ISA bus throughput

Let me add a note on the BIOS-less IDE card for your PC-AT. The biggest problem that I see is that the largest geometry that your PC-AT BIOS tables allow is 1024 cylinders, 17 sectors and 16 heads, which gives you a grand total of 139 MB total. While positively huge for the PC-AT era, it's pretty puny when it comes to IDE drives. An IDE drive in CHS mode can address 16384 cylinders, 63 sectors on 16 heads for a total of 8GB. While not huge by today's standards, it was positively enormous for the time. So, you've got two alternatives. Add a BIOS extension (usually on a disk controller card, but doesn't have to be) to plug the necessary code in during boot, or use an "overlay" (which is really the wrong word) which loads some driver code from the first track of your hard drive, then plugs itself in as sort of a TSR program (actually locates itself in high memory, then adjusts the amount of available base RAM downwards) to intercept disk requests and map them to the expanded geometry. The "overlay" mode (such as that provided by "Disk Manager") has its limitations, but should be okay on a 286. But I personally like the BIOS ROM extension myself. Cheers, Chuck