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Created: Tuesday, July 11 2006 07:53.11 CDT  
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Alignment check
Author: EliCZ # Views: 4595

AM bit of CR0 is set on Windows x64, so it is possible to generate AC faults -> it is possible to test 64bit (why only 64bit?) apps for unaligned memory access. Even MS code sometimes does mov [rxx], ryx; where rxx = x*4 (should be x*8). AC fault results in exception 0x80000002 that silently (w/o error box) terminates the process (like touching stack page before guarded one but AC fault is caught by debugger). SetErrorMode(SEM_NOALIGNMENTFAULTEXCEPT) has no effect.

/////EnableAC.bat file
;@GOTO -)
.CODE
EnableAC PROC C
  SMSW EAX
  TEST EAX, 40000h
  JE   NotEnabled
  PUSHFQ
  OR BYTE PTR [RSP+2], 4
  POPFQ
  MOV  AL, 1
  RET
NotEnabled:
  MOV  AL, 0
  RET
EnableAC ENDP
DisableAC PROC C
  PUSHFQ
  AND BYTE PTR [RSP+2], NOT 4
  POPFQ
  RET
DisableAC ENDP
END
:-)
@ECHO OFF
ML64.EXE /nologo /c EnableAC.bat
PAUSE
CLS

/////Test.c file
#include <stdio.h>
char EnableAC(void);
void DisableAC(void);
int main(void) {
  char c[16];
  printf(EnableAC() ? "AC enabled.\n" : "AC not enabled.\n");
  __try {
    *(short int *)(c+1) = 0x1234;
  }
  __except(1) {
    printf("Exception.\n");
  }
  DisableAC();
  return(printf("End.\n"));
}




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