User:Johnburger/Demo/Ints/User

To demonstrate one technique of allowing User Tasks to request functions from the Executve, I have added two INTs:

• A HLT implementation, since HLT is a Supervisor-only instruction, and User Mode code gets a GPF if it tries it;
• A Yield implementation, allowing the User Mode code to give up the rest of its time-slice.

Note that these two functions are small, since they're essentially only calling functions that are already implemented. Also note that they're single-use INTs: I could have used a register as a Function holder, similar to the way that BIOS calls use AH, but I've left that as an exercise for the reader!

Demo/Ints/User.inc

;
; Ints/User.inc
;

; This module demonstrates providing System Services to a User-mode program.
; The interrupts have been given a DPL of 3, to allow the User program access.

;...............................................................................
; HLT is a privileged instruction, since it can lock up the PC. But for demo
; purposes, an interrupt has been set up for the User program to call.

Ints.Halt:
                HLT
                IRETD                   ; That was easy!
;...............................................................................
; This interrupt does the same Task Switch that happens on the Timer call.
Ints.Yield:
                CLI                     ; Disable interrupts!
                PUSH            EAX
                CALL            Ints.Switch
                POP             EAX
                IRETD