I'm working on the virtual memory piece of a Forth for the Commodore PET 2001 and it requires both a PLAY and a RECORD deck, simultaneously available as devices #1 and #2. Fiddling around a bit with vice 2.3.dfsg-2 (Versatile Commodore Emulator) on Ubuntu Linux, I could only get one cassette drive. It works on the real hardware. Is there something I'm missing here or is it just a limitation of the emulation software?

* crickets *

I suppose I can use SEQ files on an emulated 8050 on the Vice/Xpet for development, and real datasette decks on the real hardware

I'd only be able to help by digging into VICE sources, so probably not the person to help. It sounds like an interesting idea: you're swapping to one device and using the other conventionally for data?

I would imagine that it would be straightforward to add "another deck" to the emulator, but frankly I didn't even know a PET could HAVE two cassette decks until this post showed up.

Back in the day I tried being able to append to a cassette based system, but it didn't work out. My data set was small enough to store in RAM, so I just copied over the tape each time.

The Commodore PET 2001 has two cassette ports (devices #1 and #2) while the VIC-20 & C=64 had only one (device #1) with device #2 unused (memory confidence 95% on that assertion).

The virtual memory scheme involves waiting for a FLUSH (Forth's write operation) and having the user press play on one deck and record on the other deck. Every block (1024 bytes) is copied in order from the play deck (#1) to the record deck (#2), with the in-memory modified blocks being inserted at the appropriate times to replace the (now obsolete) blocks on the play deck.

Six 192-byte data blocks per 1024 byte Forth block would mean 1152 bytes per Forth block.
2 bytes of block number (meta info)
1024 bytes of block data
126 bytes unused

At the end of copying the entire virtual memory space from one tape to the other, both decks are stopped and rewound. The next FLUSH operation would copy in the other direction from (#2) being the play deck writing to (#1) as the record deck

Hm, I wrote the xpet emulator in VICE, and IIRC I only re-used the tape code that already existed for the C64 and the other platforms.

So my guess is that you have to extend the emulator to include a second tape simulation for your requirements

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Author of the GeckOS multitasking operating system, the usb65 stack, designer of the Micro-PET and many more 6502 content: http://6502.org/users/andre/