439 lines
22 KiB
Plaintext
439 lines
22 KiB
Plaintext
# read: analogously to write, support reading from in-memory streams in
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# addition to file descriptors.
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#
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# We can pass it either a file descriptor or an address to a stream. If a
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# file descriptor is passed in, we _read from it using the right syscall. If a
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# stream is passed in (a fake file descriptor), we read from it instead. This
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# lets us initialize input for tests.
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#
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# A little counter-intuitively, the output of 'read' ends up in.. a stream. So
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# tests end up doing a redundant copy. Why? Well, consider the alternatives:
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#
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# a) Reading into a string, and returning a pointer to the end of the read
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# region, or a count of bytes written. Now this count or end pointer must be
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# managed separately by the caller, which can be error-prone.
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#
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# b) Having 'read' return a buffer that it allocates. But there's no way to
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# know in advance how large to make the buffer. If you read less than the
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# size of the buffer you again end up needing to manage initialized vs
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# uninitialized memory.
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#
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# c) Creating more helpful variants like 'read-byte' or 'read-until' which
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# also can take a file descriptor or stream, just like 'write'. But such
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# primitives don't exist in the Linux kernel, so we'd be implementing them
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# somehow, either with more internal buffering or by making multiple
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# syscalls.
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#
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# Reading into a stream avoids these problems. The buffer is externally
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# provided and the caller has control over where it's allocated, its lifetime,
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# and so on. The buffer's read and write pointers are internal to it so it's
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# easier to keep in a consistent state. And it can now be passed directly to
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# helpers like 'read-byte' or 'read-until' that only need to support streams,
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# never file descriptors.
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#
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# Like with 'write', we assume our data segment will never begin at an address
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# shorter than 0x08000000, so any smaller arguments are assumed to be real
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# file descriptors.
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#
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# As a reminder, a stream looks like this:
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# write: int # index at which to write to next
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# read: int # index at which to read next
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# data: (array byte) # prefixed by size as usual
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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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read: # f: fd or (addr stream byte), s: (addr stream byte) -> num-bytes-read/eax: int
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# . prologue
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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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# if (f < 0x08000000) return _read(f, s) # f can't be a user-mode address, so treat it as a kernel file descriptor
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81 7/subop/compare 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 0x08000000/imm32 # compare *(ebp+8)
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73/jump-if-addr>= $read:fake/disp8
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# . . push args
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ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12)
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ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8)
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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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# return
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eb/jump $read:end/disp8
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$read:fake:
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# otherwise, treat 'f' as a stream to scan from
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# . save registers
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56/push-esi
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57/push-edi
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# esi = f
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 8/disp8 . # copy *(ebp+8) to esi
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# edi = s
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 7/r32/edi 0xc/disp8 . # copy *(ebp+12) to esi
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# eax = _buffer-4(out = &s->data[s->write], outend = &s->data[s->size],
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# in = &f->data[f->read], inend = &f->data[f->write])
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# . . push &f->data[f->write]
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8b/copy 0/mod/indirect 6/rm32/esi . . . 0/r32/eax . . # copy *esi to eax
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8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/esi 0/index/eax . 0/r32/eax 0xc/disp8 . # copy esi+eax+12 to eax
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50/push-eax
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# . . push &f->data[f->read]
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8b/copy 1/mod/*+disp8 6/rm32/esi . . . 0/r32/eax 4/disp8 . # copy *(esi+4) to eax
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8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/esi 0/index/eax . 0/r32/eax 0xc/disp8 . # copy esi+eax+12 to eax
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50/push-eax
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# . . push &s->data[s->size]
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8b/copy 1/mod/*+disp8 7/rm32/edi . . . 0/r32/eax 8/disp8 . # copy *(edi+8) to eax
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8d/copy-address 1/mod/*+disp8 4/rm32/sib 7/base/edi 0/index/eax . 0/r32/eax 0xc/disp8 . # copy edi+eax+12 to eax
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50/push-eax
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# . . push &s->data[s->write]
