Re: How hardware accelerators deals with $0/$01

Am Dienstag, 24. Oktober 2017, 07:39:26 CEST schrieben Sie:
> On Fri, Oct 20, 2017 at 01:10:01AM +0200, groepaz@gmx.net wrote:
> >Am Donnerstag, 19. Oktober 2017, 22:22:44 CEST schrieben Sie:
> >> On 10/19/2017 10:03 PM, Mia Magnusson wrote:
> >> > Are we sure that the 6510 I/O port isn't accessible from external
> >> > hardware when the CPU is in DMA/three state mode?
> >> 
> >> Yes... The address lines of the 6510 are output only, same for the R/W
> >> line. Also an access by the 6510 to $00 and $01 does not appear on the
> >> address and data bus.
> >
> >not entirely true - the address appears on the bus (remember, a write to
> >$00/ $01 _does_ write to the RAM also!) only the data does not.
> 
> Right. On a C64 or C128 where the open address space (such as
> $de00-$dfff when nothing is connected to I/O1 or I/O2 on the
> cartridge port) acts deterministically, the writes to $00 or $01 would
> essentially copy the data from the previous VIC-II memory access to the
> RAM location 0 or 1. If I remember correctly, VICE correctly runs my
> program "dadb" that partially runs in the color RAM. I suppose it would
> also correctly emulate the writes to 0 and 1.
> 
> It is also possible to read from 0 or 1 with the help of the VIC-II. I
> do not remember doing this myself, but an obvious solution would be to
> read the data using the sprite collision registers.

yes yes and yes :) i wrote a test program for the latter a while ago: https://
sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/general/ram0001/

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<Jamie Zawinski>



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