[Also sent to cctalk, which appears to be down at the moment.] Fred Jan Kraan wrote: I don't know of another ROM version, but I can explain I/O port EF. It is used for drive select, density select, and side select. Bits 3 down to 0 are the drive select bits, which are active low. At any given time, either all four should be high (no drive selected), or one should be low and the other three high. Bit 6 is side select, which should be 1 for side 0, and 0 for side 1. A single-sided drive will ignore this, but for a double-sided drive it needs to be set to the correct value, even if the medium is single-sided. Bit 7 is 0 for FM (single density), and 1 for MFM (double density). The boot ROM appears to only ever write 4E or 4F (hex) to port EF. 4E will select drive 0, FM mode, side 0, while 4F selects no drive. This makes me wonder whether the documentation for bit 7 is correct; do Model II boot floppies use a single-density boot sector?

If the selected drive is double-sided, Port E0 bit 1 should read 0 for a single-sided diskette in the drive, and 1 for a double-sided diskette, as sensed by the index sensor of the drive. (Double-sided diskettes have the index hole in the jacket at a different angular position, and double-sided drives have two sensors, to distinguish single-sided from double-sided media.) Later versions of the FDC card also allow the WD1791 FDC chip to be reset by writing to port E8. The data is ignored. Aside from some of my own comments that have been added, this information is summarized from the Radio Shack "Technical Reference Manual TRS-80 Model II Catalog Number 26-4921 Revised Floppy Disk Controller Supplement". Any errors above are almost certainly mine. Eric