path: root/120allocate.subx

# Helper to dynamically allocate memory on the heap.
#
# We'd like to be able to write tests for functions that allocate memory,
# making assertions on the precise addresses used. To achieve this we'll pass
# in an *allocation descriptor* to allocate from.
#
# Allocation descriptors are also useful outside of tests. Assembly and machine
# code are of necessity unsafe languages, and one of the most insidious kinds
# of bugs unsafe languages expose us to are dangling pointers to memory that
# has been freed and potentially even reused for something totally different.
# To reduce the odds of such "use after free" errors, SubX programs tend to not
# reclaim and reuse dynamically allocated memory. (Running out of memory is far
# easier to debug.) Long-running programs that want to reuse memory are mostly
# on their own to be careful. However, they do get one bit of help: they can
# carve out chunks of memory and then allocate from them manually using this
# very same 'allocate' helper. They just need a new allocation descriptor for
# their book-keeping.

== data

# Allocations are returned in a handle, which consists of an alloc-id and a payload.
# The alloc-id helps detect use-after-free errors.
Handle-size:  # (addr int)
  8/imm32

# A default allocation descriptor for programs to use.
Heap:  # allocation-descriptor
  # curr
  0x01000000/imm32  # 16 MB
  # limit
  0x02000000/imm32  # 32 MB

Next-alloc-id:  # int
  0x100/imm32  # save a few alloc ids for fake handles

== 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

# Allocate and clear 'n' bytes of memory from an allocation-descriptor 'ad'.
# Abort if there isn't enough memory in 'ad'.
allocate:  # ad: (addr allocation-descriptor), n: int, out: (addr handle _)
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    50/push-eax
    # allocate-raw(ad, n, out)
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0x10/disp8      .                 # push *(ebp+16)
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  allocate-raw/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # eax = out->payload + 4
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0x10/disp8      .                 # copy *(ebp+16) to eax
    8b/copy                         1/mod/*+disp8   0/rm32/eax    .           .             .           0/r32/eax   4/disp8         .                 # copy *(eax+4) to eax
    05/add-to-eax  4/imm32
    # zero-out(eax, n)
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
    50/push-eax
    # . . call
    e8/call  zero-out/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
$allocate:end:
    # . restore registers
    58/pop-to-eax
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

# Claim the next 'n' bytes of memory starting at ad->curr and update ad->curr.
# Abort if there isn't enough memory in 'ad'.
allocate-raw:  # ad: (addr allocation-descriptor), n: int, out: (addr handle _)
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    50/push-eax
    51/push-ecx
    52/push-edx
    53/push-ebx
    56/push-esi
    57/push-edi
    # ecx = ad
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   8/disp8         .                 # copy *(ebp+8) to ecx
    # edx = out
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           2/r32/edx   0x10/disp8      .                 # copy *(ebp+16) to edx
    # ebx = n
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           3/r32/ebx   0xc/disp8       .                 # copy *(ebp+12) to ebx
    # out->alloc-id = Next-alloc-id
    8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           0/r32/eax   Next-alloc-id/disp32              # copy *Next-alloc-id to eax
    89/copy                         0/mod/indirect  2/rm32/edx    .           .             .           0/r32/eax   .               .                 # copy eax to *edx
    # out->payload = ad->curr
    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy *ecx to eax
$allocate-raw:save-payload-in-eax:
    89/copy                         1/mod/*+disp8   2/rm32/edx    .           .             .           0/r32/eax   4/disp8         .                 # copy eax to *(edx+4)
    # *out->payload = Next-alloc-id
    8b/copy                         1/mod/*+disp8   2/rm32/edx    .           .             .           7/r32/edi   4/disp8         .                 # copy *(edx+4) to edi
    8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           6/r32/esi   Next-alloc-id/disp32              # copy *Next-alloc-id to esi
    89/copy                         0/mod/indirect  7/rm32/edi    .           .             .           6/r32/esi   .               .                 # copy esi to *edi
$allocate-raw:increment-next-alloc-id:
    # increment *Next-alloc-id
    ff          0/subop/increment   0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32              # increment *Next-alloc-id
    # check if there's enough space
    # TODO: move this check up before any state updates when we support error recovery
    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    0/base/eax  3/index/ebx   .           0/r32/eax   4/disp8         .                 # copy eax+ebx+4 to eax
    3b/compare                      1/mod/*+disp8   1/rm32/ecx    .           .             .           0/r32/eax   4/disp8         .                 # compare eax with *(ecx+4)
    73/jump-if->=-signed  $allocate-raw:abort/disp8
$allocate-raw:commit:
    # ad->curr += n+4
    89/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy eax to *ecx
$allocate-raw:end:
    # . restore registers
    5f/pop-to-edi
    5e/pop-to-esi
    5b/pop-to-ebx
    5a/pop-to-edx
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