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8b/copy 0/mod/indirect 7/rm32/edi . . . 0/r32/eax . . # copy *edi to eax
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8d/copy-address 1/mod/*+disp8 4/rm32/sib 7/base/edi 0/index/eax . 0/r32/eax 0xc/disp8 . # copy edi+eax+12 to eax
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50/push-eax
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# . . call
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e8/call _buffer-4/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0x10/imm32 # add to esp
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# s->write += eax
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01/add 0/mod/indirect 7/rm32/edi . . . 0/r32/eax . . # add eax to *edi
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# f->read += eax
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01/add 1/mod/*+disp8 6/rm32/esi . . . 0/r32/eax 4/disp8 . # add eax to *(esi+4)
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# . restore registers
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5f/pop-to-edi
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5e/pop-to-esi
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$read:end:
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# . epilogue
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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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# - helpers
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# '_buffer' is like '_append', but silently stops instead of aborting when it runs out of space
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# 3-argument variant of _buffer
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_buffer-3: # out: address, outend: address, s: (array byte) -> num_bytes_buffered/eax
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# . prologue
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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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# eax = _buffer-4(out, outend, &s->data[0], &s->data[s->size])
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# . . push &s->data[s->size]
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . 0/r32/eax 0x10/disp8 . # copy *(ebp+16) to eax
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8b/copy 0/mod/indirect 0/rm32/eax . . . 1/r32/ecx . . # copy *eax to ecx
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8d/copy-address 1/mod/*+disp8 4/rm32/sib 0/base/eax 1/index/ecx . 1/r32/ecx 4/disp8 . # copy eax+ecx+4 to ecx
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51/push-ecx
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# . . push &s->data[0]
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8d/copy-address 1/mod/*+disp8 0/rm32/eax . . . 1/r32/ecx 4/disp8 . # copy eax+4 to ecx
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51/push-ecx
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# . . push outend
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ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12)
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# . . push out
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ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8)
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# . . call
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e8/call _buffer-4/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0x10/imm32 # add to esp
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$_buffer-3:end:
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# . restore registers
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59/pop-to-ecx
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# . epilogue
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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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# 4-argument variant of _buffer
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_buffer-4: # out: address, outend: address, in: address, inend: address -> num_bytes_buffered/eax
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# . prologue
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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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52/push-edx
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53/push-ebx
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56/push-esi
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57/push-edi
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# eax/num_bytes_buffered = 0
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b8/copy-to-eax 0/imm32
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# edi = out
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 7/r32/edi 8/disp8 . # copy *(ebp+8) to edi
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# edx = outend
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx
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# esi = in
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 0x10/disp8 . # copy *(ebp+16) to esi
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# ecx = inend
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 1/r32/ecx 0x14/disp8 . # copy *(ebp+20) to ecx
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$_buffer-4:loop:
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# if (in >= inend) break
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39/compare 3/mod/direct 6/rm32/esi . . . 1/r32/ecx . . # compare esi with ecx
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73/jump-if-addr>= $_buffer-4:end/disp8
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# if (out >= outend) break # for now silently ignore filled up buffer
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39/compare 3/mod/direct 7/rm32/edi . . . 2/r32/edx . . # compare edi with edx
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73/jump-if-addr>= $_buffer-4:end/disp8
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# *out = *in
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8a/copy-byte 0/mod/indirect 6/rm32/esi . . . 3/r32/BL . . # copy byte at *esi to BL
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88/copy-byte 0/mod/indirect 7/rm32/edi . . . 3/r32/BL . . # copy byte at BL to *edi
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# ++num_bytes_buffered
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40/increment-eax
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# ++in
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46/increment-esi
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# ++out
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47/increment-edi
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eb/jump $_buffer-4:loop/disp8
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$_buffer-4:end:
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# . restore registers
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5f/pop-to-edi
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5e/pop-to-esi
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5b/pop-to-ebx
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5a/pop-to-edx
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59/pop-to-ecx
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# . epilogue
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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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# idea: a clear-if-empty method on streams that clears only if f->read == f->write
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# Unclear how I'd use it, though. Callers seem to need the check anyway.
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# Maybe a better helper would be 'empty-stream?'
