# read: analogously to write, support reading from in-memory streams in
# addition to file descriptors.
#
# We can pass it either a file descriptor or an address to a stream. If a
# file descriptor is passed in, we _read from it using the right syscall. If a
# stream is passed in (a fake file descriptor), we read from it instead. This
# lets us initialize input for tests.
#
# A little counter-intuitively, the output of 'read' ends up in.. a stream. So
# tests end up doing a redundant copy. Why? Well, consider the alternatives:
#
# a) Reading into a string, and returning a pointer to the end of the read
# region, or a count of bytes written. Now this count or end pointer must be
# managed separately by the caller, which can be error-prone.
#
# b) Having 'read' return a buffer that it allocates. But there's no way to
# know in advance how large to make the buffer. If you read less than the
# size of the buffer you again end up needing to manage initialized vs
# uninitialized memory.
#
# c) Creating more helpful variants like 'read-byte' or 'read-until' which
# also can take a file descriptor or stream, just like 'write'. But such
# primitives don't exist in the Linux kernel, so we'd be implementing them
# somehow, either with more internal buffering or by making multiple
# syscalls.
#
# Reading into a stream avoids these problems. The buffer is externally
# provided and the caller has control over where it's allocated, its lifetime,
# and so on. The buffer's read and write pointers are internal to it so it's
# easier to keep in a consistent state. And it can now be passed directly to
# helpers like 'read-byte' or 'read-until' that only need to support streams,
# never file descriptors.
#
# Like with 'write', we assume our data segment will never begin at an address
# shorter than 0x08000000, so any smaller arguments are assumed to be real
# file descriptors.
#
# As a reminder, a stream looks like this:
# write: int # index at which to write to next
# read: int # index at which to read next
# data: (array byte) # prefixed by length as usual
== 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-'.
# 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
read: # f : fd or (address stream), s : (address stream) -> num-bytes-read/EAX
# prolog
55/push-EBP
89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
## if (f < 0x08000000) return _read(f, s) # f can't be a user-mode address, so treat it as a kernel file descriptor
81 7/subop/compare 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 8/disp8 0x08000000/imm32 # compare *(EBP+8)
7d/jump-if-greater-or-equal $read:fake/disp8
# push args
ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 0xc/disp8 . # push *(EBP+12)
ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 8/disp8 . # push *(EBP+8)
# call
e8/call _read/disp32
# discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# return
eb/jump $read:end/disp8
$read:fake:
## otherwise, treat 'f' as a stream to scan from
# save registers
56/push-ESI
57/push-EDI
# 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
# EDI = s
8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 7/r32/EDI 0xc/disp8 . # copy *(EBP+12) to ESI
# EAX = _append-4(out = &s->data[s->write], outend = &s->data[s->length],
# in = &f->data[f->read], inend = &f->data[f->write])
# push &f->data[f->write]
8b/copy 0/mod/indirect 6/rm32/ESI . . . 0/r32/EAX . . # copy *ESI to EAX
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
50/push-EAX
# push &f->data[f->read]
8b/copy 1/mod/*+disp8 6/rm32/ESI . . . 0/r32/EAX 4/disp8 . # copy *(ESI+4) to EAX
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
50/push-EAX
# push &s->data[s->length]
8b/copy 1/mod/*+disp8 7/rm32/EDI . . . 0/r32/EAX 8/disp8 . # copy *(EDI+8) to EAX
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
50/push-EAX
# push &s->data[s->write]
8b/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy *EDI to EAX
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
50/push-EAX
# call
e8/call _append-4/disp32
# discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0x10/imm32 # add to ESP
# s->write += EAX
01/add 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # add EAX to *EDI
# f.read += EAX
01/add 1/mod/*+disp8 6/rm32/ESI . . . 0/r32/EAX 4/disp8 . # add EAX to *(ESI+4)
# restore registers
5f/pop-to-EDI
5e/pop-to-ESI
$read:end:
