# 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. # Abort if there isn't enough memory in 'ad'. 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) 73/jump-if-greater-or-equal-signed $allocate:abort/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 $allocate:abort: # . _write(2/stderr, error) # . . push args 68/push "allocate: 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 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 _pending-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