303 lines
13 KiB
Plaintext
303 lines
13 KiB
Plaintext
# read-byte: one higher-level abstraction atop 'read'.
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#
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# There are many situations where 'read' is a lot to manage, and we need
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# to abstract some details away. One of them is when we want to read a file
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# character by character. In this situation we follow C's FILE data structure,
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# which manages the underlying file descriptor together with the buffer it
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# reads into. We call our version 'buffered-file'. Should be useful with other
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# primitives as well, in later layers.
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== data
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# The buffered file for standard input. Also illustrates the layout for
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# buffered-file.
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Stdin:
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# file descriptor or (address stream)
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00 00 00 00 # 0 = standard input
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# current write index
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00 00 00 00
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# current read index
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00 00 00 00
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# length (8)
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08 00 00 00
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# data
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00 00 00 00 00 00 00 00 # 8 bytes
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# TODO: 8 bytes is too small. We'll need to grow the buffer for efficiency. But
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# I don't want to type in 1024 bytes here.
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== code
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# instruction effective address register displacement immediate
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# . op subop mod rm32 base index scale r32
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# . 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
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# main:
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e8/call run-tests/disp32 # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
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#? e8/call test-read-byte-multiple/disp32
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#? e8/call test-read-byte-refills-buffer/disp32
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# syscall(exit, Num-test-failures)
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8b/copy 0/mod/indirect 5/rm32/.disp32 . . 3/r32/EBX Num-test-failures/disp32 # copy *Num-test-failures to EBX
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b8/copy-to-EAX 1/imm32/exit
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cd/syscall 0x80/imm8
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# return next byte value in EAX, with top 3 bytes cleared.
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# On EOF, return 0xffffffff.
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read-byte: # f : (address buffered-file) -> byte-or-eof/EAX
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# . prolog
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55/push-EBP
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89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
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# . save registers
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51/push-ECX
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56/push-ESI
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# ESI = f
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8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 6/r32/ESI 8/disp8 . # copy *(EBP+8) to ESI
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# ECX = f->read
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8b/copy 1/mod/*+disp8 6/rm32/ESI . . . 1/r32/ECX 8/disp8 . # copy *(ESI+8) to ECX
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# if (f->read >= f->write) populate stream from file
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3b/compare 1/mod/*+disp8 6/rm32/ESI . . . 1/r32/ECX 4/disp8 . # compare ECX with *(ESI+4)
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7c/jump-if-lesser $read-byte:from-stream/disp8
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# . clear-stream(stream = f+4)
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# . . push args
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8d/copy-address 1/mod/*+disp8 6/rm32/ESI . . . 0/r32/EAX 4/disp8 . # copy ESI+4 to EAX
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50/push-EAX
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# . f->read must now be 0; update ECX
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31/xor 3/mod/direct 1/rm32/ECX . . . 1/r32/ECX . . # clear ECX
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# . EAX = read(f->fd, stream = f+4)
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# . . push args
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50/push-EAX
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ff 6/subop/push 0/mod/indirect 6/rm32/ESI . . . . . . # push *ESI
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# . . call
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e8/call read/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
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# if EAX = 0 return 0xffffffff
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81 7/subop/compare 3/mod/direct 0/rm32/EAX . . . . . 0/imm32 # compare EAX
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75/jump-if-not-equal $read-byte:from-stream/disp8
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b8/copy-to-EAX 0xffffffff/imm32
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eb/jump $read-byte:end/disp8
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$read-byte:from-stream:
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# read byte from stream
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# AL = f->data[f->read]
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31/xor 3/mod/direct 0/rm32/EAX . . . 0/r32/EAX . . # clear EAX
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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
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# ++f->read
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ff 0/subop/increment 1/mod/*+disp8 6/rm32/ESI . . . . 8/disp8 . # increment *(ESI+8)
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$read-byte:end:
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# . restore registers
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5e/pop-to-ESI
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59/pop-to-ECX
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# . epilog
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89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP
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5d/pop-to-EBP
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c3/return
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# - tests
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test-read-byte-single:
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# - check that read-byte returns first byte of 'file'
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# setup
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# . clear-stream(_test-stream)
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# . . push args
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68/push _test-stream/imm32
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# . clear-stream(_test-buffered-file+4)
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# . . push args
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b8/copy-to-EAX _test-buffered-file/imm32
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05/add-to-EAX 4/imm32
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50/push-EAX
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# . write(_test-stream, "Ab")
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# . . push args
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68/push "Ab"/imm32
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68/push _test-stream/imm32
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# . . call
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e8/call write/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
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# read-byte(_test-buffered-file)
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# . . push args
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68/push _test-buffered-file/imm32
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# . . call
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e8/call read-byte/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# check-ints-equal(EAX, 'A', msg)
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# . . push args
