Re: CBM2 v9958 Hires-Color Graphics Card

From: Segher Boessenkool <*segher_at_kernel.crashing.org*> Date: Fri, 1 Feb 2019 00:27:39 -0600 Message-ID: <20190201062739.GV14180@gate.crashing.org> · segher_at_kernel.crashing.org

On Thu, Jan 31, 2019 at 02:55:56PM +0000, smf wrote:
> On 31/01/2019 08:24, vossi wrote:
> >My 6509R4 and the 6509R7 have both this "feature" ;)
> >Is this a know bug of the 6509?

It is documented behaviour of the 6502.

> I believe all 6502's do a dummy read before a write with STA (zp),Y
> 
> With STA $xxxx,Y you only get the dummy read if you cross a page
> boundary when adding Y, but it looks like they missed out this
> optimation on STA (zp),Y

You also get the dummy read with absolute-y-indexed addressing.
See http://archive.6502.org/books/mcs6500_family_hardware_manual.pdf
A 3.4 on page A-6 for example (and A 3.6 on page A-7 for indirect-y).

Is any of this changed with 6509?

Segher