## Checking null-terminated 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 05[0-2]*.subx -o /tmp/tmp52
# $ subx run /tmp/tmp52 # 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 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-'.
# 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
# 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
# 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
# pseudocode:
# initialize n = b.length
# initialize s1 = s
# initialize s2 = b.data
# i = 0
# for (i = 0; i < n; ++n)
# c1 = *s1
# c2 = *s2
# if c1 == 0
# return false
# if c1 != c2
# return false
# return *s1 == 0
#
# initialize s into EDI
8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 7/r32/EDI 8/disp8 . # copy *(EBP+8) to EDI
# initialize benchmark length n into EDX
8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 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
# initialize benchmark data into ESI
8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 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
# initialize loop counter i into ECX
b9/copy-to-ECX 0/imm32/exit
# while (i/ECX < n/EDX)
$kernel-string-equal:loop:
39/compare 3/mod/direct 1/rm32/ECX . . . 2/r32/EDX . . # compare ECX with EDX
74/jump-if-equal $kernel-string-equal:break/disp8
# c1/EAX, c2/EBX = *s, *benchmark
b8/copy-to-EAX 0/imm32
8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX
bb/copy-to-EBX 0/imm32
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
# ++s1, ++s2, ++i
41/inc-ECX
46/inc-ESI
47/inc-EDI
# end while
eb/jump $kernel-string-equal:loop/disp8
$kernel-string-equal:break:
# if (*s/EDI == 0) return true
b8/copy-to-EAX 0/imm32
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
# return false
$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
# call 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
# call 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(Abc-kernel-string, "Abc")
# push args
68/push "Abc"/imm32
68/push 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
# call 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(Abc-kernel-string, "Adc")
# push args
68/push "Adc"/imm32
68/push 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
# call 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(Abc-kernel-string, "")
# push args
68/push ""/imm32
68/push 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
# call check-ints-equal(EAX, 0)
# 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(Abc-kernel-string, "Ab")
# push args
68/push "Ab"/imm32
68/push 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
# call check-ints-equal(EAX, 0)
# 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(Abc-kernel-string, "Abcd")
# push args
68/push "Abcd"/imm32
68/push 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
# call check-ints-equal(EAX, 0)
# 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
== data
Null-kernel-string:
00/null
Abc-kernel-string:
41/A 62/b 63/c 00/null
# vim:nowrap:textwidth=0