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68/push "F - test-read-byte-single"/imm32
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68/push 0x41/imm32
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50/push-EAX
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# . . call
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e8/call check-ints-equal/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
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# . end
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c3/return
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test-read-byte-multiple:
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# - call read-byte twice, check that second call returns second byte
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# setup
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# . clear-stream(_test-stream)
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# . . push args
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68/push _test-stream/imm32
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# . clear-stream(_test-buffered-file+4)
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# . . push args
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b8/copy-to-EAX _test-buffered-file/imm32
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05/add-to-EAX 4/imm32
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50/push-EAX
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# . write(_test-stream, "Ab")
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# . . push args
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68/push "Ab"/imm32
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68/push _test-stream/imm32
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# . . call
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e8/call write/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
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# read-byte(_test-buffered-file)
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# . . push args
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68/push _test-buffered-file/imm32
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# . . call
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e8/call read-byte/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# read-byte(_test-buffered-file)
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# . . push args
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68/push _test-buffered-file/imm32
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# . . call
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e8/call read-byte/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# check-ints-equal(EAX, 'b', msg)
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# . . push args
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68/push "F - test-read-byte-multiple"/imm32
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68/push 0x62/imm32
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50/push-EAX
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# . . call
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e8/call check-ints-equal/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
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# . end
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c3/return
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test-read-byte-end-of-file:
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# - call read-byte on an empty 'file', check that it returns 0xffffffff
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# setup
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# . clear-stream(_test-stream)
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# . . push args
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68/push _test-stream/imm32
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# . clear-stream(_test-buffered-file+4)
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# . . push args
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b8/copy-to-EAX _test-buffered-file/imm32
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05/add-to-EAX 4/imm32
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50/push-EAX
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# read-byte(_test-buffered-file)
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# . . push args
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68/push _test-buffered-file/imm32
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# . . call
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e8/call read-byte/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# check-ints-equal(EAX, 0xffffffff, msg)
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# . . push args
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68/push "F - test-read-byte-end-of-file"/imm32
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68/push 0xffffffff/imm32
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50/push-EAX
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# . . call
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e8/call check-ints-equal/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
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# . end
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c3/return
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test-read-byte-refills-buffer:
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# - consume buffered-file's buffer, check that next read-byte still works
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# setup
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# . clear-stream(_test-stream)
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# . . push args
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68/push _test-stream/imm32
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# . clear-stream(_test-buffered-file+4)
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# . . push args
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b8/copy-to-EAX _test-buffered-file/imm32
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05/add-to-EAX 4/imm32
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50/push-EAX
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# . . call
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e8/call clear-stream/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# . write(_test-stream, "Abcdefgh")
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# . . push args
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68/push "Abcdefgh"/imm32
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68/push _test-stream/imm32
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# . . call
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e8/call write/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
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# pretend buffer is full
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# . _test-buffered-file->read = 6 # >= _test-buffered-file->length
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b8/copy-to-EAX _test-buffered-file/imm32
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c7/copy 1/mod/*+disp8 0/rm32/EAX . . . . 8/disp8 6/imm32 # copy to *(EAX+8)
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# read-byte(_test-buffered-file)
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# . . push args
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68/push _test-buffered-file/imm32
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# . . call
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e8/call read-byte/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
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# check-ints-equal(EAX, 'A', msg)
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# . . push args
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68/push "F - test-read-byte-refills-buffer"/imm32
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68/push 0x41/imm32
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50/push-EAX
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# . . call
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e8/call check-ints-equal/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
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# . end
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c3/return
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== data
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# a test buffered file for _test-stream
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_test-buffered-file:
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# file descriptor or (address stream)
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_test-stream/imm32
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# current write index
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00 00 00 00
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# current read index
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00 00 00 00
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# length (6)
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06 00 00 00
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# data
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00 00 00 00 00 00 # 6 bytes
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# . . vim:nowrap:textwidth=0
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