# epilog
89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP
5d/pop-to-EBP
c3/return
## helpers
# idea: a clear-if-empty method on streams that clears only if f.read == f.write
# Unclear how I'd use it, though. Callers seem to need the check anyway.
# Maybe a better helper would be 'empty-stream?'
_read: # fd : int, s : (address stream) -> num-bytes-read/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
52/push-EDX
53/push-EBX
56/push-ESI
# ESI = s
8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 6/r32/ESI 0xc/disp8 . # copy *(EBP+12) to ESI
# EAX = s->write
8b/copy 0/mod/indirect 6/rm32/ESI . . . 0/r32/EAX . . # copy *ESI to EAX
# EDX = s->length
8b/copy 1/mod/*+disp8 6/rm32/ESI . . . 2/r32/EDX 8/disp8 . # copy *(ESI+8) to EDX
# syscall(read, fd, &s->data[s->write], s->length - s->write)
# fd : EBX
8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 3/r32/EBX 8/disp8 . # copy *(EBP+8) to EBX
# data : ECX = &s->data[s->write]
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
# size : EDX = s->length - s->write
29/subtract 3/mod/direct 2/rm32/EDX . . . 0/r32/EAX . . # subtract EAX from EDX
# syscall
b8/copy-to-EAX 3/imm32/read
cd/syscall 0x80/imm8
# add the result EAX to s->write
01/add 0/mod/indirect 6/rm32/ESI . . . 0/r32/EAX . . # add EAX to *ESI
# restore registers
5e/pop-to-ESI
5b/pop-to-EBX
5a/pop-to-EDX
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
# Two options:
# 1 (what we have above):
# ECX = s
# EAX = s->write
# EDX = s->length
# # syscall
# ECX = lea ECX+EAX+12
# EDX = sub EDX EAX
#
# 2:
# ECX = s
# EDX = s->length
# ECX = &s->data
# # syscall
# ECX = add ECX, s->write
# EDX = sub EDX, s->write
#
# Not much to choose between the two? Option 2 performs a duplicate load to
# use one less register, but doesn't increase the amount of spilling (ECX
# and EDX must be used, and EAX must be clobbered anyway).
## tests
test-read-single:
# 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-stream-buffer)
# push args
68/push _test-stream-buffer/imm32
# 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(_test-stream, _test-stream-buffer)
# push args
68/push _test-stream-buffer/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, 2)
# push args
68/push "F - test-read-single: return EAX"/imm32
68/push 2/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
# check-ints-equal(*_test-stream-buffer->data, 41/A 62/b 00 00, msg)
# push args
68/push "F - test-read-single"/imm32
68/push 0x006241/imm32/Ab
# push *_test-stream-buffer->data
b8/copy-to-EAX _test-stream-buffer/imm32
ff 6/subop/push 1/mod/*+disp8 0/rm32/EAX . . . . 0xc/disp8 . # push *(EAX+12)
# 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-is-stateful:
## make two consecutive reads, check that their results are appended
# 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-stream-buffer)
# push args
68/push _test-stream-buffer/imm32
# 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, "C")
# push args
68/push "C"/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(_test-stream, _test-stream-buffer)
# push args
68/push _test-stream-buffer/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
# write(_test-stream, "D")
# push args
68/push "D"/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(_test-stream, _test-stream-buffer)
# push args
68/push _test-stream-buffer/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(*_test-stream-buffer->data, 43/C 44/D 00 00, msg)
# push args
68/push "F - test-read-is-stateful"/imm32
68/push 0x00004443/imm32/C-D
# push *_test-stream-buffer->data
b8/copy-to-EAX _test-stream-buffer/imm32
ff 6/subop/push 1/mod/*+disp8 0/rm32/EAX . . . . 0xc/disp8 . # push *(EAX+12)
# 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-returns-0-on-end-of-file:
## read after hitting end-of-file, check that result is 0
# 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-stream-buffer)
# push args
68/push _test-stream-buffer/imm32
# 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
## first read gets to end-of-file
# read(_test-stream, _test-stream-buffer)
# push args
68/push _test-stream-buffer/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-stream-buffer)
# push args
68/push _test-stream-buffer/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)
# 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-stream-buffer:
# 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