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# 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
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