[Dr_Jefyll]

> it might be helpful to have an example or two

Apologies. The original source code from the early 1980's is stored on a non-DOS floppy. Maybe someday I'll retrieve and publish it. But meanwhile here's a taste:

One of the tasks is to activate a solenoid to ink up the lithographic plate after a certain number of revolutions of the press. That number, range 0 to 9, is read from a 4-bit, binary weighted thumbwheel switch which has been set by the press operator.

The code uses 1 instruction to test bit0 of the thumbwheel switch. If bit0 is true, we fall through to another test that waits (jumps to self) until the tachometer pulse goes high; this eventually falls through to another test that waits for the tachometer pulse to go low. So, that's one revolution. But if bit0 of the thumbwheel switch is false, we jump past all this (ie, don't wait for one revolution).

Bit1 of the thumbwheel switch is similarly tested, except the tach must go hi-lo twice (2 revolutions); and so on for bits 2 and 3 (4 revs and 8 revs). Then, with all the counting complete, the solenoid gets turned on.

I have simplified somewhat; actually the rev-counting portion is a subroutine, called from two different places in the code. The (very primitive!) subroutine calling convention is explained in the article. HTH.

[mbreese]

That does help! Thanks for writing this up. It’s really quite interesting.