andrew kelley <piclist@mit.edu> wrote:

> How would automatic intelligent banking for call's and goto's work
> multiple code page PIC's (in a linker or assembler)? I mean, if
> you were to have a piece of code on the boundary that would be
> pushed onto the next page with the addition of banking
> instructions,  how would the assembler know what to do for max
> efficiency? 

Andrew:

The piclist.com list archive seems to be offline at the moment, so 
I'll just copy an email I sent a few years ago...

------- Forwarded Message Follows ------

Steve Hardy <PICLIST@MITVMA.MIT.EDU> wrote:

> Yes, the perennial phase error.
> ....
> I encountered this sort of problem when writing a Z80 assembler.
> The Z80 has two different flavours of branch: JR and JP.  JR only
> had a short range (and takes 2 bytes) whereas JP could jump
> anywhere (3 byte instruction).  I wanted to implement a pseudo-op
> called 'J' which used the shortest possible opcode JR or JP.  It
> turned out that to optimally implement this, the assembler would
> potentially have to make a large number of passes, refining its
> estimate of the value of each label each time.  It is also
> conceivable that the assembler could get stuck in an infinite
> loop. 

Steve:

This is slightly off-topic, but there's a really simple algorithm 
that solves the "branch/jump optimization" problem in linear time:  

    1.  Start with all branches set to the shortest size. 

    2.  Put all the branches on a stack. 

    3.  Pull a branch off the stack; if it's out of range, increase
    it to the next-larger size (some processors have 3 or more branch
    sizes) and put all the branches that SPAN the just-increased
    branch on the stack. 

    4.  Repeat step 3 until everything's stable. 

This algorithm was first shown to me by Dr. Cliff Click, who works in 
Motorola's PowerPC compiler group.  As I said, it runs in linear time 
and is guaranteed not to get stuck in an infinite loop.  

I can send you the rigorous mathematical proof if you're 
interested... It depends on the fact that monotonic functions over 
complete lattices have a unique minimal fixed point.  

-Andy  

------- End of Forwarded Message ------

=== Andrew Warren - fastfwd@ix.netcom.com

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