MegaST v1.1: floppy disk support (internal drive, read only)

Floppy disk support: the MEGA65's internal 3.5" drive works as the ST's drive A: (read only, #15). Original Atari ST disks, including copy protected games, are read as on a real ST. Please report which disks work and which don't in #15.

New

• "Real floppy drive as A:" in "System settings" (off by default), then "Reset Atari ST": drive A: is the disk in the MEGA65's drive. The core has a complete WD1772 floppy controller (WF1772IP by Wolfgang Förster), so copy protections and formats with 10 sectors and 82 tracks work. Drive B: and the floppy images are not used while it is on.
• "Floppy read assist" in "System settings" (on by default): the MEGA65 drive's speed varies by up to 8 % within each turn of the disk, and the WD1772 then misreads a sector now and then; some loaders keep the wrong bytes and hang. A data separator in front of the controller re-times the drive's pulses, and a sector that fails its checksum is read again (up to 15 more turns) before the ST gets it.
• Tested on an R6 with an original-format Jim Power disk (82 tracks, 10 sectors): boots and loads the game with no read errors.

Limits: read only (the core never writes to the disk; games that save to the disk get an error), DD disks only. A worn or badly written disk can still fail even if it reads in a PC drive: an older copy of the test disk had a weak spot that needed 26 turns. The R3/R3A, R4 and R5 cores are built from the same source but not tested with the drive yet.

Files

The menu has two new lines: copy the new 104 byte atarist/stcfg from the zip to the SD card. The 102 byte file of 1.0 is reported as corrupt and the settings are not saved. The 1536 byte atarist/stmount stays valid.

File	Board	Status
MegaST-R6-v1.1.cor	MEGA65 R6	developer's board, floppy drive tested
MegaST-R5-v1.1.cor	MEGA65 R5	not tested yet, please report
MegaST-R4-v1.1.cor	MEGA65 R4	not tested yet, please report
MegaST-R3-v1.1.cor	MEGA65 R3/R3A (no SDRAM)	1.0 working on an R3A (#2), floppy drive not tested; 512 KB ST RAM, no Viking, no STEroids

Known limitations: the right button of an Atari mouse is a short click only and works about once every 10 seconds (#17); a tester saw some programs crash with 8 MB of ST RAM on an STe (4 MB works).

All changes: VERSIONS.md, manual: README.

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