ericr@ipmoea.UUCP (Eric Roskos) (10/18/87)
>Does anyone know how to format floppies to a size of 400K? > > I know it was done under some P-system (not Pecan) which did not depend on > MS-DOS. I suppose that one formats 10 sectors on a track instead of the > standard 8 or 9, (though I wonder whether 5 doubly long sectors could have > been used instead). The ROM BIOS supplies an interrupt service for formatting > various sector sizes and numbers, but it seems to depend on a mysterious > disk parameter table with abstruse entries for intersector gaps and other > arcana not documented. That was done on NCI's version of the UCSD p-System; you are correct that it formatted the disks with 10 sectors/track instead of 9 by changing the format table. To format disks this way, you had to change the "gap length for format" and "gap length" parameters (offsets 5 and 7 in the table) to a smaller value. Unfortunately, that was 4 years or so ago, and I don't remember what the smaller values were... I think they were roughly half of the values the BIOS uses (2AH and 50H). I don't know why there are different gap lengths for Format vs Read/Write operations... ? The Intel manual is fairly cryptic, it just says that the number is the "gap 3 size," which is the number of bytes of gap between sectors (according to the manual). On a 96TPI drive, you could get 800K on a floppy disk (this was back before the AT; you had to buy a Tandon TM100-4 drive.) The SAGE (now Stride) machines also used this diskette format. (NCI was a firm in Vancouver, Canada; I still have their manuals, their address is A106-1093 W Broadway, Vancouver, BC Canada, V6H 1E2. I don't know if they are still in existence. They produced a faster and somewhat more enhanced version of the p-System than was available from SofTech; this was around 1982.)