1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
|
# Null-terminated vs length-prefixed ascii strings.
#
# By default we create strings with a 4-byte length prefix rather than a null suffix.
# However we still need null-prefixed strings when interacting with the Linux
# kernel in a few places. This layer implements a function for comparing
# a null-terminated 'kernel string' with a length-prefixed 'SubX string'.
#
# To run (from the subx directory):
# $ ./subx translate examples/ex11.subx -o examples/ex11
# $ ./subx run examples/ex11 # runs a series of tests
# ...... # all tests pass
#
# (We can't yet run the tests when given a "test" commandline argument,
# because checking for it would require the function being tested! Breakage
# would cause tests to not run, rather than to fail as we'd like.)
== 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-'.
# syscall(exit, EAX)
89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX
b8/copy-to-EAX 1/imm32/exit
cd/syscall 0x80/imm8
# compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
# reason for the name: the only place we should have null-terminated ascii strings is from commandline args
kernel-string-equal?: # s : null-terminated ascii string, benchmark : length-prefixed ascii string -> EAX : boolean
# pseudocode:
# n = benchmark->length
# s1 = s
# s2 = benchmark->data
# i = 0
# while (i < n)
# c1 = *s1
# c2 = *s2
# if (c1 == 0) return false
# if (c1 != c2) return false
# ++s1, ++s2, ++i
# return *s1 == 0
#
# registers:
# i: ECX
# n: EDX
# s1: EDI
# s2: ESI
# c1: EAX
# c2: EBX
#
# . 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
57/push-EDI
# s1/EDI = s
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 7/r32/EDI 8/disp8 . # copy *(EBP+8) to EDI
# n/EDX = benchmark->length
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 2/r32/EDX 0xc/disp8 . # copy *(EBP+12) to EDX
8b/copy 0/mod/indirect 2/rm32/EDX . . . 2/r32/EDX . . # copy *EDX to EDX
# s2/ESI = benchmark->data
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 6/r32/ESI 0xc/disp8 . # copy *(EBP+12) to ESI
81 0/subop/add 3/mod/direct 6/rm32/ESI . . . . . 4/imm32 # add to ESI
# i/ECX = c1/EAX = c2/EBX = 0
b9/copy-to-ECX 0/imm32/exit
b8/copy-to-EAX 0/imm32
bb/copy-to-EBX 0/imm32
$kernel-string-equal?:loop:
# if (i >= n) break
39/compare 3/mod/direct 1/rm32/ECX . . . 2/r32/EDX . . # compare ECX with EDX
7d/jump-if-greater-or-equal $kernel-string-equal?:break/disp8
# c1 = *s1
8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX
# c2 = *s2
8a/copy 0/mod/indirect 6/rm32/ESI . . . 3/r32/EBX . . # copy byte at *ESI to lower byte of EBX
# if (c1 == 0) return false
3d/compare-EAX 0/imm32
74/jump-if-equal $kernel-string-equal?:false/disp8
# if (c1 != c2) return false
39/compare 3/mod/direct 0/rm32/EAX . . . 3/r32/EBX . . # compare EAX with EBX
75/jump-if-not-equal $kernel-string-equal?:false/disp8
# ++i
41/inc-ECX
# ++s1
47/inc-EDI
# ++s2
46/inc-ESI
eb/jump $kernel-string-equal?:loop/disp8
$kernel-string-equal?:break:
# return *s1 == 0
8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX
3d/compare-EAX 0/imm32
75/jump-if-not-equal $kernel-string-equal?:false/disp8
$kernel-string-equal?:true:
b8/copy-to-EAX 1/imm32
eb/jump $kernel-string-equal?:end/disp8
$kernel-string-equal?:false:
b8/copy-to-EAX 0/imm32
$kernel-string-equal?:end:
# . restore registers
5f/pop-to-EDI
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
# - tests
test-compare-null-kernel-string-with-empty-array:
# EAX = kernel-string-equal?(Null-kernel-string, "")
# . . push args
68/push ""/imm32
68/push Null-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 1, msg)
# . . push args
68/push "F - test-compare-null-kernel-string-with-empty-array"/imm32
68/push 1/imm32/true
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
c3/return
test-compare-null-kernel-string-with-non-empty-array:
# EAX = kernel-string-equal?(Null-kernel-string, "Abc")
# . . push args
68/push "Abc"/imm32
68/push Null-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/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-compare-null-kernel-string-with-non-empty-array"/imm32
68/push 0/imm32/false
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
c3/return
test-compare-kernel-string-with-equal-array:
