On Mon, 2 Feb 2004, Kroko wrote:
> I have finally finished my first flashcart prototype and I have a question to
> the programming experts :-)
>
> There is a bit of empty space on the CPLD. There are 128K SRAM on the Cartridge.
> I would like this RAM to be available for programming. In theory I could enhance the
> 3F bankswitching to support RAM. We could call it 3F+. 128K/256 that would be 256
> blocks of RAM each with a size of 512Bytes which would be mapped into the first 1K
> of the cartridges address space. (512 bytes write port and 512 bytes read port). A write
> to 3F selects a 2K Rom slice into the first 2K of the cartridge. A write to 3E could select
> a 512Byte RAM slice into the first 1K of the cartridge. So the first 1K could be either
> ROM or RAM, dependent on what address you were last writing to (3E or 3F). What do
> you think about this idea ?
>
> Would this RAM-Slice size be good, or should I make it smaller, lets say 256
> slices of 256Byte ....
Try this:
Think of the SRAM as 128 blocks each of 1024 bytes.
Let 3F control the first 2K of the cart's addressable space:
Writing a value N with the MSB cleared (i.e. from 0 to 127) there maps
blocks N and N+1 into the first 2K, read only (i.e. as if it were ROM).
Writing a value N with the MSB set (i.e. from 128 to 255) there maps
block N as RAM, with the first 1K as read and the second 1K as write.
Let 3E control the second 2K in a similar manner:
Writing a value N with the MSB cleared (i.e. from 0 to 127) there maps
blocks N and N+1 into the second 2K, read only (i.e. as if it were ROM).
Writing a value N with the MSB set (i.e. from 128 to 255) there maps
block N as RAM, with the third 1K as read and the fourth 1K as write.
Did that make sense? Now any part of the 128K SRAM can be mapped as
RAM or ROM.
--
adam@xxxxxxxxxxxxxxxx http://cuddlepuddle.org/~adam/pgp.html
http://cuddlepuddle.org/~adam/resume.html
"The dinosaurs are not around today because they did not have a space program."
-- Arthur C. Clarke
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