Re: CBM2 v9958 Hires-Color Graphics Card

Den Thu, 31 Jan 2019 02:24:32 -0600 (CST) skrev vossi <vossi@ceffy.de>:
> Hi,
> at testing my new CBM2-v9958 graphics card I had a problem to write
> to the VDP-VRAM.
> After some non-successful testing with longer delays between reg and
> vram access, I started my LA and found an interesting fact:
> The 6509 performs a short read access before it writes with STA(ZP),Y
> from bank1 to bank15 (screenshot)!
> I tried again with code in bank15 $0400 using STA absolute,X and the
> 6509 only writes correctly.
> I also measured a read from bank1 to 15 and thats only a plain read.
> 
> My 6509R4 and the 6509R7 have both this "feature" ;)
> Is this a know bug of the 6509?

What does a standard 6502 do in this situation?

STA (ZP),Y afaik takes 6 cycles, but it only needs to do four reads and
one write cycle, so there is one cycle where it only does internal
operations, and since the 6800/6502 architecture lacks any way to tell
the world that it doesn't want to do a bus access at all during a
cycle, it will of course do a read and discarding the result.

If a 6502 does the same thing (except for the bank switching, of
course) this should be considered expected, although unwanted.

If a 6502 does this too, then someone might have stumbled upon it
in some other situation.

I can't think on any existing hardware on a 6502 based computer that
would be affected by the thing you have observed. All general RAM can
tolerate an extra dummy read, and I/O chips where each access
increments a counter (like the VDP in a C128) would probably always be
accessed with absolute addressing mode.
 
> PS: After setting up the v9958 VRAM-address I write to the VRAM via
> port #0, but the VDP saw the read access and increases the address
> counter so that the write-byte land one byte later in the VRAM and I
> had a displacement by one char on the screen.
> 
> BTW: The v9958 has 128k dedicated VRAM and does 256x212 with 256
> colors, 512x212 with 16 colors, 512x424 interlaced and 256x212 with
> 19k colors. It also supports 32 sprites, sprites collision detection,
> hardware commands like higspeed-move, line, pixel, search and
> H/V-softs-crolling. It has direct RGB output with PAL or NTSC
> software selectable ans screen adjusta.
> 
> <http://cbm-hackers.2304266.n4.nabble.com/file/t374762/6509indirect.jpg> 


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