Byte-addressable memory?

[jgalak]

Is there any way to have memory that is byte-addressable rather than word-addressable?

I'm trying to build a very simple RISC-V processor, which calls for Byte-addressable memory and I can't figure out a way to do it. Ideally I'd want both ROM and RAM.

I suppose it can be built up from individual register or even flip-flops, but aside from the complexity of doing this for a large memory space, I wouldn't be able to load a hex file into such memory...

[mengstr]

Not sure exactly what you want, but what I would start looking into is simply taking four of 8-bit RAMs, connect the address lines in parallel on all of them (except maybe A0 and A1) depending on how I would like to address the memories in non-32 bit mode.

Then I would put some multiplexers at the data outputs of the RAMs. In normal mode the four RAMs would connect their data to a separate part of the data-bus (0-7, 7-15, 16-23, 24-31). Or let one (any) of them be muxxed into D0-D7 (and the others be disconnected).

Of courser there will be some issues with the chip selects and the A0 and A1 depending on which mode you are access ing the memory in in any particular moment. But a bit of logic and decoders connected to your MOP control lines would solve that quire easily as soon as you have defined at a low level exactly what to do in each mode.

[hneemann]

You can find a RAM32Bit in Library->Rams.
It is a 32-bit RAM with a byte address and an AM flag that selects the word, half-word or byte mode. It also has a Sign flag to enable sign extension. Remember to set the size in the generic section of the component.

[jgalak]

Oh, that's perfect for RAM, and I see it's actually built up from smaller RAMs. That solves half of my problem, but I would also really like a ROM that could do this, one that could load a HEX file. I don't imagine the same approach would work here, at least not unless I "striped" my file across 4 separate files....

[jgalak]

Maybe I need to build a bootstrapper circuit that loads the contents of a word-addressable ROM into this RAM , at startup...

[hneemann]

This approach also works for ROMs, as the HEX loader can also distribute the read file to several ROMs. The ROMs must all be marked as “program memory” and have names whose alphabetical order determines the byte order.

[jgalak]

Oh, that sounds excellent. I'll play around with that.

[jgalak]

After much fiddling, got it working. Thank you