1 # write: like _write, but also support in-memory output streams (`ostream`s)
  2 # in addition to file descriptors.
  3 #
  4 # Our first dependency-injected and testable primitive. We can pass it either
  5 # a file descriptor or an address to an output stream or ostream. If a file
  6 # descriptor is passed in, we _write to it using the right syscall. If a 'fake
  7 # file descriptor' or ostream is passed in, we append to the output stream.
  8 # This lets us redirect output in tests and check it later.
  9 #
 10 # We assume our data segment will never begin at an address shorter than
 11 # 0x08000000, so any smaller arguments are assumed to be real file descriptors.
 12 #
 13 # An ostream looks like this:
 14 #   write: int  # index at which writes go
 15 #   data: (array byte)  # prefixed by length as usual
 16 
 17 == data
 18 
 19 # In-memory ostream for tests to write() to.
 20 # Also illustrates the layout of ostreams.
 21 Test-ostream:
 22   # current write index
 23   00 00 00 00
 24   # length (= 8)
 25   08 00 00 00
 26   # data
 27   00 00 00 00 00 00 00 00  # 8 bytes
 28 
 29 == code
 30 
 31 # instruction                     effective address                                                   operand     displacement    immediate
 32 # op          subop               mod             rm32          base        index         scale       r32
 33 # 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
 34 
 35 # main:  (manual test if this is the last file loaded)
 36 #?   e8/call  test-write-appends/disp32
 37   e8/call  run-tests/disp32  # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
 38   # syscall(exit, Num-test-failures)
 39   8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           1/r32/EBX   Num-test-failures/disp32          # copy *Num-test-failures to EBX
 40   b8/copy-to-EAX  1/imm32
 41   cd/syscall  0x80/imm8
 42 
 43 write:  # f : fd or (address stream), s : (address array byte) -> <void>
 44   # prolog
 45   55/push-EBP
 46   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
 47   # save registers
 48   50/push-EAX
 49   51/push-ECX
 50   52/push-EDX
 51   53/push-EBX
 52   56/push-ESI
 53   # if (f < 0x08000000) _write(f, s), return  # f can't be a user-mode address, so treat it as a kernel file descriptor
 54   81          7/subop/compare     1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           8/disp8         0x08000000/imm32  # compare *(EBP+8)
 55   7f/jump-if-greater  $write:else/disp8
 56     # push args
 57   ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           8/disp8         .                 # push *(EBP+8)
 58   ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0xc/disp8       .                 # push *(EBP+12)
 59     # call
 60   e8/call  _write/disp32
 61     # discard args
 62   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
 63   eb/jump  $write:end/disp8
 64 $write:else:
 65   # otherwise, treat 'f' as a stream to append to
 66   #
 67   # pseudocode:
 68   #   destend = &f.data[t.length]
 69   #   oldw = f.write
 70   #   if s.length == 0 return
 71   #   f.write += s.length
 72   #   dest = &f.data[oldw]
 73   #   srcend = &s.data[s.length]
 74   #   src = &s.data[0]
 75   #   while true:
 76   #     if src >= srcend break
 77   #     if dest >= destend break  # for now silently ignore filled up ostream buffer
 78   #     *dest = *src
 79   #     ++src
 80   #     ++dest
 81   #
 82   # key registers to set up for the loop:
 83   #   EAX/dest, ECX/destend, EBX/src, ESI/srcend
 84   # we save EDX for byte operations (has to be one of the first 4 registers)
 85   #
 86   # register setup before the loop:
 87   #   EAX = *(EBP+8)    # f
 88   #   EBX = *(EBP+12)   # s
 89   #   ECX = *(EAX+4)    # f.length
 90   #   ECX = EAX+8+ECX  # destend = &f.data[f.length]
 91   #   ESI = *EAX        # oldw = f.write
 92   #   EDX = *EBX        # s.length
 93   #   *EAX = *EAX + EDX # update f.write  (allowed to go past f.length)
 94   #                     # do this here just because it's convenient
