VAX/VMS program to put current time in your DCL prompt

  A while back (a good while back, in the 80s) I was experimenting with "process permanent" things in VAX/VMS - ways to make code and data that survives the rundown of an image, and can do things while your process is just sitting at the DCL prompt or running other normal programs. You know, like what the DOS era PC types called "terminate and stay resident".

  In service to this idea, I wrote a simple program that loads into your process's P1 space, and at one minute or one second intervals, formats the current time and stores in in your DCL prompt. I had grown weary of having people at work that would say  "I  can  do it at home on my PeeCee, why can't we do it on the VAX?"

  The program allocates some space in your process's P1 space, and copies the code to do the work to it. That code calls $settimr to schedule an AST that modifies your prompt with the current time. It also reschedules itself to run again after a minute (or second) have elapsed.

  P1 space doesn't get run down when an image exits, so all of this endures through the life of your process. Not really all that much to it. 

  Since the code is copied to a different address than it was linked at, is is necessary for it to be PIC - position independent code. That's not too tough to do on a VAX - you access locations that are within the code being moved using addressing modes that are relative to the PC, and you access locations that are outside of the code being moved by absolute addresses.

  That is. inside the code to be relocated, use

              mov    zeep,r0

 

       (other code and data)

zeep:     long    666

  The mov     zeep,r0 uses relative mode, and after relocation, zeep is still the same distance away from the instruction as it was when it was assembled.

  If zeep is not part of the code being relocated, but is a location outside of it, use absolute mode to access it.

    mov    @#zeep,r0

  That's it in a nutshell. Register mode is always good

    mov    r2,r5 

  Is PIC,.But, if you are moving or using addresses in the registers, it will take a little thought to keep it  PIC.

  SImple, nicht wahr? Well, it gets easier with practice.

    So here it is - minute_clock will update every minute. Second_clock will update every second.

minute_clock.mar

second_clock.mar

  To use...

$ mac minute_clock

$ mac second_clock

$ link minute_clock

$ link second_clock

$ run minute_clock

13:29>

or

$ run second_clock

$ 13:29:23>

  Per login session, don't run more than once, or run both versions - these programs don't unload until you log out, so it would be wasteful to get more than one copy running in your process.