jobs.doc

MMURTL(tm) Computer Operating System Ver x0.8, source code.

Copyright 1991-1993, R.A. Burgess

JOB Management

Each program that is loaded in MMURTL is called a JOB.  A structure call a Job Control Block (JCB) is used to keep track of the job and some of its resources.  The JCB also keeps track of some context information such as the job's current path and the command line passed in to the application.

Job Path

Don't confuse the term Path for the MS-DOS equivilent.  In MMURTL, a job's path is simply a prefix used by the file system to expand and access files.  With any filesystem operation (opening, renaming, listing, etc.), if you don't specify a full file specification (including the drive), the file system appends your specification to your current path.

Using Job Management Calls

The calls Chain(), ExitJob(), LoadNewJob(), and SetExitJob() all work together to allow you load applications either by replacing yourself (the application that is calling Job calls), or by creating new application in the context of new job number (a separate Job Control Block.

Chain() replaces your application by allowing you to specifiy the name of a MMURTL run file to load replacing the program that called Chain().  Context information is saved in the JCB so you can set the Command Line and the Path in advance for the application.

SetExitJob allows you to specify what runfile (if any) will be executed when you call ExitJob().  If there is no valid run file specified in the JCB, the JOB it terminated.


Theory of operation

Understanding how MMURTL manages jobs will assist you in operations such as Chain or LoadNewJob.

How a New Job is Created

Each job is allocated and assigned the following resources when it is loaded:

1) A Page Directory to manage its linear memory space.  
2) An initial page table.
3) A 4K virtual video text screen
4) An initial stack (size specified in RUN file)
5) Code pages (size specified in RUN file)
6) Data pages (size specified in RUN file)
7) Initial task (with start address specifed in RUN file)


The File JOBC.C contains the public code that actually allocates the resources, loads, and finally executes the new job.

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