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Diffstat (limited to 'subx/069allocate.subx')
| -rw-r--r-- | subx/069allocate.subx | 207 |
1 files changed, 207 insertions, 0 deletions
diff --git a/subx/069allocate.subx b/subx/069allocate.subx new file mode 100644 index 00000000..37b01d21 --- /dev/null +++ b/subx/069allocate.subx @@ -0,0 +1,207 @@ +# 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 + +# The 'global' allocation descriptor. Pass this into 'allocate' to claim a +# hitherto unused bit of memory. +Heap: + Start-of-heap/imm32 # curr + 00 00 00 0b # limit = 0x0b000000; keep sync'd with DATA_SEGMENT + SEGMENT_ALIGNMENT + +== 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 + +# main: + 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 + +# 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) + 7c/jump-if-lesser $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 + 81 7/subop/compare 3/mod/direct 0/rm32/EAX . . . . . 0/imm32 # compare EAX + 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"/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 |