$allocate-raw:abort:
    (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 "allocate: failed" 3 0)  # 3=cyan
    {
      eb/jump loop/disp8
    }
    # never gets here

test-allocate-raw-success:
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # var ad/ecx: allocation-descriptor containing 16 bytes
    # . var end/ecx: (addr byte)
    89/<- %ecx 4/r32/esp
    # . var start/edx: (addr byte) = end - 16
    81 5/subop/subtract %esp 0x10/imm32
    89/<- %edx 4/r32/esp
    # . ad = {start, end}
    51/push-ecx
    52/push-edx
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
    # var expected-payload/ebx = ad->curr
    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           3/r32/ebx   .               .                 # copy *ecx to ebx
    # var h/edx: handle = {0, 0}
    68/push  0/imm32
    68/push  0/imm32
    89/copy                         3/mod/direct    2/rm32/edx    .           .             .           4/r32/esp   .               .                 # copy esp to edx
    # *Next-alloc-id = 0x34
    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32  0x34/imm32  # copy to *Next-alloc-id
    # allocate-raw(ad, 3, h)
    # . . push args
    52/push-edx
    68/push  3/imm32
    51/push-ecx
    # . . call
    e8/call  allocate-raw/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->alloc-id, 0x34, msg)
    # . . push args
    68/push  "F - test-allocate-raw-success: sets alloc-id in handle"/imm32
    68/push  0x34/imm32
    ff          6/subop/push        0/mod/indirect  2/rm32/edx    .           .             .           .           .               .                 # push *edx
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->payload, expected-payload, msg)
    # . . push args
    68/push  "F - test-allocate-raw-success: sets payload in handle"/imm32
    53/push-ebx
    ff          6/subop/push        1/mod/*+disp8   2/rm32/edx    .           .             .           .           4/disp8         .                 # push *(edx+4)
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->payload->alloc-id, 0x34, msg)
    # . . push args
    68/push  "F - test-allocate-raw-success: sets alloc-id in payload"/imm32
    68/push  0x34/imm32
    ff          6/subop/push        0/mod/indirect  3/rm32/ebx    .           .             .           .           .               .                 # push *ebx
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(*Next-alloc-id, 0x35, msg)
    # . . push args
    68/push  "F - test-allocate-raw-success: increments Next-alloc-id"/imm32
    68/push  0x35/imm32
    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32              # push *Next-alloc-id
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(ad->curr - expected-payload, 3 + 4 for alloc-id, msg)
    # . . push args
    68/push  "F - test-allocate-raw-success: updates allocation descriptor"/imm32
    68/push  7/imm32
    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy *ecx to eax
    29/subtract                     3/mod/direct    0/rm32/eax    .           .             .           3/r32/ebx   .               .                 # subtract ebx from eax
    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
    # clean up
    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32  0x100/imm32 # copy to *Next-alloc-id
    # . reclaim locals
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0x20/imm32        # add to esp
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

lookup:  # h: (handle _T) -> result/eax: (addr _T)
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    51/push-ecx
    # eax = 0
    31/xor                          3/mod/direct    0/rm32/eax    .           .             .           0/r32/eax   .               .                 # clear eax
    # ecx = handle->alloc_id
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   8/disp8         .                 # copy *(ebp+8) to ecx
    # if (ecx == 0) return 0
    81          7/subop/compare     3/mod/direct    1/rm32/ecx    .           .             .           .           .               0/imm32           # compare ecx
    74/jump-if-=  $lookup:end/disp8
    # eax = handle->address (payload)
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0xc/disp8       .                 # copy *(ebp+12) to eax
    # if (ecx != *eax) abort
    39/compare                      0/mod/indirect  0/rm32/eax    .           .             .           1/r32/ecx   .               .                 # compare *eax and ecx
    75/jump-if-!=  $lookup:abort/disp8
    # add 4 to eax
    05/add-to-eax  4/imm32
$lookup:end:
    # . restore registers
    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

