edx
    53/push-ebx
    56/push-esi
    57/push-edi
    # var s1/edi: (addr byte) = s
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           7/r32/edi   8/disp8         .                 # copy *(ebp+8) to edi
    # var n/edx: int = benchmark->length
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           2/r32/edx   0xc/disp8       .                 # copy *(ebp+12) to edx
    8b/copy                         0/mod/indirect  2/rm32/edx    .           .             .           2/r32/edx   .               .                 # copy *edx to edx
    # var s2/esi: (addr byte) = benchmark->data
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           6/r32/esi   0xc/disp8       .                 # copy *(ebp+12) to esi
    81          0/subop/add         3/mod/direct    6/rm32/esi    .           .             .           .           .               4/imm32           # add to esi
    # var i/ecx: int = 0
    b9/copy-to-ecx  0/imm32/exit
    # var c1/eax: byte = 0
    b8/copy-to-eax  0/imm32
    # var c2/ebx: byte = 0
    bb/copy-to-ebx  0/imm32
$kernel-string-equal?:loop:
    # if (i >= n) break
    39/compare                      3/mod/direct    1/rm32/ecx    .           .             .           2/r32/edx   .               .                 # compare ecx with edx
    7d/jump-if->=  $kernel-string-equal?:break/disp8
    # c1 = *s1
    8a/copy-byte                    0/mod/indirect  7/rm32/edi    .           .             .           0/r32/AL    .               .                 # copy byte at *edi to AL
    # c2 = *s2
    8a/copy-byte                    0/mod/indirect  6/rm32/esi    .           .             .           3/r32/BL    .               .                 # copy byte at *esi to BL
    # if (c1 == 0) return false
    3d/compare-eax-and  0/imm32/null
    74/jump-if-=  $kernel-string-equal?:false/disp8
    # if (c1 != c2) return false
    39/compare                      3/mod/direct    0/rm32/eax    .           .             .           3/r32/ebx   .               .                 # compare eax and ebx
    75/jump-if-!=  $kernel-string-equal?:false/disp8
    # ++i
    41/increment-ecx
    # ++s1
    47/increment-edi
    # ++s2
    46/increment-esi
    eb/jump  $kernel-string-equal?:loop/disp8
$kernel-string-equal?:break:
    # return *s1 == 0
    8a/copy-byte                    0/mod/indirect  7/rm32/edi    .           .             .           0/r32/AL    .               .                 # copy byte at *edi to AL
    3d/compare-eax-and  0/imm32/null
    75/jump-if-!=  $kernel-string-equal?:false/disp8
$kernel-string-equal?:true:
    b8/copy-to-eax  1/imm32
    eb/jump  $kernel-string-equal?:end/disp8
$kernel-string-equal?:false:
    b8/copy-to-eax  0/imm32
$kernel-string-equal?:end:
    # . restore registers
    5f/pop-to-edi
    5e/pop-to-esi
    5b/pop-to-ebx
    5a/pop-to-edx
    59/pop-to-ecx
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

# - tests

test-compare-null-kernel-string-with-empty-array:
    # eax = kernel-string-equal?(Null-kernel-string, "")
    # . . push args
    68/push  ""/imm32
    68/push  Null-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-ints-equal(eax, 1, msg)
    # . . push args
    68/push  "F - test-compare-null-kernel-string-with-empty-array"/imm32
    68/push  1/imm32/true
    50/push-eax
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    c3/return

test-compare-null-kernel-string-with-non-empty-array:
    # eax = kernel-string-equal?(Null-kernel-string, "Abc")
    # . . push args
    68/push  "Abc"/imm32
    68/push  Null-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-ints-equal(eax, 0, msg)
    # . . push args
    68/push  "F - test-compare-null-kernel-string-with-non-empty-array"/imm32
    68/push  0/imm32/false
    50/push-eax
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    c3/return

test-compare-kernel-string-with-equal-array:
    # eax = kernel-string-equal?(_test-Abc-kernel-string, "Abc")
    # . . push args
    68/push  "Abc"/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-ints-equal(eax, 1, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
    68/push  1/imm32/true
    50/push-eax
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    c3/return

test-compare-kernel-string-with-inequal-array:
    # eax = kernel-string-equal?(_test-Abc-kernel-string, "Adc")
    # . . push args
    68/push  "Adc"/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-ints-equal(eax, 0, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
    68/push  0/imm32/false
    50/push-eax
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    c3/return

test-compare-kernel-string-with-empty-array:
    # eax = kernel-string-equal?(_test-Abc-kernel-string, "")
    # . . push args
    68/push  ""/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-ints-equal(eax, 0, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
    68/push  0/imm32/false
    50/push-eax
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    c3/return

test-compare-kernel-string-with-shorter-array:
    # eax = kernel-string-equal?(_test-Abc-kernel-string, "Ab")
    # . . push args
    68/push  "Ab"/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-ints-equal(eax, 0, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-shorter-array"/imm32
    68/push  0/imm32/false
    50/push-eax
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    c3/return

test-compare-kernel-string-with-longer-array:
    # eax = kernel-string-equal?(_test-Abc-kernel-string, "Abcd")
    # . . push args
    68/push  "Abcd"/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-ints-equal(eax, 0, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-longer-array"/imm32
    68/push  0/imm32/false
    50/push-eax
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    c3/return

== data

Null-kernel-string:  # (addr kernel-string)
    00/null

_test-Abc-kernel-string:  # (addr kernel-string)
    41/A 62/b 63/c 00/null

# . . vim:nowrap:textwidth=0