# EAX = kernel-string-equal?(_test-Abc-kernel-string, "Abc")
# . . push args
68/push "Abc"/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 1, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-equal-array"/imm32
68/push 1/imm32/true
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
c3/return
test-compare-kernel-string-with-inequal-array:
# EAX = kernel-string-equal?(_test-Abc-kernel-string, "Adc")
# . . push args
68/push "Adc"/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/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-compare-kernel-string-with-equal-array"/imm32
68/push 0/imm32/false
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
c3/return
test-compare-kernel-string-with-empty-array:
# EAX = kernel-string-equal?(_test-Abc-kernel-string, "")
# . . push args
68/push ""/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/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-compare-kernel-string-with-equal-array"/imm32
68/push 0/imm32/false
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
c3/return
test-compare-kernel-string-with-shorter-array:
# EAX = kernel-string-equal?(_test-Abc-kernel-string, "Ab")
# . . push args
68/push "Ab"/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/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-compare-kernel-string-with-shorter-array"/imm32
68/push 0/imm32/false
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
c3/return
test-compare-kernel-string-with-longer-array:
# EAX = kernel-string-equal?(_test-Abc-kernel-string, "Abcd")
# . . push args
68/push "Abcd"/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/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-compare-kernel-string-with-longer-array"/imm32
68/push 0/imm32/false
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
c3/return
# - helpers
# print msg to stderr if a != b, otherwise print "."
check-ints-equal: # (a : int, b : int, msg : (address array byte)) -> boolean
# . prolog
55/push-EBP
89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
# . save registers
51/push-ECX
53/push-EBX
# load args into EAX, EBX and ECX
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 0/r32/EAX 8/disp8 . # copy *(EBP+8) to EAX
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 3/r32/EBX 0xc/disp8 . # copy *(EBP+12) to EBX
# if (EAX == b/EBX) print('.') and return
39/compare 3/mod/direct 0/rm32/EAX . . . 3/r32/EBX . . # compare EAX and EBX
75/jump-if-unequal $check-ints-equal:else/disp8
# . write-stderr('.')
# . . push args
68/push "."/imm32
# . . call
e8/call write-stderr/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
# . return
eb/jump $check-ints-equal:end/disp8
# otherwise print(msg)
$check-ints-equal:else:
# copy msg into ECX
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 1/r32/ECX 0x10/disp8 . # copy *(EBP+16) to ECX
# print(ECX)
# . . push args
51/push-ECX
# . . call
e8/call write-stderr/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
# print newline
# . . push args
68/push Newline/imm32
# . . call
e8/call write-stderr/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
$check-ints-equal:end:
# . restore registers
5b/pop-to-EBX
59/pop-to-ECX
# end
89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP
5d/pop-to-EBP
c3/return
write-stderr: # s : (address array byte) -> <void>
# . prolog
55/push-EBP
89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
# . save registers
50/push-EAX
51/push-ECX
52/push-EDX
53/push-EBX
# syscall(write, 2/stderr, (data) s+4, (size) *s)
# . . fd = 2 (stderr)
bb/copy-to-EBX 2/imm32
# . . x = s+4
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 1/r32/ECX 8/disp8 . # copy *(EBP+8) to ECX
81 0/subop/add 3/mod/direct 1/rm32/ECX . . . . . 4/imm32 # add to ECX
# . . size = *s
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 2/r32/EDX 8/disp8 . # copy *(EBP+8) to EDX
8b/copy 0/mod/indirect 2/rm32/EDX . . . 2/r32/EDX . . # copy *EDX to EDX
# . . syscall
b8/copy-to-EAX 4/imm32/write
cd/syscall 0x80/imm8
# . restore registers
5b/pop-to-EBX
5a/pop-to-EDX
59/pop-to-ECX
58/pop-to-EAX
# . end
89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP
5d/pop-to-EBP
c3/return
== data
Newline:
# size
01 00 00 00
# data
0a/newline
# for kernel-string-equal tests
Null-kernel-string:
00/null
_test-Abc-kernel-string:
41/A 62/b 63/c 00/null
# . . vim:nowrap:textwidth=0
|