[OpenBIOS] PATCH: RFC: Rework control flow stack words to use 2 data stack items

Mark Cave-Ayland mark.cave-ayland at siriusit.co.uk
Mon Dec 7 12:20:52 CET 2009

Hi all,

After some more debugging this weekend, I've found that r638 is not the 
correct fix for resolving backwards branches. Re-reading the OF spec 
lots of times and searching the internet for information on the Forth 
control flow stack has now changed my understanding of how this should work.

Currently, OpenBIOS uses 1 stack item per control flow stack item which 
works fine when executing control flow words directly in the 
interpreter. However, Fcode evaluation requires slightly different 
semantics for bbranch and b?branch (since they can be used as part of 
different control flow words with slightly different behaviours) and so 
in order for this to work, we also need to know the *type* of a control 
flow stack item. In particular, we need to know if it is a source 
address or a dest address in order to be able to resolve nested control 
flow statements correctly.

Please find attached my current working patch for extending the control 
flow stack so that each item now takes 2 data stack words: an address, 
followed by an address type. I have a separate patch for the Fcode 
evaluator, which when applied in conjunction with this patch actually 
gets to the point where it attempts to read the file from the ISO image. 

However, since this patch involves some reasonably low level tinkering, 
I'd like to put it out for review first. Please can people with some 
Forth experience (e.g. Stefan, Tarl, Laurent) take a look at this and 
check that it's okay to apply?



Mark Cave-Ayland - Senior Technical Architect
PostgreSQL - PostGIS
Sirius Corporation plc - control through freedom
t: +44 870 608 0063

Sirius Labs: http://www.siriusit.co.uk/labs
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