i386 System Calls

Software Interrupts

Probably the easiest way to implement system calls on the i386 is via software interrupts. If you add an interrupt or trap gate to the IDT with a DPL of 3, an application can call this interrupt and it will trap into the kernel routine specified.

Arguments for a local function call are usually passed on the application's stack. However, the kernel and application will usually have seperate stacks for purposes of protection. The i386 actually provides a mechanism for automatically transferring arguments between stacks of different protection levels, but for now we'll just pass a pointer to the arguments in a general purpose register (in this case, eax). The return value for the system call will be stored by the kernel into eax before it returns control to the application.

To make a system call (assuming that the software interrupt vector is 0x30), call this function in an application, making sure the parameters are already set up:

void *appSysCall(void *args[]) {   
   void *appReturnValue;
   asm ("int $0x30":"=eax" (appReturnValue):"eax" ((int) args));
   return appReturnValue;

And on the system side, we need to write an assembly wrapper to pass args legitimately into the kernelSysCall() function.

/* assembler follows */

.globl asmkernelSysCall
    pusha                  /* save all registers */
    pushl %eax             /* pass args[] parameter to sysCall() */
    call kernelSysCall    
    addl $4, %esp          /* remove parameter */
    popa                   /* restore registers */
    iret                   /* and return to the application */

/* assembler ends */

void kernelSysCall(void *args[], struct regs r) 
   /*  does stuff according to args[]  */

   r.eax = returnValue;

setExceptionHandler(0x30, &asmkernelSysCall);