For anyone who wants to test if your custom mapper does things in the order you expect I have created a mapper tester.
You can download the 128kb SCC mapper compatible ROM file here.
If all goes well the output looks like this:
If you get another order after flashing your ROM your mapper does not work with sequential segments. In other words when you select segment 0 you get 4, etc. This could be because the higher address lines that address the segments or not in the sequential order we would expect. It could be mirrored or flipped.
P.S. this is best visible when flashing on a separate programmer and then booting on the MSX.
P.S.2 you can find the code on my GitHub to make it compatible with ASCII8 or 16 if you want. Compile with sjasmplus.
MSXUSB PCB issue solved.
While working with gaduke on his MSXUSB build I checked what was causing his problems. The ROM mapper somehow was stuck on segment 0. The above tester ROM showed for every bank segment 0.
Then I remembered that in an earlier revision of the PCB I added a botch wire to solve similar issues. Then I got thinking. I had my new KiCAD design files on GitHub. But had I generated new Gerbers? I found out I had not.
So if you are still planning building a MSXUSB then make sure you are downloading v1.0. This is mentioned on the silkscreen. If you have the older CPLD version you can fix your PCB by added a botch wire as shown here:
I am sorry if you got stuck in your own build because of this.
