Curiosity

[John Woods]

> All right, you guys!  I've been watching this conversation fly by for
> some weeks now, and I have one burning question to ask:
> 	What the #%$@ does the MARK instruction do??!?

[Hoping no one else answers...]
MARK N (bit pattern 0064NN) does the following:

SP <- PC + 2 x NN
PC <- R5
R5 <- (SP) + NN	= number of parameters
		(This according to my Microcomputer Processor Handbook.
		 I think the + on the last line may be spurious)
		 
The way to use this instruction to facilitate function return is "obvious".
(and if you believe THAT, I have this bridge...:-)

Having never seen it used, I can't offer examples of how to use it (and
thinking about it makes my brain hurt!), but a hint: the MARK N instruction
goes somewhere in the stack frame, and you return directly to it, rather
than directly to the subroutine did the calling.

Just goes to show you what happens when you use microcode roms that are too
big :-)
-- 
John Woods, Charles River Data Systems, Framingham MA, (617) 626-1101
...!decvax!frog!john, ...!mit-eddie!jfw, jfw%mit-ccc at MIT-XX.ARPA

Sorry, I don't feel deep right now.