[Coco] Pop PC with empty stack?

[Bob Devries]

Hi Lothan,

If your code sample is at the end of a subroutine, the PULS PC will 
effectively be an RTS.

Regards, Bob Devries

--
Besides a mathematical inclination, an exceptionally good mastery of one's 
native tongue is the most vital asset of a competent programmer.

Edsger W.Dijkstra, 18 June 1975

----- Original Message ----- 
From: "Lothan" <lothan at newsguy.com>
To: "CoCo List" <coco at maltedmedia.com>
Sent: Tuesday, April 06, 2010 2:08 PM
Subject: [Coco] Pop PC with empty stack?



> I'm hoping someone can help refresh my memory on the internals of OS-9. 

> Specifically, I'm seeing this code:

>

> PSHS Y

> ...

> PSHS X

> ...

> LDA ,-X

> CMPA #$30

> BCS L016C

> CMPA #$39

> BHI L016C

> ...

> L016C LDD 2,U

> PULS X

> PULS Y,PC

>

> That it's popping X and Y from the stack makes perfect sense because those 

> two registers were pushed onto the stack previously. At this point the 

> stack should be empty, so what happens when it attempts to pop PC from the 

> stack? Is this effectively the same as OS9 F$Exit?

>

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