Re: Hardware-based vs software-based emulation

On 02/18/2017 12:08 PM, groepaz@gmx.net wrote:
> On Saturday 18 February 2017, 12:01:05 Gerrit Heitsch
> <gerrit@laosinh.s.bawue.de> wrote:
>> On 02/18/2017 09:38 AM, groepaz@gmx.net wrote:
>>> On Saturday 18 February 2017, 08:24:26 smf <smf@null.net> wrote:
>>>> On 17/02/2017 20:26, Gerrit Heitsch wrote:
>>>>> That can be fixed by using a static RAM instead of DRAMs.
>>>>
>>>> It can be fixed by latching signals as well.
>>>>
>>>> My point is that a 100% VIC II in an FPGA would have the VSP problem, or
>>>> it's not 100%. AFAIK vice doesn't have the VSP problem either.
>>>
>>> actually x64sc can emulate it :) (its optional, "VSP Bug Emulation" in
>>> VICII settings)
>>>
>>> and no, the problem can _not_ be fixed (only) by using SRAM - that was
>>> tried and proven wrong long ago. its an urban myth that doesn't want to
>>> die :)
>> Well, the VSP-Problem happens because DRAMs use destructive readout,
>> meaning even a read cycle has to write the data back to the cells. This
>> happens inside the DRAM once /RAS goes high. This is not the case with
>> SRAM, a read cycle will just read the contens of a cell without
>> destroying them. So when you have a problem with address lines changing
>> while /RAS goes down and your latch latches the wrong address, you might
>> read the wrong data, but you CANNOT destroy the RAM contents this way as
>> you can with DRAM when you confuse the row decoder.
>>
>> I have done a SRAM replacement for the C64 using a 128KB SRAM, a
>> 74HCT573 and a 74HCT32 and so far no one was able to detect the VSP bug
>> when using it.
>
> as said, others have done it too and found that the VSP bug still exists - to
> remove it for certain you must add an additional circuit that prevents those
> writes to wrong addresses from happening. (but then, with such circuit, you
> dont need sram)

Again, there are NO writes, VIC only does read cycles, its R/_W line is 
input only and you cannot destroy data in an SRAM with a read cycle.

VSP destroys data due to the way DRAM works.

Got a link to where someone claims to use an SRAM and still have VSP 
problems?

  Gerrit



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