# write: like _write, but also support in-memory streams in addition to file # descriptors. # # Our first dependency-injected and testable primitive. We can pass it either # a file descriptor or an address to a stream. If a file descriptor is passed # in, we _write to it using the right syscall. If a 'fake file descriptor' or # stream is passed in, we append to the stream. This lets us redirect output # in tests and check it later. # # We assume our data segment will never begin at an address shorter than # 0x08000000, so any smaller arguments are assumed to be real file descriptors. # # A stream looks like this: # read: int # index at which to read next # write: int # index at which writes go # data: (array byte) # prefixed by length as usual == 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 # TODO: come up with a way to signal when a write to disk fails write: # f : fd or (address stream), s : (address array byte) -> # . prolog 55/push-EBP 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP # if (f < 0x08000000) _write(f, s) and return # f can't be a user-mode address, so treat it as a kernel file descriptor 81 7/subop/compare 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 0x08000000/imm32 # compare *(EBP+8) 7d/jump-if-greater-or-equal $write:fake/disp8 # . . 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 _write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP eb/jump $write:end/disp8 $write:fake: # otherwise, treat 'f' as a stream to append to # . save registers 50/push-EAX 51/push-ECX 52/push-EDX 53/push-EBX # ECX = f 8b/copy 1/mod/*+disp8 5/rm32/EBP . . 1/r32/ECX 8/disp8 . # copy *(EBP+8) to ECX # EDX = f->write 8b/copy 0/mod/indirect 1/rm32/ECX . . . 2/r32/EDX . . # copy *ECX to EDX # EBX = f->length 8b/copy 1/mod/*+disp8 1/rm32/ECX . . . 3/r32/EBX 8/disp8 . # copy *(ECX+8) to EBX # EAX = _append-3(&f->data[f->write], &f->data[f->length], s) # . . push s ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 0xc/disp8 . # push *(EBP+12) # . . push &f->data[f->length] 8d/copy-address 1/mod/*+disp8 4/rm32/sib 1/base/ECX 3/index/EBX . 3/r32/EBX 0xc/disp8 . # copy ECX+EBX+12 to EBX 53/push-EBX # . . push &f->data[f->write] 8d/copy-address 1/mod/*+disp8 4/rm32/sib 1/base/ECX 2/index/EDX . 3/r32/EBX 0xc/disp8 . # copy ECX+EDX+12 to EBX 53/push-EBX # . . call e8/call _append-3/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP # f-