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_read: # fd: int, s: (addr stream byte) -> num-bytes-read/eax: int
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# . prologue
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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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52/push-edx
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53/push-ebx
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56/push-esi
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# esi = s
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 0xc/disp8 . # copy *(ebp+12) to esi
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# eax = s->write
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8b/copy 0/mod/indirect 6/rm32/esi . . . 0/r32/eax . . # copy *esi to eax
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# edx = s->size
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8b/copy 1/mod/*+disp8 6/rm32/esi . . . 2/r32/edx 8/disp8 . # copy *(esi+8) to edx
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# syscall_read(fd, &s->data[s->write], s->size - s->write)
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# . . fd: ebx
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 3/r32/ebx 8/disp8 . # copy *(ebp+8) to ebx
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# . . data: ecx = &s->data[s->write]
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8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/esi 0/index/eax . 1/r32/ecx 0xc/disp8 . # copy esi+eax+12 to ecx
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# . . size: edx = s->size - s->write
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29/subtract 3/mod/direct 2/rm32/edx . . . 0/r32/eax . . # subtract eax from edx
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# . . syscall
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e8/call syscall_read/disp32
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# add the result eax to s->write
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01/add 0/mod/indirect 6/rm32/esi . . . 0/r32/eax . . # add eax to *esi
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$_read:end:
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# . restore registers
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5e/pop-to-esi
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5b/pop-to-ebx
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5a/pop-to-edx
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59/pop-to-ecx
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# . epilogue
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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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# Two options:
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# 1 (what we have above):
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# ecx = s
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# eax = s->write
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# edx = s->size
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# # syscall
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# ecx = lea ecx+eax+12
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# edx = sub edx eax
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#
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# 2:
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# ecx = s
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# edx = s->size
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# ecx = &s->data
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# # syscall
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# ecx = add ecx, s->write
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# edx = sub edx, s->write
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#
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# Not much to choose between the two? Option 2 performs a duplicate load to
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# use one less register, but doesn't increase the amount of spilling (ecx
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# and edx must be used, and eax must be clobbered anyway).
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# - tests
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test-read-single:
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# - write a single character into _test-stream, then read from it
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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-tmp-stream)
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# . . push args
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68/push _test-tmp-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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# 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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# eax = read(_test-stream, _test-tmp-stream)
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# . . push args
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68/push _test-tmp-stream/imm32
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68/push _test-stream/imm32
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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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# check-ints-equal(eax, 2, msg)
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# . . push args
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68/push "F - test-read-single: return value"/imm32
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68/push 2/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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# check-stream-equal(_test-tmp-stream, "Ab", msg)
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# . . push args
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68/push "F - test-read-single"/imm32
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68/push "Ab"/imm32
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68/push _test-tmp-stream/imm32
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# . . call
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e8/call check-stream-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-is-stateful:
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# - make two consecutive reads, check that their results are appended
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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-tmp-stream)
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# . . push args
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68/push _test-tmp-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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# write(_test-stream, "C")
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# . . push args
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68/push "C"/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(_test-stream, _test-tmp-stream)
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# . . push args
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68/push _test-tmp-stream/imm32
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68/push _test-stream/imm32
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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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# write(_test-stream, "D")
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# . . push args
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68/push "D"/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(_test-stream, _test-tmp-stream)
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# . . push args
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68/push _test-tmp-stream/imm32
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68/push _test-stream/imm32
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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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# check-stream-equal(_test-tmp-stream, "CD", msg)
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# . . push args
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68/push "F - test-read-is-stateful"/imm32
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68/push "CD"/imm32
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68/push _test-tmp-stream/imm32
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# . . call
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e8/call check-stream-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-returns-0-on-end-of-file:
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# - read after hitting end-of-file, check that result is 0
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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-tmp-stream)
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# . . push args
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68/push _test-tmp-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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|
# . 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
|
|
# . . call
|
|
e8/call write/disp32
|
|
# . . discard args
|
|
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
|
|
# first read gets to end-of-file
|
|
# . read(_test-stream, _test-tmp-stream)
|
|
# . . push args
|
|
68/push _test-tmp-stream/imm32
|
|
68/push _test-stream/imm32
|
|
# . . call
|
|
e8/call read/disp32
|
|
# . . discard args
|
|
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
|
|
# second read
|
|
# . read(_test-stream, _test-tmp-stream)
|
|
# . . push args
|
|
68/push _test-tmp-stream/imm32
|
|
68/push _test-stream/imm32
|
|
# . . call
|
|
e8/call read/disp32
|
|
# . . discard args
|
|
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
|
|
# check-ints-equal(eax, 0, msg)
|
|
# . . push args
|
|
68/push "F - test-read-returns-0-on-end-of-file"/imm32
|
|
68/push 0/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-tmp-stream: # (stream byte)
|
|
# current write index
|
|
0/imm32
|
|
# current read index
|
|
0/imm32
|
|
# size
|
|
8/imm32
|
|
# data
|
|
00 00 00 00 00 00 00 00 # 8 bytes
|
|
|
|
# . . vim:nowrap:textwidth=0
|