Interesting that that works before any access is attempted! I’d have expected it to need a starting guess for the serial bus address at least. Or does it only work for the default drive that is accessed if you press the RUN key, or perhaps if you OPEN a file to the device first?
Either way, it won’t catch multiple drives. For that you need to loop on such a “read the error channel” operation, once per bus ID, which will error on anything that is absent or, we desperately hope, not actually a storage device — and give you the DOS version message, error 73, for anything that IS a drive. This technique works on the C64’s DOS 2.0 as well if you do the error channel access by hand.
Last Edit: Jun 27, 2015 2:11:42 GMT by gsteemso: Realized I may have misinterpreted the original post.
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Agitator-in-chief for the Seattle Retro-Computing Society