# 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/EBP . . # copy EBP to ESP 5d/pop-to-EBP c3/return ## tests test-read-byte-single: ## check that read-byte returns first byte of 'file' # clear-stream(_test-stream) # push args 68/push _test-stream/imm32 # call e8/call clear-stream/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # clear-stream(_test-buffered-file+4) # push args b8/copy-to-EAX _test-buffered-file/imm32 05/add-to-EAX 4/imm32 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 # write(_test-stream, "Ab") # push args 68/push "Ab"/imm32 68/push _test-stream/imm32 # call e8/call write/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # read-byte(_test-buffered-file) # push args 68/push _test-buffered-file/imm32 # call e8/call read-byte/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # check-ints-equal(EAX, 'A') # push args 68/push "F - test-read-byte-single"/imm32 68/push 0x41/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 # end c3/return test-read-byte-multiple: ## call read-byte twice, check that second call returns second byte # clear-stream(_test-stream) # push args 68/push _test-stream/imm32 # call e8/call clear-stream/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # clear-stream(_test-buffered-file+4) # push args b8/copy-to-EAX _test-buffered-file/imm32 05/add-to-EAX 4/imm32 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 # write(_test-stream, "Ab") # push args 68/push "Ab"/imm32 68/push _test-stream/imm32 # call e8/call write/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # read-byte(_test-buffered-file) # push args 68/push _test-buffered-file/imm32 # call e8/call read-byte/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # read-byte(_test-buffered-file) # push args 68/push _test-buffered-file/imm32 # call e8/call read-byte/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # check-ints-equal(EAX, 'b') # push args 68/push "F - test-read-byte-multiple"/imm32 68/push 0x62/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 # end c3/return test-read-byte-end-of-file: ## call read-byte on an empty 'file', check that it returns -1 # clear-stream(_test-stream) # push args 68/push _test-stream/imm32 # call e8/call clear-stream/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # clear-stream(_test-buffered-file+4) # push args b8/copy-to-EAX _test-buffered-file/imm32 05/add-to-EAX 4/imm32 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 # read-byte(_test-buffered-file) # push args 68/push _test-buffered-file/imm32 # call e8/call read-byte/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # check-ints-equal(EAX, -1) # push args 68/push "F - test-read-byte-end-of-file"/imm32 68/push -1/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 # end c3/return == data _test-buffered-file: # file descriptor or (address stream) _test-stream/imm32 # 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 # vim:nowrap:textwidth=0