ISA MFM disk controllers

On 24 Jun 2009 at 10:43, Dave McGuire wrote: I've always been a little peeved at the disk drive utilities that determine "optimum" interleave for an AT-style MFM controller. There are really two dimensions to the problem--and no interleave "optimizer" that I know of deals effectively with them. The first, of course, is how fast your PC and controller can get data off of a drive. Choosing a too-small interleave can have disastrous effects on performance because you can get to the point where only a single sector per rev is being transferred. A too-large interleave has a much lesser effect on performance, since the PC is waiting only a sector time or two to get the next sector. In other words, on may not notice a large subjective difference in performance if the interleave is at 4:1 instead of 3:1. The other aspect of the problem is how fast an application can read data--that is, how much processing is going on between disk accesses. Setting the interleave to a value that an optimizer comes up with can have the same effect as a too-small interleave. That is, the application must wait a whole revolution before data is available. Some old CP/M systems used three interleaves (I've got at least one sample of this in my collection). The boot tracks are formatted with a 1:1 interleave, as they're simply streamed into memory with no processing; the directory area is formatted at 2:1 because directory searches aren't usually processor-intensive and the remainder (data area) of the disk is formatted at at 3:1. Ultimately, the solution is a controller with a large buffer or cache or a cache driver in the PC. Then the controller essentially becomes the limiting factor and the "optimum" interleave determined by a utility is essentially the correct one, since reads are occurring a track at a time. But for non-cache, sector-at-a-time setups, it's better to choose an interleave that's slightly larger than recommended. Similarly, if a controller can support multi-sector reads at unity interleave, it makes sense to interleave clusters rather than sectors. FWIW, --Chuck