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From: Ettore Perazzoli
Date: 1998-09-17
Subject: Re: Tagged format was: Snapshot files for emulators

Ettore> 21 16 Machine ID

Rajnai> What is this for Plus/4? And are those values (and other
Rajnai> values) little of big endian?

You decide the ID for the Plus/4. :-) The ID can just be a string,
padded with zeroes; x64 simply uses `C64'.

Values are little endian (LSB first).

Ettore> Every module has a 16-byte long signature which is unique
Ettore> to that type of module (e.g. `MAINCPU', `1541CPU', `CIA1',
Ettore> `CIA2' etc.); for now, order does matter (so the signature
Ettore> is just used as a safety check),

Rajnai> No, please do not depend on position. This will cause a
Rajnai> really hard way to expand format. (Always should be sure:
Rajnai> you coordinate the whole development, and no additional
Rajnai> informations will be available.) And: if there is an
Rajnai> unknown "module" (let's call hunk, as on Amiga always
Rajnai> did), you can simply skip.

This has just been fixed. I was aware it was not a good thing to do;
just the first try was done this way to slightly simplify coding.

Ettore> 32 4 info about the last opcode

Rajnai> Is this necessary?

Yes. If a branch instruction such as BNE is executed, branch is taken and no page boundary is crossed, there is an extra cycle of delay for dispatching the interrupt. Moreover, we have to know whether the last opcode has enabled (previously disabled) IRQs: this adds another extra delay.

I could kludge this by increasing the IRQ/NMI clock tick value, but I think it's cleaner this way.

Rajnai> What are these fields? I don't know what BA means.

The BA line is used by the VIC-II to steal cycles to the CPU during bad lines, or for fetching sprite data. When BA is down, the CPU cannot do any read accesses, and as soon as it tries to read from the memory, it is blocked until BA goes up again. Write accesses happen as usual instead.

I need to know about the last BA fetch, again for IRQ/NMI bookkeeping.

Rajnai> What about other hunks?

Do you want me to list all the other modules? These are probably not relevant to the Plus/4, as the hardware is different.

I could also list the 1541 format, but it is not decided yet; Andreas
(Boose) is taking care of it.

--
Ettore