42S local label caching

[Sylvain Cote]

Hello Dave,

That is a lot of questions

I do not remember the details of the HP-42S implementation, but since it is a close simulation of the HP-41, here are the guidelines for the HP-41 ...

(11-11-2019 05:54 PM)Dave Britten Wrote:  Is that distance literally stored directly in the GTO or XEQ instruction inside the program?

Yep! ... for local labels only, 2 bytes GTO/XEQ for short distance LBL 00 to LBL 14 and 3 bytes GTO/XEQ for long distance for LBL 15 and up

(11-11-2019 05:54 PM)Dave Britten Wrote:  Or is it held elsewhere in memory?

Nop! ... see above

(11-11-2019 05:54 PM)Dave Britten Wrote:  How long are these cached branching distances kept?

Until the program is modified or packed, when that happens, all local GTO/XEQ distances are reset to zero

(11-11-2019 05:54 PM)Dave Britten Wrote:  Do they persist between multiple invocations of a program?

Yep!

(11-11-2019 05:54 PM)Dave Britten Wrote:  How does this work exactly?

When a GTO/XEQ is encountered and the distance field is set to zero, then the FOCAL interpreter then do a scan in search of the requested LBL, if label is found and the distance is within allowed reach then the number of bytes to jump is stored in the GTO/XEQ itself.

(11-11-2019 05:54 PM)Dave Britten Wrote:  What about if you XEQ a global label in a different program area? Will local label lookups inside that program be cached if you call that same external program multiple times inside your program?

Global GTO/XEQ are different beasts and global LBL are looked up every time.

Sylvain

Ref.: Synthetic Programming on the HP-41C, by W.C Wickes, pages 16 & 17