262 lines
12 KiB
Plaintext
262 lines
12 KiB
Plaintext
# Checking null-terminated ascii strings.
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#
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# By default we create strings with a 4-byte length prefix rather than a null suffix.
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# However we still need null-prefixed strings when interacting with the Linux
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# kernel in a few places. This layer implements a function for comparing
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# a null-terminated 'kernel string' with a length-prefixed 'SubX string'.
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#
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# To run (from the subx directory):
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# $ subx translate 05[0-2]*.subx -o /tmp/tmp52
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# $ subx run /tmp/tmp52 # runs a series of tests
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# ...... # all tests pass
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#
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# (We can't yet run the tests when given a "test" commandline argument,
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# because checking for it would require the function being tested! Breakage
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# would cause tests to not run, rather than to fail as we'd like.)
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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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# main:
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e8/call run-tests/disp32 # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
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# syscall(exit, Num-test-failures)
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8b/copy 0/mod/indirect 5/rm32/.disp32 . . 3/r32/EBX Num-test-failures/disp32 # copy *Num-test-failures to EBX
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b8/copy-to-EAX 1/imm32
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cd/syscall 0x80/imm8
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# compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
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# reason for the name: the only place we should have null-terminated ascii strings is from commandline args
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kernel-string-equal: # s : null-terminated ascii string, benchmark : length-prefixed ascii string -> EAX : boolean
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# . prolog
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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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# pseudocode:
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# initialize n = b->length
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# initialize s1 = s
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# initialize s2 = b->data
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# i = 0
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# for (i = 0; i < n; ++n)
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# c1 = *s1
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# c2 = *s2
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# if c1 == 0
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# return false
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# if c1 != c2
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# return false
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# return *s1 == 0
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#
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# initialize s into EDI
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8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 7/r32/EDI 8/disp8 . # copy *(EBP+8) to EDI
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# initialize benchmark length n into EDX
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8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 2/r32/EDX 0xc/disp8 . # copy *(EBP+12) to EDX
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8b/copy 0/mod/indirect 2/rm32/EDX . . . 2/r32/EDX . . # copy *EDX to EDX
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# initialize benchmark data into ESI
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8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 6/r32/ESI 0xc/disp8 . # copy *(EBP+12) to ESI
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81 0/subop/add 3/mod/direct 6/rm32/ESI . . . . . 4/imm32 # add to ESI
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# initialize loop counter i into ECX
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b9/copy-to-ECX 0/imm32/exit
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# while (i/ECX < n/EDX)
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$kernel-string-equal:loop:
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39/compare 3/mod/direct 1/rm32/ECX . . . 2/r32/EDX . . # compare ECX with EDX
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74/jump-if-equal $kernel-string-equal:break/disp8
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# c1/EAX, c2/EBX = *s, *benchmark
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b8/copy-to-EAX 0/imm32
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8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX
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bb/copy-to-EBX 0/imm32
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8a/copy 0/mod/indirect 6/rm32/ESI . . . 3/r32/EBX . . # copy byte at *ESI to lower byte of EBX
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# if (c1 == 0) return false
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3d/compare-EAX 0/imm32
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74/jump-if-equal $kernel-string-equal:false/disp8
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# if (c1 != c2) return false
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39/compare 3/mod/direct 0/rm32/EAX . . . 3/r32/EBX . . # compare EAX with EBX
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75/jump-if-not-equal $kernel-string-equal:false/disp8
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# ++s1, ++s2, ++i
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41/inc-ECX
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46/inc-ESI
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47/inc-EDI
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# end while
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eb/jump $kernel-string-equal:loop/disp8
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$kernel-string-equal:break:
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# if (*s/EDI == 0) return true
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b8/copy-to-EAX 0/imm32
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8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX
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3d/compare-EAX 0/imm32
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75/jump-if-not-equal $kernel-string-equal:false/disp8
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$kernel-string-equal:true:
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b8/copy-to-EAX 1/imm32
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eb/jump $kernel-string-equal:end/disp8
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# return false
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$kernel-string-equal:false:
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b8/copy-to-EAX 0/imm32
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$kernel-string-equal: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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# . epilog
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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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# - tests
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test-compare-null-kernel-string-with-empty-array:
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# EAX = kernel-string-equal(Null-kernel-string, "")
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# . . push args
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68/push ""/imm32
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68/push Null-kernel-string/imm32
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# . . call
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e8/call kernel-string-equal/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, 1, msg)
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# . . push args
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68/push "F - test-compare-null-kernel-string-with-empty-array"/imm32
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68/push 1/imm32/true
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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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c3/return
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test-compare-null-kernel-string-with-non-empty-array:
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# EAX = kernel-string-equal(Null-kernel-string, "Abc")
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# . . push args
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68/push "Abc"/imm32
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68/push Null-kernel-string/imm32
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# . . call
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e8/call kernel-string-equal/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, 0, msg)
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# . . push args
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68/push "F - test-compare-null-kernel-string-with-non-empty-array"/imm32
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68/push 0/imm32/false
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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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c3/return
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test-compare-kernel-string-with-equal-array:
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# EAX = kernel-string-equal(Abc-kernel-string, "Abc")
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# . . push args
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68/push "Abc"/imm32
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68/push Abc-kernel-string/imm32
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# . . call
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e8/call kernel-string-equal/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, 1, msg)
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# . . push args
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68/push "F - test-compare-kernel-string-with-equal-array"/imm32
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68/push 1/imm32/true
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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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c3/return
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test-compare-kernel-string-with-inequal-array:
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# EAX = kernel-string-equal(Abc-kernel-string, "Adc")
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# . . push args
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68/push "Adc"/imm32
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68/push Abc-kernel-string/imm32
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# . . call
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e8/call kernel-string-equal/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, 0, msg)
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# . . push args
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68/push "F - test-compare-kernel-string-with-equal-array"/imm32
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68/push 0/imm32/false
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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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c3/return
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test-compare-kernel-string-with-empty-array:
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# EAX = kernel-string-equal(Abc-kernel-string, "")
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# . . push args
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68/push ""/imm32
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68/push Abc-kernel-string/imm32
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# . . call
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e8/call kernel-string-equal/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, 0, msg)
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# . . push args
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68/push "F - test-compare-kernel-string-with-equal-array"/imm32
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68/push 0/imm32/false
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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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c3/return
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test-compare-kernel-string-with-shorter-array:
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# EAX = kernel-string-equal(Abc-kernel-string, "Ab")
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# . . push args
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68/push "Ab"/imm32
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68/push Abc-kernel-string/imm32
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# . . call
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e8/call kernel-string-equal/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, 0, msg)
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# . . push args
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68/push "F - test-compare-kernel-string-with-shorter-array"/imm32
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68/push 0/imm32/false
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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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c3/return
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test-compare-kernel-string-with-longer-array:
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# EAX = kernel-string-equal(Abc-kernel-string, "Abcd")
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# . . push args
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68/push "Abcd"/imm32
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68/push Abc-kernel-string/imm32
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# . . call
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e8/call kernel-string-equal/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, 0, msg)
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# . . push args
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68/push "F - test-compare-kernel-string-with-longer-array"/imm32
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68/push 0/imm32/false
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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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c3/return
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== data
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Null-kernel-string:
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00/null
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Abc-kernel-string:
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41/A 62/b 63/c 00/null
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# . . vim:nowrap:textwidth=0
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