System Calls

Aquila Kernel is monolithic, the only way to request a kernel function is through a system call.

System Calls are requested in arch. specific manner, each arch. should export arch_syscall_return which is responsible for returning value to the calling process upon syscall satisfaction.

All kernel syscall are defined in kernel/sys/syscall.c in syscall_table with their numbers, arch calls a syscall by executing syscall_table[syscall_nr](arg0, arg1, ...)

x86

On x86, syscalls are requested using int 0x80 instruction with syscall number in eax and arguments in ebx, ecx and edx. The interrupt is caught by interrupt function defined in kernel/arch/x86/cpu/isr.c and passed to arch_syscall function defined in kernel/arch/x86/sys/syscall.c which then calls the relevant syscall, its implementation looks like:

void arch_syscall(regs_t *r)
{
    /* Sanity Checking */
    void (*syscall)() = syscall_table[r->eax];
    syscall(r->ebx, r->ecx, r->edx);
}

Implemented syscalls

POSIX standard

void exit(int status)

• Standard
• Number: 1
• Arguments:
  • int status: Termination status of process
• Returns: N/A
• Errors: N/A

int close(int fildes)

• Standard
• Number: 2
• Arguments:
  • int fildes: File Descriptor to close
• Returns: int
  • Success: 0
  • Error: -errno
• Errors:
  • -EBADFD: Bad File Descriptor