[Info-vax] How much of VMS is still in MACRO-32?

[Arne Vajhøj]

On 6/1/2021 8:33 PM, Michael Moroney wrote:
> On 5/31/2021 9:07 AM, Simon Clubley wrote:
> 
>> Is there any self-modifying code in VMS ? (I hope the answer to that
>> is no, BTW. :-))
> 
> There's self-modifying code on the CDC 6x00 systems -- IN THE HARDWARE.
> You see, the procedure call instruction (RJ nnn) jumps to address nnn+1 
> (60 bits) and writes a branch to the instruction after the RJ 
> instruction at address nnn.  To return from the procedure, branch to the 
> entry point nnn and execute the (branch) instruction written by the RJ 
> instruction.
> 
> Obviously this prevents recursive code or anything like that.
> 
> If I recall correctly, a register save/restore procedure (ab)used the RJ 
> instruction to write a bunch of self-modifying code.

I remember that.

Pascal could do recursion.

Fortran could not.

Unless one wrote two small Compass routines to get and set the
saved address and managed that in a small array.

Arne