Page 185 - DCAP507_SYSTEM_SOFTWARE

Unit 11: Programming Languages Concept (II)

11.3 Multitasking Notes

To understand the power of assembly language we discuss how to execute multitasking. In the
debugger our thread of instructions was broken by the debugger; it got the control, used all
registers, exhibited an detailed interface, waited for the key, and then reinstated processor state
to what was instantly before disruption. Our program resumed as if nothing happened. The
program execution was in the similar logical flow.
If we have two dissimilar programs A and B. Program A is busted, its state saved, and returned
to B rather than A. By looking at the instruction set, we can instantly say that nothing can prevent
us from doing that. IRET will return to whatever CS and IP it locates on the stack. Now B is
interrupted someway, its state saved, and we return back to A. A will have no way of recognizing
that it was interrupted as its whole environment has been restored. It never knew the debugger
took control when it was debugged. It sill has no way of gaining this knowledge. If this function
of breaking and restoring programs is performed at high speed the user will sense that all the
programs are executing simultaneously where actually they are being switched to and forth at
high speed.

In real meaning multitasking is simple, even though we have to be enormously cautious when
implementing it. The surroundings of a program in the very simple case is all its registers and
stack. Now to obtain control from the program without the program knowing regarding it, we
can utilize the IRQ 0 highest priority interrupt that is from time to time coming to the processor.

Example: Now we show a very basic example of multitasking. We have two subroutines
written in assembly language. All the techniques discussed here are pertinent to code written in
higher level languages also. However, the code to control this multitasking cannot be simply
written in a higher level language so we write it in assembly language. The two subroutines
rotate bars by altering characters at the two corners of the screen and have infinite loops. By
hooking the timer interrupt and saving and reinstating the registers of the tasks one by one, it
occurs that both tasks are executing concurrently.

Example:
001 ; elementary multitasking of two threads[org 0x0100] jmp start ;
002 ax,bx,ip,cs,flags storage areataskstates: dw 0, 0, 0, 0, 0 ; task0 regsdw 0, 0, 0, 0,
003 0 ; task1 regsdw 0, 0, 0, 0, 0 ; task2 regs current: db 0 ; index of current
004 taskchars: db ‘\|/-’ ; shapes to form a bar ; one task to be multitaskedtaskone:
005 mov al, [chars+bx] ; read the next shapemov [es:0], al ; write at top left of
006 screen
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Contd...

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