Deunan wrote on 2024-07-25, 16:12:
You understand that you are not measuring the ISA bus speed here, but the overall performance of the system, right? Is that a fast 486 with on-chip L1 talking to the bus, or something else? Pure DOS/BIOS with it's code path meant to run on 8086, or some optimized device driver?
So I'm working with an AMD 486 @ 133 Mhz w/ 0 wait state memory, 256KB Cache, shadowed ROMS, no Himem or EMM386.
All ISA hard drive transfers at <= 2MB/s when using a non caching ISA storage controller, even when reading from the drive cache. This is with the stock BIOS, MrBios, XTIDE, SIIG Enhanced BIOS, EZDrive overlay, etc.
When using a caching ISA IDE controller, it can read at 4MB/s from the controller cache, but gets something like 1.4MB/s when reading directly from the drive, as is the nature with those caching cards.
The computer is fast enough to transfer at 4MB/s. The ISA is fast enough to transfer at 4MB/s. The drives are fast enough when operating in a fancier ATA mode.
Just seems like it must be ATA overhead that is keeping things a lot lower than they theoretically could be, but maybe I don't understand the issue.