ZX Club #05 29 октября 1997

Программирование

Club IM2 - Terminology and theoretical aspects of interrupts and multitasking.

          IM2 and multitasking



  (C) Dr.Gluker 1997



      The article, "club im2" electronic newspaper ZX_Club N4 
emerged some terminological confusion, which requires, in my 
humble opinion, some clarification. Let me remind you, it was 
about interrupts and multitasking. In this article I would like

to refer to a purely terminological and
theoretical aspects of these issues.


      Firstly: the interruption - just
mechanism that can be used for
his work application (or,
speaking a language other problem) and, by and large, no 
relation to multi-tasking is not available; and multitasking  - 
The ability to run two (or more) independent (!) Applications 
at the same time. Another thing that interrupt timer than it 
actually is and is im 2, can be used as

mechanism for multi-tasking.


      Why did not consider running in interrupt procedures 
independent tasks? All the matter in the very mechanism 
interrupts. This question can pereinachit: why interrupt is 
named interrupts. Actually, the answer is

in the question itself. Preyvaniya interrupt the execution of 
the basic (!) Program to perform some kind of short-term 
procedure, that can not be inserted into the main loop

program. In this case (by convention) the interrupt handler 
must save the environment and the main program after execution 
recover as the judges. This makes every programmer, while 
maintaining the stack registers team push and restoring their 
team pop.



      Another thing - the problem, it can be divided into three 
main sections: software stack segment isponyaemogo code segment

data. Any full-fledged, working program always (!) Carries 
three of these element. Moreover, the Z80 processor can

perform only one task and have only
a stack: refers to the hardware implementation. For example, 
Motorolla 68000 is two stacks and the possibility of switching

therefore, to realize multi-tasking on
Z80 can only programmatically by using the interrupt mechanism
timer.


      Next: what would finally clarify the issue of preemption, 
it is necessary touch on some aspects of the hardware

implementation of the Speccy. The fact that the hardware 
interrupts our favorite made very inflexible, I would even say 
that - after sleeves, in connection with which we are deprived 
of a very many of the features that provide

standard interrupts. That is, on the Speccy
there is only one worthy of mention
type of interrupt - im2, which happens
every 1 / 50 second, and third is not given.
Since we previously known clock
CPU frequency (3.5 MHz), we can uniquely calculate how many 
cycles can make a processor for one interrupt, and

namely - 70000. This refers to variant
The Pentagon, on the other machines can be
smaller but no less than 68000. What we said is the "magic" 
number? A case in that the more cycles expended

procedure is interrupted, the less they will remain the main 
program, and not excluded Alternatively, when the interruption, 
"eat" all machine cycles and not one "tick" will not leave the 
main program, which will lead to it stops.



      Now, returning to the multi-tasking, we explain the 
theoretical implementation thereof by the example of 
multi-tasking BASIC. So, we need to Speccy at once performed 
two independent (!) BASIC program. To implement, we need The 
following mechanisms: 

 -Sohrannenie environment to perform tasks in

  extended memory;

 -Restoration of the environment problem and satisfied
  of thereof;

 -Sharing mechanism of computer time

  between two tasks.

So, what should be preserved? Yes all
system! Ie - the domain of variables
text of the program, the stack of the program.
With the restoration of the problems seem to be too
should not be. Then you should pay
attention to the mechanism of task switching.
Here it will be useful to us and im2. Organize a counter which 
is set a certain number of interruptions, during which the 
first program will be executed, and interview him. As soon as 
the ripe, we retain all engine parameters, to stop the task, 
copy it to not available (!) other programs area

memory, load the following programs and
run it on the execution of the interrupted
moment. As you can see, to realize multi-tasking BASIC is not 
so difficult - in theory. But in practice, will encounter Deep 
problems: lack of memory very slow switching, slow execution of 
every program, the restriction on the amount of occupied 
memory, inability to use the procedure im2, and etc., etc. 
Thus, I can guarantee you the futility and senselessness of 
such an undertaking. 


      Actually, that's all I
wanted to say about multi-tasking on
"Antiquity" Speccy.


                . Oooo

                () Oooo.

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