Multitask OS  (MOS)

MOS is a simple round-robin scheduling preemptive OS.  I will show how to use interrupt to write a concurrent program supported by a task-switcher. 

A process consists of its data structure called Process Control Block (PCB).  PCB contains process's PC, process's SP (and other attributes). Each process has its own Stack.

The main part of MOS is its scheduler.  The scheduler has a queue of the pointer of PCB. 

In the following example, I will show how two processes are created and then the operating system starts.
 
process1()
  ...
process2()
  ...

main()
  p = newp()   // create a new process
  p.PC = &process1
  p.SP = newStack()
  enqueue(p)   // put it in the process queue
  p = newp()
  p.PC = &process2
  p.SP = newStack()
  enqueue(p)
  boot()       // start the scheduler

We create two PCBs for process1() and process2() and put them in the process queue.  The scheduler is an Interrupt Service Routine.  It is invoked when an interrupt occurs.  The task of the scheduler is to save the current active process (its PC and SP including local states, all registers) and set up for the next process to run by restoring its PC, SP, registers.  How the actual execution of the real processor instruction occurs will be shown. It required the knowledge of low level instruction of a real processor.

// this is invoked when interrupt occurs
tswitch()
  p = current process
  save current context of p
  p = nextp()                //  get the next one in the process queue
  restore the context of p   // it uses p.SP
  PC = p.PC
  reti                       //   return from int, jump to the current p

That is all for a task-switcher.  When an interrupt occurs, the current process is interrupted. Its context is saved in its Stack.  The next process is selected and its context is restored. Then, finally, jump to that process which make it active until the next interrupt occurs. 

The rest is just the handling of data structure and operations on them that are required.

newp()
  allocate a block of memory for storing PC, SP and other attributes
  return a pointer to this block (called PCB)

newStack()
  allocate a block of memory as the stack for a process
  return a pointer to this block

Next is the operations on the process queue.  The process queue is a fixed size array storing pointers to PCBs.  A global variable "nump" stores the number of process in the queue.  The end of queue is denoted by 0.  The terminated process is denoted by 1. 

nextp()
  from the present queue index
  scan for the next process
     skip the "1"
     if the "0" is found go back to the beginning of the queue.
  update the queue index
  return the pointer to PCB

enqueue(p)
  put p to the end of the queue
  nump++

The last mysterious thing is boot(). Actually it is rather simple. It works in a part just like the switcher.  It sets up PC and SP then jumps to start the first process in the queue.

boot()
  PC = currentp.PC
  SP = currentp.SP
  jump to PC        // this act needs low level instruction sequence

The remaining question is "What happen when all processes terminate?".  Some how we must check that there is at least one process in the queue (during nextp()). When "nump" is zero the operating system should shut down (the simulation stops).

Let us makes all the above code concrete by coding in S2 assembly language.

Extended Instructions

To simplify the writing of MOS, those support functions for MOS are implemented as extended instructions using "trap n" in S2.1.  The following functions are included in the simulator s21mos.  You need to read mos.c in the tool package s21mos.zip to see how they are written.

The process queue is managed internally inside the simulator (that is, it is outside of S2 space).

The rest are for supporting auxiliary functions:

Memory Map

Task Switcher

The main part of MOS in the task switcher. The implementation is S2 assembly language follows the pseudo code explained earlier.

Then we get the next process (trap nextp returns r27) and check whether there is any process in the queue.

    trap nextp
    jf r27 exit       ;; no process in the queue

And restore the context of the next process.  r27 holds the pointer to the process (PCB).

    st r27 currentp   ;; update currentp
    ld sp @1 r27      ;; get p.SP
    resr sp           ;; restore context

Then switch to the next process using "rest r20" follows by "reti"

    ld r20 @0 r27     ;; get p.PC
    rest r20          ;; RetAds = p.PC
    trap ei   
    reti

Critical Section

The task-switcher is not interruptible, so we must protect this section of the code.  Using Disable Interrupt (trap di) and Enable Interrupt (trap ei) to enclose the code to prevent interrupt during running of this code.  This is called "critical section".

Example

I create two processes, one process counts to 5, another process counts to 10.  The interrupt interval is set to 30. The output shows the distinction between process1 <.> and process2 [.].  Please observe the frequency of interrupt and the concurrency of two processes.  Once the process1 is terminated, process2 continues to its end.

The full assembly code is here (mos.txt).  Here is the screen dump.

C:\s21i\test>sim21i mos.obj
load program, last address 62
>g
<1> interrupt
[1] [2] interrupt
<2> <3> interrupt
[3] [4] interrupt
<4> <5> interrupt
[5] [6] [7] interrupt
[8] [9] interrupt
[10] interrupt
stop, execute 248 instructions
>q

C:\s21i\test>

You can trace the execution of this program step-by-step and watch what happen when the interrupt occurs, how the task-switch works.  I put additional information on the simulator when run it single step.  It shows all relevant register values, process queue, and two PCBs.   Here is some screen dump.

MOS in Assembly

Here is the assembly code (mos2.txt) that implement  newp(), newStack(), enqueue(), terminate(), nextp() in S2 assembly code (instead of special trap instructions).  The way to boot MOS is slightly improved.  We made a dummy zeroth process which is not in the process queue and start the task switcher to switch to the task in the process queue by software interrupt, int 0.  Please use the s21mos2.zip  package to run the code.

    jal link newp
    st sp @1 retval
    st retval currentp  ;; zeroth p
    trap ei
    int 0             ;; start by switch p

Please try it.  I set the interrupt interval to 100 (in s21.h).  If you change it, you can observe the change in the number of interrupt. 

Enjoy!

last update  13 Apr 2016  Happy Songkran!