Today, I learned about two uncommon but risky situations with the VidHD card for the Apple II that can result in data loss. John Brooks and I discussed the details and I’ve edited and updated the manual on our website. In brief:
- Do not have an unprotected disk in your 5.25″ drive during power up, or data loss can result.
- Avoid accessing the VidHD Control Panel during disk writes, or data loss can result.
Most users do not encounter the first problem as they boot to a menu, or boot from a hard drive or other mass storage in the first six seconds, so the 5.25″ drive is not active during the VidHD DMA. If the boot disk is write-protected, the startup DMA may cause an I/O error, but it will not write to the floppy.
For those booting an unprotected floppy disk, workarounds include:
1. Press Ctrl-Reset at startup to turn off the drive, or
2. Hold Ctrl-Reset for six seconds while powering on the Apple II (delaying floppy boot until after VidHD DMA).
Unfortunately, a firmware update will not fix this, as the problem is in VidHD’s bus bridge chip and/or inherent in doing DMA independently of the 6502 CPU, while the 6502 is doing critically-timed floppy access. John also said that it may be possible for data loss to occur on an Apple IIGS with a 3.5″ drive, but no problems have been reported to Blue Shift Inc.