$lookup:abort:
    (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 "lookup: failed" 3 0)  # 3=cyan
    {
      eb/jump loop/disp8
    }
    # never gets here

test-lookup-success:
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # var ad/ebx: allocation-descriptor containing 16 bytes
    # . var end/ecx: (addr byte)
    89/<- %ecx 4/r32/esp
    # . var start/edx: (addr byte) = end - 16
    81 5/subop/subtract %esp 0x10/imm32
    89/<- %edx 4/r32/esp
    # . ad = {start, end}
    51/push-ecx
    52/push-edx
    89/copy                         3/mod/direct    3/rm32/ebx    .           .             .           4/r32/esp   .               .                 # copy esp to ebx
    # var handle/ecx: handle
    68/push  0/imm32/address
    68/push  0/imm32/alloc-id
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
    # var old-top/edx = ad->curr
    8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           2/r32/edx   .               .                 # copy *ebx to edx
    # allocate-raw(ad, 2, handle)
    # . . push args
    51/push-ecx
    68/push  2/imm32/size
    53/push-ebx
    # . . call
    e8/call  allocate-raw/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # eax = lookup(handle)
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   1/rm32/ecx    .           .             .           .           4/disp8         .                 # push *(ecx+4)
    ff          6/subop/push        0/mod/indirect  1/rm32/ecx    .           .             .           .           .               .                 # push *ecx
    # . . call
    e8/call  lookup/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # eax contains old top of heap, except skipping the alloc-id in the payload
    # . check-ints-equal(eax, old-top+4, msg)
    # . . push args
    68/push  "F - test-lookup-success"/imm32
    81          0/subop/add         3/mod/direct    2/rm32/edx    .           .             .           .           .               4/imm32           # add to edx
    52/push-edx
    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
    # clean up
    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32  0x100/imm32 # copy to *Next-alloc-id
    # . reclaim locals
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0x20/imm32        # add to esp
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

test-lookup-null-returns-null:
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # var handle/ecx: handle
    68/push  0/imm32/address
    68/push  0/imm32/alloc-id
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
    # eax = lookup(handle)
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   1/rm32/ecx    .           .             .           .           4/disp8         .                 # push *(ecx+4)
    ff          6/subop/push        0/mod/indirect  1/rm32/ecx    .           .             .           .           .               .                 # push *ecx
    # . . call
    e8/call  lookup/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-lookup-null-returns-null"/imm32
    68/push  0/imm32
    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
    # . reclaim locals
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

_pending-test-lookup-failure:
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # var ad/ecx: allocation-descriptor containing 16 bytes
    # . var end/ecx: (addr byte)
    89/<- %ecx 4/r32/esp
    # . var start/edx: (addr byte) = end - 16
    81 5/subop/subtract %esp 0x10/imm32
    89/<- %edx 4/r32/esp
    # . ad = {start, end}
    51/push-ecx
    52/push-edx
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
    # var h1/ecx: handle
    68/push  0/imm32/address
    68/push  0/imm32/alloc-id
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
    # var old_top/ebx = ad->curr
    8b/copy                         0/mod/indirect  6/rm32/esi    .           .             .           3/r32/ebx   .               .                 # copy *esi to ebx
    # first allocation, to h1
    # . allocate(ad, 2, h1)
    # . . push args
    51/push-ecx
    68/push  2/imm32/size
    56/push-esi
    # . . call
    e8/call  allocate/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # reset ad->curr to mimic reclamation
    89/copy                         0/mod/indirect  6/rm32/esi    .           .             .           3/r32/ebx   .               .                 # copy ebx to *esi
    # second allocation that returns the same address as the first
    # var h2/edx: handle
    68/push  0/imm32/address
    68/push  0/imm32/alloc-id
    89/copy                         3/mod/direct    2/rm32/edx    .           .             .           4/r32/esp   .               .                 # copy esp to edx
    # . allocate(ad, 2, h2)
    # . . push args
    52/push-edx
    68/push  2/imm32/size
    56/push-esi
    # . . call
    e8/call  allocate/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h1->address, h2->address, msg)
    # . . push args
    68/push  "F - test-lookup-failure"/imm32
    ff          6/subop/push        1/mod/*+disp8   2/rm32/ecx    .           .             .           .           4/disp8         .                 # push *(edx+4)
    ff          6/subop/push        1/mod/*+disp8   1/rm32/ecx    .           .             .           .           4/disp8         .                 # push *(ecx+4)
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # lookup(h1) should crash
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   1/rm32/ecx    .           .             .           .           4/disp8         .                 # push *(ecx+4)
    ff          6/subop/push        0/mod/indirect  1/rm32/ecx    .           .             .           .           .               .                 # push *ecx
    # . . call
    e8/call  lookup/disp32
    # should never get past this point
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # clean up
    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32  0x100/imm32 # copy to *Next-alloc-id
    # . reclaim locals
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0x20/imm32        # add to esp
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

