1 # Checking null-terminated ascii strings.
  2 #
  3 # By default we create strings with a 4-byte length prefix rather than a null suffix.
  4 # However we still need null-prefixed strings when interacting with the Linux
  5 # kernel in a few places. This layer implements a function for comparing
  6 # a null-terminated 'kernel string' with a length-prefixed 'SubX string'.
  7 #
  8 # To run (from the subx directory):
  9 #   $ ./subx translate 05[0-2]*.subx -o /tmp/tmp52
 10 #   $ ./subx run /tmp/tmp52  # runs a series of tests
 11 #   ......  # all tests pass
 12 #
 13 # (We can't yet run the tests when given a "test" commandline argument,
 14 # because checking for it would require the function being tested! Breakage
 15 # would cause tests to not run, rather than to fail as we'd like.)
 16 
 17 == code
 18 #   instruction                     effective address                                                   register    displacement    immediate
 19 # . op          subop               mod             rm32          base        index         scale       r32
 20 # . 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
 21 
 22 Entry:  # run all tests
 23     e8/call  run-tests/disp32  # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
 24     # syscall(exit, Num-test-failures)
 25     8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           3/r32/EBX   Num-test-failures/disp32          # copy *Num-test-failures to EBX
 26     b8/copy-to-EAX  1/imm32/exit
 27     cd/syscall  0x80/imm8
 28 
 29 # compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
 30 # reason for the name: the only place we should have null-terminated ascii strings is from commandline args
 31 kernel-string-equal?:  # s : null-terminated ascii string, benchmark : length-prefixed ascii string -> EAX : boolean
 32     # pseudocode:
 33     #   n = benchmark->length
 34     #   s1 = s
 35     #   s2 = benchmark->data
 36     #   i = 0
 37     #   while (i < n)
 38     #     c1 = *s1
 39     #     c2 = *s2
 40     #     if (c1 == 0) return false
 41     #     if (c1 != c2) return false
 42     #     ++s1, ++s2, ++i
 43     #   return *s1 == 0
 44     #
 45     # registers:
 46     #   i: ECX
 47     #   n: EDX
 48     #   s1: EDI
 49     #   s2: ESI
 50     #   c1: EAX
 51     #   c2: EBX
 52     #
 53     # . prolog
 54     55/push-EBP
 55     89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
 56     # . save registers
 57     51/push-ECX
 58     52/push-EDX
 59     53/push-EBX
 60     56/push-ESI
 61     57/push-EDI
 62     # s1/EDI = s
 63     8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           7/r32/EDI   8/disp8         .                 # copy *(EBP+8) to EDI
 64     # n/EDX = benchmark->length
 65     8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           2/r32/EDX   0xc/disp8       .                 # copy *(EBP+12) to EDX
 66     8b/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           2/r32/EDX   .               .                 # copy *EDX to EDX
 67     # s2/ESI = benchmark->data
 68     8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           6/r32/ESI   0xc/disp8       .                 # copy *(EBP+12) to ESI
 69     81          0/subop/add         3/mod/direct    6/rm32/ESI    .           .             .           .           .               4/imm32           # add to ESI
 70     # i/ECX = c1/EAX = c2/EBX = 0
 71     b9/copy-to-ECX  0/imm32/exit
 72     b8/copy-to-EAX  0/imm32
 73     bb/copy-to-EBX  0/imm32
 74 $kernel-string-equal?:loop:
 75     # if (i >= n) break
 76     39/compare                      3/mod/direct    1/rm32/ECX    .           .             .           2/r32/EDX   .               .                 # compare ECX with EDX
 77     7d/jump-if-greater-or-equal  $kernel-string-equal?:break/disp8
 78     # c1 = *s1
 79     8a/copy-byte                    0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/AL    .               .                 # copy byte at *EDI to AL
 80     # c2 = *s2
 81     8a/copy-byte                    0/mod/indirect  6/rm32/ESI    .           .             .           3/r32/BL    .               .                 # copy byte at *ESI to BL
 82     # if (c1 == 0) return false
 83     3d/compare-EAX-and  0/imm32
 84     74/jump-if-equal  $kernel-string-equal?:false/disp8
 85     # if (c1 != c2) return false
 86     39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # compare EAX and EBX
 87     75/jump-if-not-equal  $kernel-string-equal?:false/disp8
 88     # ++i
 89     41/increment-ECX
 90     # ++s1
 91     47/increment-EDI
 92     # ++s2
 93     46/increment-ESI
 94     eb/jump  $kernel-string-equal?:loop/disp8
 95 $kernel-string-equal?:break:
 96     # return *s1 == 0
 97     8a/copy-byte                    0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/AL    .               .                 # copy byte at *EDI to AL
 98     3d/compare-EAX-and  0/imm32
 99     75/jump-if-not-equal  $kernel-string-equal?:false/disp8
100 $kernel-string-equal?:true:
101     b8/copy-to-EAX  1/imm32
102     eb/jump  $kernel-string-equal?:end/disp8
103 $kernel-string-equal?:false:
104     b8/copy-to-EAX  0/imm32
105 $kernel-string-equal?:end:
106     # . restore registers
107     5f/pop-to-EDI
108     5e/pop-to-ESI
109     5b/pop-to-EBX
110     5a/pop-to-EDX
111     59/pop-to-ECX
112     # . epilog
