OS dev: Procedure that ends a task.

[Alex_]

Hello.
I have an assignment to make a procedure that will end a task. The manual from the university isn't quite clear about how this works. So mainly i have this problem: How does a procedure end a working task?
1) Does the bios call the procedure ? or,
2) Will my current operating system call my procedure? or,
3) I must make myself an os that boots, launches a task, and then after an amount of time calls the procedure to end the task and bail out.
I have no idea!
The manual does give some sort of code(which should be changed by me to work properly) that shows how it should look like. But once i don't understand the way it works, how can i know what to change?
The code with comments (translated by me) from the manual:

PROC	newint20h		; int 20h
	call	_di

;keeping back any interruptions so that the task may not be launched when the processor is taken away (me: what does "taken away" mean????)
 	pop	ax
	pop	ax	;Extracting from stiva (me: I don't know the true translation of stiva. But one thing for sure: it's a FILO type memory acces)
	pop	ax

;extracting the leftovers that are left from the execution of int 20h (we don't need the CS,IP indicators any more) (me: What int 20h? Are they talking about THIS procedure or the system's one.)
	push 	cs
	pop	ds
	lea	di,[cs:firsttask]	; DI = Offset of TASK
	mov	al,[cs:numtask]	; AL = Task's ID
	xor	ah,ah		; AX = Task's ID
	push	 dx
	mov	dx,515
	mul	dx	; AX = the starting index of firsttask
	pop	dx
	add	di,ax	; DI = putting the top of the stiva in di

;Putting a 255 value into task's ID , marking it as non-loaded.
	mov	[Byte Ptr cs:di],255
	mov	dx,[cs:di+3]
	call	FreeMemory
	call	_ei
	jmp	int08new
; Freeing the memory, and beginning the execution of int 8;
	ENDP	newint20

[NotNull]

It might be that the 16-bit code you have been given
pertains to when a task requests to terminate on a
single tasking operating system.

Or if it refers to multi-tasking, then it is not just an
intermediary but free's a task from a list of tasks
to execute and free's its memory, and transfers
control back to the task-scheduler.

INT 08 is called by the system clock on IBM-PCs.

I think it might be for multi-tasking.
It is possible in 16-bit assembly to multi-task applications.

[NotNull]

push cs ; if CS==DS then [firsttask] same as [cs:firsttask]
pop ds
lea di,[cs:firsttask]   ; Offset of some array of data structures. See below
mov al,[cs:numtask] ; Current task ID
xor ah,ah 
push dx  
mov dx,515 ; hmmm. ID is index into 515 byte data items??? 
mul dx         
pop dx
add di,ax    ; It must be... Because FIRSTTASK+(ID*515)
mov [Byte Ptr cs:di],255 ;clears first byte of data struct to 255
mov dx,[cs:di+3]  ; retrieves the address of some of its memory.
                            ; in DOS a data struct is made for each
                            ; block of memory, for ex.
call FreeMemory
call _ei
jmp int08new  ; if int08 is on the same segment, if not jmp far

Same thing happens when you index into an array of
words:

lea di, [myarr]
mov bx, index
shl bx, 1
mov word [myarr+bx], 1234h

[NotNull]

Since its getting rid of FLAGS, CS, and IP on the stack [SP=SP+6],
I would assume this doesn't pertain to multi-tasking but
a procedure of similar function to the INT 20h DOS interrupt.
This would mean the TASK calls the procedure in order to terminate.
It looks like its just keeping information about each task in a
large contiguous array of data structures, and keeping track of
the task that was currently yeilding system resources, instead
of using a PSP (like DOS), so it doesn't need the caller's CS value.

[Alex_]

Thanks very much notnull. But i still don't understad how to make from all that a procedure that ends a task.
:(

[NotNull]

Let's see.
Does it have to be based on the code you showed?
Does it have to be a working program?
Does it have to manage multiple tasks?

If all else fails try making a program that loads a binary image
as an overlay, and set up an interrupt handler
for termination that points into your loading program.

INT 21/48-4A Memory Management
INT 21/4B03 Load Overlay
INT 21/25 SetIVT

You'll have to allocate the block of memory for the
overlay yourself.
Use a indirect far jump into the overlay, starting at whatever
offset is the entry point:

push es
xor ax, ax
push ax
retf
;or
push es
xor ax, ax
push ax
mov bp, sp
jmp far [bp]

Make sure to include a call to your termination interrupt within
the overlay.

[Alex_]

Uhm. I'm confused.
I actually understood my assignment wrong. So i don't have to do that anymore... I'm sorry for bothering. But i appreciate your help! Thanx