# when comparing handles, just treat them as pure values
handle-equal?:  # a: (handle _T), b: (handle _T) -> result/eax: boolean
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    51/push-ecx
    # eax = false
    b8/copy-to-eax  0/imm32/false
$handle-equal?:compare-alloc-id:
    # ecx = a->alloc_id
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   8/disp8         .                 # copy *(ebp+8) to ecx
    # if (ecx != b->alloc_id) return false
    39/compare                      1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0x10/disp8      .                 # compare ecx and *(ebp+16)
    75/jump-if-!=  $handle-equal?:end/disp8
$handle-equal?:compare-address:
    # ecx = handle->address
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0xc/disp8       .                 # copy *(ebp+12) to ecx
    # if (ecx != b->address) return false
    39/compare                      1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0x14/disp8      .                 # compare ecx and *(ebp+20)
    75/jump-if-!=  $handle-equal?:end/disp8
$handle-equal?:return-true:
    # return true
    b8/copy-to-eax  1/imm32/true
$handle-equal?:end:
    # . restore registers
    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

copy-handle:  # src: (handle _T), dest: (addr handle _T)
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    50/push-eax
    51/push-ecx
    # ecx = dest
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0x10/disp8      .                 # copy *(ebp+16) to ecx
    # *dest = src
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   8/disp8         .                 # copy *(ebp+8) to eax
    89/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy eax to *ecx
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0xc/disp8       .                 # copy *(ebp+12) to eax
    89/copy                         1/mod/*+disp8   1/rm32/ecx    .           .             .           0/r32/eax   4/disp8         .                 # copy eax to *(ecx+4)
$copy-handle:end:
    # . restore registers
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

# helper: create a nested allocation descriptor (useful for tests)
allocate-region:  # ad: (addr allocation-descriptor), n: int, out: (addr handle allocation-descriptor)
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    50/push-eax
    51/push-ecx
    # allocate(ad, n, out)
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0x10/disp8      .                 # push *(ebp+16)
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  allocate/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # eax = out->payload
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0x10/disp8      .                 # copy *(ebp+16) to eax
    8b/copy                         1/mod/*+disp8   0/rm32/eax    .           .             .           0/r32/eax   4/disp8         .                 # copy *(eax+4) to eax
    # skip payload->allocid
    05/add-to-eax  4/imm32
    # if (eax == 0) abort
    3d/compare-eax-and  0/imm32
    74/jump-if-=  $allocate-region:abort/disp8
    # earmark 8 bytes at the start for a new allocation descriptor
    # . *eax = eax + 8
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy eax to ecx
    81          0/subop/add         3/mod/direct    1/rm32/ecx    .           .             .           .           .               8/imm32           # add to ecx
    89/copy                         0/mod/indirect  0/rm32/eax    .           .             .           1/r32/ecx   .               .                 # copy ecx to *eax
    # . *(eax+4) = eax + n
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy eax to ecx
    03/add                          1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0xc/disp8       .                 # add *(ebp+12) to ecx
    89/copy                         1/mod/*+disp8   0/rm32/eax    .           .             .           1/r32/ecx   4/disp8         .                 # copy ecx to *(eax+4)
    # . restore registers
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

