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return during "int"
Posted: Tue Jun 28, 2005 5:39 pm
by GLneo
my system call set up works like this:
Code: Select all
_int_handler_set:
push eax
push ebx
push ecx
push edx
mov eax, _int_handler
call eax
iret
which gets called after int 30h, which in turn calls:
Code: Select all
void int_handler(volatile unsigned int EDX, volatile unsigned int ECX,
volatile unsigned int EBX, volatile unsigned int EAX)
{
if(EAX == 1)
free((void *)EBX);
else if(EAX == 2)
EAX = (unsigned int)malloc(EBX);
else if(EAX == 3)
cls();
else if(EAX == 4)
putchar((char)EBX);
else if(EAX == 5)
putint(EBX);
else if(EAX == 6)
puts((unsigned char *)EBX);
else if(EAX == 7)
attr((short)EBX);
}
but i think whats wrong is the functions get called and after they do what they do, they "ret" but because it has been after an "int" it should be a "iret" so the sys goes crazy, help???
Re:return during "int"
Posted: Tue Jun 28, 2005 9:21 pm
by Brendan
Hi,
GLneo wrote:but i think whats wrong is the functions get called and after they do what they do, they "ret" but because it has been after an "int" it should be a "iret" so the sys goes crazy, help???
The C functions get "CALLed" from within the assembly stub, and therefore should use a "RET" to return to the assembly stub.
The problem is your assembly stub, which pushes a pile of general registers onto the stack that are never popped off again. Try changing to:
Code: Select all
_int_handler_set:
pushad
push eax
push ebx
push ecx
push edx
call _int_handler
add esp,16
popad
iret
Cheers,
Brendan
Re:return during "int"
Posted: Wed Jun 29, 2005 3:08 am
by Pype.Clicker
oh, btw, you might want to write free, malloc, etc. into a syscall_table[] and issue
rather than having them in a bunch of "if ... elsif .. elsif ..."
Re:return during "int"
Posted: Wed Jun 29, 2005 7:54 am
by GLneo
thx, brendan, that fixed it!
@Pype.Clicker: how would i do that, just have "syscall_table" have an array of pointers to functions?
Re:return during "int"
Posted: Wed Jun 29, 2005 8:40 am
by Pype.Clicker
yes, that's roughly the idea ...