 95   #   EAX = EAX+8+ESI  # dest = &f.data[oldw]
 96   #   ESI = EBX+4+EDX   # srcend = &s.data[s.length]
 97   #   EBX = EBX+4       # src = &s.data[0]
 98   #
 99   # EAX = f
100   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none              0/r32/EAX   8/disp8         .                 # copy *(EBP+8) to EAX
101   # EBX = s
102   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none              3/r32/EBX   0xc/disp8       .                 # copy *(EBP+12) to EBX
103   # ECX = f.length
104   8b/copy                         1/mod/*+disp8   0/rm32/EAX    .           .             .           1/r32/ECX   4/disp8         .                 # copy *(EAX+4) to ECX
105   # ECX/destend = &f.data[f.length]
106   8d/copy-address                 1/mod/*+disp8   4/rm32/sib    0/base/EAX  1/index/ECX   .           1/r32/ECX   8/disp8         .                 # copy EAX+ECX+8 to ECX
107   # ESI/oldw = f.write
108   8b/copy                         0/mod/indirect  0/rm32/EAX    .           .             .           6/r32/ESI   .               .                 # copy *EAX to ESI
109   # EDX = s.length
110   8b/copy                         0/mod/indirect  3/rm32/EBX    .           .             .           2/r32/EDX   .               .                 # copy *EBX to EDX
111   # if EDX == 0 return
112   81          7/subop/compare     3/mod/direct    2/rm32/EDX    .           .             .           .           .               0/imm32           # compare EDX
113   74/jump-if-equal  $write:end/disp8
114   # f.write += s.length
115   01/add                          0/mod/indirect  0/rm32/EAX    .           .             .           2/r32/EDX   .               .                 # add EDX to *EAX
116   # EAX/dest = &f.data[oldw]
117   8d/copy-address                 1/mod/*+disp8   4/rm32/sib    0/base/EAX  6/index/ESI   .           0/r32/EAX   8/disp8         .                 # copy EAX+ESI+8 to EAX
118   # ESI/srcend = &s.data[s.length]
119   8d/copy-address                 1/mod/*+disp8   4/rm32/sib    3/base/EBX  2/index/EDX   .           6/r32/ESI   4/disp8         .                 # copy EBX+EDX+4 to ESI
120   # EBX/src = &s.data[0]
121   81          0/subop/add         3/mod/direct    3/rm32/EBX    .           .             .           .           .               4/imm32           # add to EBX
122   # while (true)
123 $write:loop:
124   # if EBX/src >= ESI/srcend break
125   39/compare                      3/mod/direct    3/rm32/EBX    .           .             .           6/r32/ESI   .               .                 # compare EBX with ESI
126   7d/jump-if-greater-or-equal  $write:end/disp8
127   # if EAX/dest >= ECX/destend break  (for now silently ignore filled up ostream buffer)
128   39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           1/r32/ECX   .               .                 # compare EAX with ECX
129   7d/jump-if-greater-or-equal  $write:end/disp8
130   # copy one byte
131   8a/copy-byte                    0/mod/indirect  3/rm32/EBX    .           .             .           2/r32/DL    .               .                 # copy byte at *EBX to DL
132   88/copy-byte                    0/mod/indirect  0/rm32/EAX    .           .             .           2/r32/DL    .               .                 # copy byte at DL to *EAX
133   # updates
134   40/increment-EAX
135   43/increment-EBX
136   eb/jump  $write:loop/disp8
137 $write:end:
138   # restore registers
139   5e/pop-to-ESI
140   5b/pop-to-EBX
141   5a/pop-to-EDX
142   59/pop-to-ECX
143   58/pop-to-EAX
144   # epilog
145   89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
146   5d/pop-to-EBP
147   c3/return
148 
149 clear-ostream:  # f : (address ostream)
150   # prolog
151   55/push-EBP
152   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
153   # save registers
154   50/push-EAX
155   51/push-ECX
156   # EAX = f
157   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none              0/r32/EAX   8/disp8         .                 # copy *(EBP+8) to EAX