# We could create a more general '$abort' jump target, but then we'd need to do
# a conditional jump followed by loading the error message and an unconditional
# jump. Or we'd need to unconditionally load the error message before a
# conditional jump, even if it's unused the vast majority of the time. This way
# we bloat a potentially cold segment in RAM so we can abort with a single
# instruction.
$allocate-region:abort:
    (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 "allocate-region: failed to allocate" 3 0)  # 3=cyan
    {
      eb/jump loop/disp8
    }
    # never gets here

# Claim the next 'n+4' bytes of memory and initialize the first 4 to n.
# Abort if there isn't enough memory in 'ad'.
allocate-array:  # ad: (addr allocation-descriptor), n: int, out: (addr handle _)
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    50/push-eax
    51/push-ecx
    52/push-edx
    # ecx = n
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0xc/disp8       .                 # copy *(ebp+12) to ecx
    # var size/edx: int = n+4
    8d/copy-address                 1/mod/*+disp8   1/rm32/ecx    .           .             .           2/r32/edx   4/disp8         .                 # copy ecx+4 to edx
    # allocate(ad, size, out)
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0x10/disp8      .                 # push *(ebp+16)
    52/push-edx
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  allocate/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # *out->payload = n
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0x10/disp8      .                 # copy *(ebp+16) to eax
    8b/copy                         1/mod/*+disp8   0/rm32/eax    .           .             .           0/r32/eax   4/disp8         .                 # copy *(eax+4) to eax
    # . skip payload->allocid
    05/add-to-eax  4/imm32
    # .
    89/copy                         0/mod/indirect  0/rm32/eax    .           .             .           1/r32/ecx   .               .                 # copy ecx to *eax
$allocate-array:end:
    # . restore registers
    5a/pop-to-edx
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

test-allocate-array:
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # var ad/ecx: allocation-descriptor containing 16 bytes
    # . var end/ecx: (addr byte)
    89/<- %ecx 4/r32/esp
    # . var start/edx: (addr byte) = end - 16
    81 5/subop/subtract %esp 0x10/imm32
    89/<- %edx 4/r32/esp
    # . ad = {start, end}
    51/push-ecx
    52/push-edx
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
    # var expected-payload/ebx = ad->curr
    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           3/r32/ebx   .               .                 # copy *ecx to ebx
    # var h/edx: handle = {0, 0}
    68/push  0/imm32
    68/push  0/imm32
    89/copy                         3/mod/direct    2/rm32/edx    .           .             .           4/r32/esp   .               .                 # copy esp to edx
    # *Next-alloc-id = 0x34
    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32  0x34/imm32  # copy to *Next-alloc-id
    # allocate-array(ad, 3, h)
    # . . push args
    52/push-edx
    68/push  3/imm32
    51/push-ecx
    # . . call
    e8/call  allocate-array/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->alloc-id, 0x34, msg)
    # . . push args
    68/push  "F - test-allocate-array: sets alloc-id in handle"/imm32
    68/push  0x34/imm32
    ff          6/subop/push        0/mod/indirect  2/rm32/edx    .           .             .           .           .               .                 # push *edx
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->payload, expected-payload, msg)
    # . . push args
    68/push  "F - test-allocate-array: sets payload in handle"/imm32
    53/push-ebx
    ff          6/subop/push        1/mod/*+disp8   2/rm32/edx    .           .             .           .           4/disp8         .                 # push *(edx+4)
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->payload->alloc-id, 0x34, msg)
    # . . push args
    68/push  "F - test-allocate-array: sets alloc-id in payload"/imm32
    68/push  0x34/imm32
    ff          6/subop/push        0/mod/indirect  3/rm32/ebx    .           .             .           .           .               .                 # push *ebx
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->payload->size, 3, msg)
    # . . push args
    68/push  "F - test-allocate-array: sets array size in payload"/imm32
    68/push  3/imm32
    ff          6/subop/push        1/mod/*+disp8   3/rm32/ebx    .           .             .           .           4/disp8         .                 # push *(ebx+4)
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(*Next-alloc-id, 0x35, msg)
    # . . push args
    68/push  "F - test-allocate-array: increments Next-alloc-id"/imm32
    68/push  0x35/imm32
    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32              # push *Next-alloc-id
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(ad->curr - expected-payload, 3 + 4 for alloc-id + 4 for array size, msg)
    # . . push args
    68/push  "F - test-allocate-array: updates allocation descriptor"/imm32
    68/push  0xb/imm32
    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy *ecx to eax
    29/subtract                     3/mod/direct    0/rm32/eax    .           .             .           3/r32/ebx   .               .                 # subtract ebx from eax
    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
    # clean up
    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32  1/imm32     # copy to *Next-alloc-id
    # . reclaim locals
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0x20/imm32        # add to esp
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

