# 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.

== 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

# Claim the next 'n' bytes of memory starting at ad->curr and update ad->curr.
# If there isn't enough memory before ad->limit, return 0 and leave 'ad' unmodified.
allocate:  # ad : (address allocation-descriptor), n : int -> address-or-null/EAX
    # . prolog
    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
    # ECX = ad
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           1/r32/ECX   8/disp8         .                 # copy *(EBP+8) to ECX
    # save ad->curr
    8b/copy                         0/mod/indirect  1/rm32/ECX    .           .             .           0/r32/EAX   .               .                 # copy *ECX to EAX
    # check if there's enough space
    # . EDX = ad->curr + n
    89/copy                         3/mod/direct    2/rm32/EDX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EDX
    03/add                          1/mod/*+disp8   5/rm32/EBP    .           .             .           2/r32/EDX   0xc/disp8       .                 # add *(EBP+12) to EDX
    3b/compare                      1/mod/*+disp8   1/rm32/ECX    .           .             .           2/r32/EDX   4/disp8         .                 # compare EDX with *(ECX+4)
    72/jump-if-lesser-signed  $allocate:commit/disp8
    # return null if not
    b8/copy-to-EAX  0/imm32
    eb/jump  $allocate:end/disp8
$allocate:commit:
    # update ad->curr
    89/copy                         0/mod/indirect  1/rm32/ECX    .           .             .           2/r32/EDX   .               .                 # copy EDX to *ECX
$allocate:end:
    # . restore registers
    5a/pop-to-EDX
    59/pop-to-ECX
    # . epilog
    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    5d/pop-to-EBP
    c3/return

test-allocate-success:
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # var ad/ECX : (address allocation-descriptor) = {11, 15}
    68/push  0xf/imm32/limit
    68/push  0xb/imm32/curr
    89/copy                         3/mod/direct    1/rm32/ECX    .           .             .           4/r32/ESP   .               .                 # copy ESP to ECX
    # EAX = allocate(ad, 3)
    # . . push args
    68/push  3/imm32
    51/push-ECX
    # . . call
    e8/call  allocate/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # check-ints-equal(EAX, 11, msg)
    # . . push args
    68/push  "F - test-allocate-success: returns current pointer of allocation descriptor"/imm32
    68/push  0xb/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
    # check-ints-equal(ad->curr, 14, msg)
    # . . push args
    68/push  "F - test-allocate-success: updates allocation descriptor"/imm32
    68/push  0xe/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
    # . epilog
    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    5d/pop-to-EBP
    c3/return

test-allocate-failure:
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # var ad/ECX : (address allocation-descriptor) = {11, 15}
    68/push  0xf/imm32/limit
    68/push  0xb/imm32/curr
    89/copy                         3/mod/direct    1/rm32/ECX    .           .             .           4/r32/ESP   .               .                 # copy ESP to ECX
    # EAX = allocate(ad, 6)
    # . . push args
    68/push  6/imm32
    51/push-ECX
    # . . call
    e8/call  allocate/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-allocate-failure: 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
    # no change to ad->curr
    # . check-ints-equal(ad->curr, 11)
    # . . push args
    68/push  "F - test-allocate-failure: updates allocation descriptor"/imm32
    68/push  0xb/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
    # . epilog
    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 : (address allocation-descriptor), n : int -> new-ad : (address allocation-descriptor)
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # . save registers
    51/push-ECX
    # EAX = allocate(ad, n)
    # . . push args
    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    .           .             .           .           .               8/imm32           # add to ESP
    # if (EAX == 0) abort
    3d/compare-EAX-and  0/imm32
    74/jump-if-equal  $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
    # . epilog
    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:
    # . _write(2/stderr, error)
    # . . push args
    68/push  "allocate-region: failed to allocate\n"/imm32
    68/push  2/imm32/stderr
    # . . call
    e8/call  _write/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # . syscall(exit, 1)
    bb/copy-to-EBX  1/imm32
    b8/copy-to-EAX  1/imm32/exit
    cd/syscall  0x80/imm8
    # never gets here

# . . vim:nowrap:textwidth=0