# read-byte: one higher-level abstraction atop 'read'. # # There are many situations where 'read' is a lot to manage, and we need # to abstract some details away. One of them is when we want to read a file # character by character. In this situation we follow C's FILE data structure, # which manages the underlying file descriptor together with the buffer it # reads into. We call our version 'buffered-file'. Should be useful with other # primitives as well, in later layers. == data # The buffered file for standard input. Also illustrates the layout for # buffered-file. Stdin: # file descriptor or (address stream) 00 00 00 00 # 0 = standard input # current write index 00 00 00 00 # current read index 00 00 00 00 # length (8) 08 00 00 00 # data 00 00 00 00 00 00 00 00 # 8 bytes # TODO: 8 bytes is too small. We'll need to grow the buffer for efficiency. == code # instruction effective address operand 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-'. #? e8/call test-read-byte-multiple/disp32 # 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 cd/syscall 0x80/imm8 # return next byte value in EAX, with top 3 bytes cleared. # On EOF, return 0xffffffff. read-byte: # f : (address buffered-file) -> byte/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 56/push-ESI # ESI = f 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 6/r32/ESI 8/disp8 . # copy *(EBP+8) to ESI # ECX = f->read 8b/copy 1/mod/*+disp8 6/rm32/ESI . . . 1/r32/ECX 8/disp8 . # copy *(ESI+8) to ECX ## if (f->read < f->write) read byte from stream 3b/compare 1/mod/*+disp8 6/rm32/ESI . . . 1/r32/ECX 4/disp8 . # compare ECX with *(ESI+4) 7c/jump-if-lesser $read-byte:from-stream/disp8 # clear-stream(stream = f+4) # push args 8d/copy-address 1/mod/*+disp8 6/rm32/ESI . . . 0/r32/EAX 4/disp8 . # copy ESI+4 to EAX 50/push-EAX # call e8/call clear-stream/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # EAX = read(f->fd, stream = f+4) # push args 50/push-EAX ff 6/subop/push 0/mod/indirect 6/rm32/ESI . . . . . . # push *ESI # call e8/call read/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # if EAX = 0 return 0xffffffff 81 7/subop/compare 3/mod/direct 0/rm32/EAX . . . . . 0/imm32 # compare EAX 75/jump-if-not-equal $read-byte:from-stream/disp8 b8/copy-to-EAX 0xffffffff/imm32 eb/jump $read-byte:end/disp8 $read-byte:from-stream: # AL = f->data[f->read] 31/xor 3/mod/direct 0/rm32/EAX . . . 0/r32/EAX . . # clear EAX 8a/copy-byte 1/mod/*+disp8 4/rm32/sib 6/base/ESI 1/index/ECX . 0/r32/AL 0x10/disp8 . # copy *(ESI+ECX+16) to AL # ++f->read ff 0/subop/increment 1/mod/*+disp8 6/rm32/ESI . . . . 8/disp8 . # increment *(ESI+8) $read-byte:end: # restore registers 5e/pop-to-ESI 59/pop-to-ECX # epilog 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32
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