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# 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. But
# I don't want to type in 1024 bytes here.
== 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-'.
#? e8/call test-read-byte-multiple/disp32
#? e8/call test-read-byte-refills-buffer/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/exit
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-or-eof/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 5/rm32/EBP . . . 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) populate stream from file
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
# . f->read must now be 0; update ECX
31/xor 3/mod/direct 1/rm32/ECX . . . 1/r32/ECX . . # clear ECX
# . 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:
# 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 byte at *(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'
# setup
# . 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', msg)
# . . 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
# setup
# . 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', msg)
# . . 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 0xffffffff
# setup
# . 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, 0xffffffff, msg)
# . . push args
68/push "F - test-read-byte-end-of-file"/imm32
68/push 0xffffffff/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-refills-buffer:
# - consume buffered-file's buffer, check that next read-byte still works
# setup
# . 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, "Abcdefgh")
# . . push args
68/push "Abcdefgh"/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
# pretend buffer is full
# . _test-buffered-file->read = 6 # >= _test-buffered-file->length
b8/copy-to-EAX _test-buffered-file/imm32
c7 0/subop/copy 1/mod/*+disp8 0/rm32/EAX . . . . 8/disp8 6/imm32 # copy to *(EAX+8)
# 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', msg)
# . . push args
68/push "F - test-read-byte-refills-buffer"/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
== data
# a test buffered file for _test-stream
_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 (6)
06 00 00 00
# data
00 00 00 00 00 00 # 6 bytes
# . . vim:nowrap:textwidth=0
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