copy-array:  # ad: (addr allocation-descriptor), src: (addr array _T), out: (addr handle array _T)
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    50/push-eax
    51/push-ecx
    52/push-edx
    56/push-esi
    # esi = src
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           6/r32/esi   0xc/disp8       .                 # copy *(ebp+12) to esi
    # var size/ecx: int = src->size+4
    8b/copy                         0/mod/indirect  6/rm32/esi    .           .             .           1/r32/ecx   .               .                 # copy *esi to ecx
    81          0/subop/add         3/mod/direct    1/rm32/ecx    .           .             .           .           .               4/imm32           # add to ecx
    # allocate(ad, size, out)
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0x10/disp8      .                 # push *(ebp+16)
    51/push-ecx
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  allocate/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # var payload/eax: (addr byte) = out->payload
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0x10/disp8      .                 # copy *(ebp+16) to eax
    8b/copy                         1/mod/*+disp8   0/rm32/eax    .           .             .           0/r32/eax   4/disp8         .                 # copy *(eax+4) to eax
    # . skip payload->allocid
    05/add-to-eax  4/imm32
    # var max/ecx: (addr byte) = payload + size
    01/add                          3/mod/direct    1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # add eax to ecx
    # _append-4(payload, max, src, &src->data[src->size])
    # . . push &src->data[src->size]
    8b/copy                         0/mod/indirect  6/rm32/esi    .           .             .           2/r32/edx   .               .                 # copy *esi to edx
    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    6/base/esi  2/index/edx   .           2/r32/edx   4/disp8         .                 # copy esi+edx+4 to edx
    52/push-edx
    # . . push src
    56/push-esi
    # . . push max
    51/push-ecx
    # . . push payload
    50/push-eax
    # . . call
    e8/call  _append-4/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0x10/imm32        # add to esp
$copy-array:end:
    # . restore registers
    5e/pop-to-esi
    5a/pop-to-edx
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

