[stella] New jumptable implementation.

Subject: [stella] New jumptable implementation.
From: R Mundschau <rmundsc@xxxxxxxxx>
Date: Thu, 02 Jan 2003 21:20:52 -0600

This is my first post, so let me start by introducing myself.  My name is
Rob Mundschau.  I never programmed for the 2600 before, but I do have
considerable experience programming the Commodore 64.  Lately I have grown
interested in the 2600 Homebrew scene, and I hope to make some games.

I have been browsing the archives, and I haven't found any mention of
technique I know of for implementing a jump table.  Excuse me is this was
mentioned before, and I missed it.  The smallest implementation I saw on the
list so far is:

# Assume index is in X
    LDA TableLo,X
    STA Vector
    LDA TableHi,X
    STA Vector+1
    JMP (Vector)

# Put Lo and Hi jump tables here.

As an alternative we can use the stack in combination with a RTS command:

# Assume index is in X
    LDA TableHi,X
    LDA TableLo,X   # I hope I pushed these in the right order...

# Put Lo and Hi Jump tables here:

Using the PHA instructions saves 2 bytes of ROM.  The RTS takes one more
clock cycle than the indirect JMP instruction so the routine takes one clock
more overall.  There is one more catch.  You must subtract 1 from the
addresses in the Jump table because the RTS instruction adds 1 to the PC
value it loads from the stack.  If the Lo byte is $00, then you must
subtract 1 from the Hi byte as the borrow for the subtraction.


Rob M.

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