158   # ECX = f.length
159   8b/copy                         1/mod/*+disp8   0/rm32/EAX    .           .             .           1/r32/ECX   4/disp8         .                 # copy *(EAX+4) to ECX
160   # ECX = &f.data[f.length]
161   8d/copy-address                 1/mod/*+disp8   4/rm32/sib    0/base/EAX  1/index/ECX   .           1/r32/ECX   8/disp8         .                 # copy EAX+ECX+8 to ECX
162   # f.write = 0
163   c7/copy                         0/mod/direct    0/rm32/EAX    .           .             .           .           .               0/imm32           # copy to *EAX
164   # EAX = f.data
165   81          0/subop/add         3/mod/direct    0/rm32/EAX    .           .             .           .           .               8/imm32           # add to EAX
166   # while (true)
167 $clear-ostream:loop:
168   # if EAX >= ECX break
169   39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           1/r32/ECX   .               .                 # compare EAX with ECX
170   7d/jump-if-greater-or-equal  $clear-ostream:end/disp8
171   # *EAX = 0
172   c7/copy                         0/mod/direct    0/rm32/EAX    .           .             .           .           .               0/imm32           # copy to *EAX
173   # EAX += 4
174   81          0/subop/add         3/mod/direct    0/rm32/EAX    .           .             .           .           .               4/imm32           # add to EAX
175   eb/jump  $clear-ostream:loop/disp8
176 $clear-ostream:end:
177   # restore registers
178   59/pop-to-ECX
179   58/pop-to-EAX
180   # epilog
181   89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
182   5d/pop-to-EBP
183   c3/return
184 
185 test-write-single:
186   # clear-ostream(Test-ostream)
187     # push args
188   68/push  Test-ostream/imm32
189     # call
190   e8/call  clear-ostream/disp32
191     # discard args
192   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
193   # write(Test-ostream, "Ab")
194     # push args
195   68/push  "Ab"/imm32
196   68/push  Test-ostream/imm32
197     # call
198   e8/call  write/disp32
199     # discard args
200   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
201   # check-ints-equal(*Test-ostream.data, 41/A 62/b 00 00, msg)
202     # push args
203   68/push  "F - test-write-single"/imm32
204   68/push  0x006241/imm32/Ab
205     # push *Test-ostream.data
206   b8/copy-to-EAX  Test-ostream/imm32
207   ff          6/subop/push        1/mod/*+disp8   0/rm32/EAX    .           .             .           .           8/disp8         .                 # push *(EAX+8)
208     # call
209   e8/call  check-ints-equal/disp32
210     # discard args
211   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
212   # end
213   c3/return
214 
215 test-write-appends:
216   # clear-ostream(Test-ostream)
217     # push args
218   68/push  Test-ostream/imm32
219     # call
220   e8/call  clear-ostream/disp32
221     # discard args
222   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
223   # write(Test-ostream, "C")
224     # push args
225   68/push  "C"/imm32
226   68/push  Test-ostream/imm32
227     # call
228   e8/call  write/disp32
229     # discard args
230   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
231   # write(Test-ostream, "D")
232     # push args
233   68/push  "D"/imm32
234   68/push  Test-ostream/imm32
235     # call
236   e8/call  write/disp32
237     # discard args
238   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
239   # check-ints-equal(*Test-ostream.data, 43/C 44/D 00 00, msg)
240     # push args
241   68/push  "F - test-write-appends"/imm32
242   68/push  0x00004443/imm32/C-D
243     # push *Test-ostream.data
244   b8/copy-to-EAX  Test-ostream/imm32
245   ff          6/subop/push        1/mod/*+disp8   0/rm32/EAX    .           .             .           .           8/disp8         .                 # push *(EAX+8)
246     # call
247   e8/call  check-ints-equal/disp32
248     # discard args
249   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
250   # end
251   c3/return