test-copy-array:
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # var src/esi: (addr array int) = [3, 4, 5]
    68/push  5/imm32
    68/push  4/imm32
    68/push  3/imm32
    68/push  0xc/imm32/size
    89/copy                         3/mod/direct    6/rm32/esi    .           .             .           4/r32/esp   .               .                 # copy esp to esi
    # var ad/ecx: allocation-descriptor containing 32 bytes
    # . var end/ecx: (addr byte)
    89/<- %ecx 4/r32/esp
    # . var start/edx: (addr byte) = end - 32
    81 5/subop/subtract %esp 0x20/imm32
    89/<- %edx 4/r32/esp
    # . ad = {start, end}
    51/push-ecx
    52/push-edx
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
    # var expected-payload/ebx = ad->curr
    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           3/r32/ebx   .               .                 # copy *ecx to ebx
    # var h/edx: handle = {0, 0}
    68/push  0/imm32
    68/push  0/imm32
    89/copy                         3/mod/direct    2/rm32/edx    .           .             .           4/r32/esp   .               .                 # copy esp to edx
    # *Next-alloc-id = 0x34
    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32  0x34/imm32  # copy to *Next-alloc-id
    # copy-array(ad, src, h)
    # . . push args
    52/push-edx
    56/push-esi
    51/push-ecx
    # . . call
    e8/call  copy-array/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->alloc-id, 0x34, msg)
    # . . push args
    68/push  "F - test-copy-array: sets alloc-id in handle"/imm32
    68/push  0x34/imm32
    ff          6/subop/push        0/mod/indirect  2/rm32/edx    .           .             .           .           .               .                 # push *edx
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->payload, expected-payload, msg)
    # . . push args
    68/push  "F - test-copy-array: sets payload in handle"/imm32
    53/push-ebx
    ff          6/subop/push        1/mod/*+disp8   2/rm32/edx    .           .             .           .           4/disp8         .                 # push *(edx+4)
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(h->payload->alloc-id, 0x34, msg)
    # . . push args
    68/push  "F - test-copy-array: sets alloc-id in payload"/imm32
    68/push  0x34/imm32
    ff          6/subop/push        0/mod/indirect  2/rm32/edx    .           .             .           .           .               .                 # push *edx
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # var payload/eax: (addr int) = lookup(h)
    # . . push args
    ff          6/subop/push        1/mod/*+disp8   2/rm32/edx    .           .             .           .           4/disp8         .                 # push *(edx+4)
    ff          6/subop/push        0/mod/indirect  2/rm32/edx    .           .             .           .           .               .                 # push *edx
    # . . call
    e8/call  lookup/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-ints-equal(payload->size, 0xc, msg)
    # . . push args
    68/push  "F - test-copy-array: sets array size in payload"/imm32
    68/push  0xc/imm32
    ff          6/subop/push        0/mod/indirect  0/rm32/eax    .           .             .           .           .               .                 # 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
    # check-ints-equal(*Next-alloc-id, 0x35, msg)
    # . . push args
    68/push  "F - test-copy-array: increments Next-alloc-id"/imm32
    68/push  0x35/imm32
    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32              # push *Next-alloc-id
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # check-ints-equal(ad->curr - expected-payload, 12 + 4 for alloc-id + 4 for size, msg)
    # . . push args
    68/push  "F - test-copy-array: updates allocation descriptor"/imm32
    68/push  0x14/imm32
    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy *ecx to eax
    29/subtract                     3/mod/direct    0/rm32/eax    .           .             .           3/r32/ebx   .               .                 # subtract ebx from eax
    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
    # clean up
    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           Next-alloc-id/disp32  1/imm32     # copy to *Next-alloc-id
    # . reclaim locals
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0x40/imm32        # add to esp
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

# Fill a region of memory with zeroes.
zero-out:  # start: (addr byte), size: int
    # pseudocode:
    #   curr/esi = start
    #   i/ecx = 0
    #   while true
    #     if (i >= size) break
    #     *curr = 0
    #     ++curr
    #     ++i
    #
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    50/push-eax
    51/push-ecx
    52/push-edx
    56/push-esi
    # curr/esi = start
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           6/r32/esi   8/disp8         .                 # copy *(ebp+8) to esi
    # var i/ecx: int = 0
    31/xor                          3/mod/direct    1/rm32/ecx    .           .             .           1/r32/ecx   .               .                 # clear ecx
    # edx = size
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           2/r32/edx   0xc/disp8       .                 # copy *(ebp+12) to edx
$zero-out:loop:
    # if (i >= size) break
    39/compare                      3/mod/direct    1/rm32/ecx    .           .             .           2/r32/edx   .               .                 # compare ecx with edx
    7d/jump-if->=  $zero-out:end/disp8
    # *curr = 0
    c6          0/subop/copy-byte   0/mod/direct    6/rm32/esi    .           .             .           .           .               0/imm8            # copy byte to *esi
    # ++curr
    46/increment-esi
    # ++i
    41/increment-ecx
    eb/jump  $zero-out:loop/disp8
$zero-out:end:
    # . restore registers
    5e/pop-to-esi
    5a/pop-to-edx
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

test-zero-out:
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # region/ecx = 34, 35, 36, 37
    68/push  0x37363534/imm32
    89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
    # zero-out(ecx, 3)
    # . . push args
    68/push  3/imm32/size
    51/push-ecx
    # . . call
    e8/call  zero-out/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # first 3 bytes cleared, fourth left alone
    # . check-ints-equal(*ecx, 0x37000000, msg)
    # . . push args
    68/push  "F - test-zero-out"/imm32
    68/push  0x37000000/imm32
    ff          6/subop/push        0/mod/indirect  1/rm32/ecx    .           .             .           .           .               .                 # push *ecx
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # . reclaim locals
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

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