Date   : Tue, 13 Sep 1994 08:31:24 -0600
From   : barnett@... (David Barnett)
Subject: Address modes

Chris lam wrote:
>
>on a more technical note, a 6502 emulator should strictly check all
>absolute addresses to see if they fall on paged I/O. in Frak! i've seen
>
>  BIT &FE68
>  BIT &70
>
>i.e. the BIT &FE68 serves purely to access &FE68 and so reset the T2
>interrupt flag in the user VIA.
>
>at the moment, i'm only checking absolute addressing modes. i hope this
>is adequate!
>

The best way not to miss anything is to make your 6502 emulation work the
way the chip does:  Make all memory references go through one process which
then checks for memory mapped addresses.  That way, you can cover obscure
effects (including an indexed addressing bug in the chip on an indexed read
which can touch two locations a page apart).  You are also covered for the
extended opcodes of the 65c02 used in the Master.

David Barnett