113     89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
114     5d/pop-to-EBP
115     c3/return
116 
117 # - tests
118 
119 test-compare-null-kernel-string-with-empty-array:
120     # EAX = kernel-string-equal?(Null-kernel-string, "")
121     # . . push args
122     68/push  ""/imm32
123     68/push  Null-kernel-string/imm32
124     # . . call
125     e8/call  kernel-string-equal?/disp32
126     # . . discard args
127     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
128     # check-ints-equal(EAX, 1, msg)
129     # . . push args
130     68/push  "F - test-compare-null-kernel-string-with-empty-array"/imm32
131     68/push  1/imm32/true
132     50/push-EAX
133     # . . call
134     e8/call  check-ints-equal/disp32
135     # . . discard args
136     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
137     c3/return
138 
139 test-compare-null-kernel-string-with-non-empty-array:
140     # EAX = kernel-string-equal?(Null-kernel-string, "Abc")
141     # . . push args
142     68/push  "Abc"/imm32
143     68/push  Null-kernel-string/imm32
144     # . . call
145     e8/call  kernel-string-equal?/disp32
146     # . . discard args
147     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
148     # check-ints-equal(EAX, 0, msg)
149     # . . push args
150     68/push  "F - test-compare-null-kernel-string-with-non-empty-array"/imm32
151     68/push  0/imm32/false
152     50/push-EAX
153     # . . call
154     e8/call  check-ints-equal/disp32
155     # . . discard args
156     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
157     c3/return
158 
159 test-compare-kernel-string-with-equal-array:
160     # EAX = kernel-string-equal?(_test-Abc-kernel-string, "Abc")
161     # . . push args
162     68/push  "Abc"/imm32
163     68/push  _test-Abc-kernel-string/imm32
164     # . . call
165     e8/call  kernel-string-equal?/disp32
166     # . . discard args
167     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
168     # check-ints-equal(EAX, 1, msg)
169     # . . push args
170     68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
171     68/push  1/imm32/true
172     50/push-EAX
173     # . . call
174     e8/call  check-ints-equal/disp32
175     # . . discard args
176     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
177     c3/return
178 
179 test-compare-kernel-string-with-inequal-array:
180     # EAX = kernel-string-equal?(_test-Abc-kernel-string, "Adc")
181     # . . push args
182     68/push  "Adc"/imm32
183     68/push  _test-Abc-kernel-string/imm32
184     # . . call
185     e8/call  kernel-string-equal?/disp32
186     # . . discard args
187     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
188     # check-ints-equal(EAX, 0, msg)
189     # . . push args
190     68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
191     68/push  0/imm32/false
192     50/push-EAX
193     # . . call
194     e8/call  check-ints-equal/disp32
195     # . . discard args
196     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
197     c3/return
198 
199 test-compare-kernel-string-with-empty-array:
200     # EAX = kernel-string-equal?(_test-Abc-kernel-string, "")
201     # . . push args
202     68/push  ""/imm32
203     68/push  _test-Abc-kernel-string/imm32
204     # . . call
205     e8/call  kernel-string-equal?/disp32
206     # . . discard args
207     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
208     # check-ints-equal(EAX, 0, msg)
209     # . . push args
210     68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
211     68/push  0/imm32/false
212     50/push-EAX
213     # . . call
214     e8/call  check-ints-equal/disp32
215     # . . discard args
216     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
217     c3/return
218 
219 test-compare-kernel-string-with-shorter-array:
220     # EAX = kernel-string-equal?(_test-Abc-kernel-string, "Ab")
221     # . . push args
222     68/push  "Ab"/imm32
223     68/push  _test-Abc-kernel-string/imm32
224     # . . call
225     e8/call  kernel-string-equal?/disp32
226     # . . discard args
227     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
228     # check-ints-equal(EAX, 0, msg)
229     # . . push args
230     68/push  "F - test-compare-kernel-string-with-shorter-array"/imm32
231     68/push  0/imm32/false
232     50/push-EAX
233     # . . call
234     e8/call  check-ints-equal/disp32
235     # . . discard args
236     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
237     c3/return
238 
239 test-compare-kernel-string-with-longer-array:
240     # EAX = kernel-string-equal?(_test-Abc-kernel-string, "Abcd")
241     # . . push args
242     68/push  "Abcd"/imm32
243     68/push  _test-Abc-kernel-string/imm32
244     # . . call
245     e8/call  kernel-string-equal?/disp32
246     # . . discard args
247     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
248     # check-ints-equal(EAX, 0, msg)
249     # . . push args
250     68/push  "F - test-compare-kernel-string-with-longer-array"/imm32
251     68/push  0/imm32/false
252     50/push-EAX
253     # . . call
254     e8/call  check-ints-equal/disp32
255     # . . discard args
256     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
257     c3/return
258 
259 == data
260 
261 Null-kernel-string:
262     00/null
263 
264 _test-Abc-kernel-string:
265     41/A 62/b 63/c 00/null
266 
267 # . . vim:nowrap:textwidth=0