#425 FDC CPU emulation for PET style drives (4040 etc...)

[Daniel Kahlin]

Currently the FDC CPU (6504 in the case of the 4040) is stubbed of in src/drive/ieee/fdc.c and emulated as job-codes only with a special case for executing the downloaded formatting code.
To be complete, the actual FDC CPU should be emulated.

There is useful information at: https://www.pagetable.com/docs/anatomy-4040.html