KERNEL SYSTEM CALL INTERFACE:
The kernel system calls are all done through interrupt 0x80
All parameters are passed in the 'AX,BX,CX,DX,DI,SI' registers.
The AX register contains the system call number.
The BX,CX,DX,DI,SI registers contain the first 5 arguments from the stack.
(NB If the syscall is know to have less than 5 args the rest are not loaded)
On return from the syscall AX has the return value.
If AX is -ve then errno= -AX and return val = -1;

The system calls are named in syscall/syscall.dat.
There is a script syscall/mksyscall which generates the assembler for the
system calls, near the top there is a line:
  COMPACT=1
If this is changed to
  COMPACT=0
the code generated will be slightly faster and larger.

-RDB

KERNEL SIGNAL INTERFACE:
  It is assumed the kernel will never pass a signal to the userspace
  routine unless it's _explicitly_ asked for!

The Kernel need only save space for _one_ function pointer
(to system_signal) and must deal with SIG_DFL and SIG_IGN
in kernel space.

When a signal is required the kernel must set all the registers as if
returning from a interrupt normally then push the number of the signal
to be generated, push the current pc value, then set the pc to the
address of the 'system_signal' function.

This is in syscall/signal.c

-RDB