Date   : Wed, 20 Jul 1994 21:03:33 +0100
From   : jfid@... (James Fidell)
Subject: Re: Calculating branch addresses

JAMES WARD wrote:

> Isn't there a bug in the 6502, in that it doesn't carry when it adds
> up indirect address and thus pulls the indices from the wrong pair of
> memory locations?  I seem to remember reading something to this
> effect in the Advanced User Guide.
> I can't remember the details off the top of my head, but if this is
> true then it's worth being aware of.  I am not sure whether 65C02
> etc. duplicate this bug/feature or not.

I think there's a problem that occurs with the JMP (indirect)
instruction when it's given an address which is the last byte in a page.
In this case it will take the low byte of the jump address from the last
byte in the page and the high byte of the jump address from the first.

There's also some oddness with certain zero page instructions which
causes the address register to wrap around to the start of the page
rather than continuing into page one, but I can't remember what these
are off the top of my head.

James.

-- 
 "Yield to temptation --             |
  it may not pass your way again"    |     jfid@...        
                                     |
        - Lazarus Long               |              James Fidell