Z1 Bus signals

[tonyw]

I'm building an external Flash ROM reader/writer/programmer to plug into an A500 (look, I've got one already, OK?). Most of the bus signals are pretty mundane, but what about the UDS and LDS signals? LDS means Low Data Select (bits 7:0), UDS means Upper Data Select (bits 15:.

When do I need to use them? Is it during a Write (Byte) operation, or a Read (Byte) operation, or when?

tony

[tonyw]

Thanks for that, Cyb.
 
That's more or less what I understood. Since I'm going to write the code anyway, I can guarantee the transfer type.

What determines whether it's going to use the high or low byte? Is it an even/odd address, like this?

If I read.b 0x0080 0000, CPU asserts LDS and only looks at bits 0-7;
If I read.b 0x0080 0001, CPU asserts UDS and only looks at bits 8-15;
If I read.w 0x0080 0000, CPU asserts both and looks at the whole word.

If that is right, then I need not worry, since I'm not going to use the upper 8 bits at all. I'm just going to map the bus address to the ROM address so that each 16-bit bus address maps to an 8-bit ROM byte.

Hopefully, I can just use LDS as the strobe for everything.

tony