john@hp-pcd.UUCP (john) (07/26/85)
<<<< < I remember when HP first brought out the HP 150, it used 3.5" floppies < that handled only 256K per disk. Then along came the Macintosh, < which used the same 3.5" disks, but by varying the speed of the drive < depending on the seek address, the Mac was able to get 400K on each floppy. The 150 used the HP-9121 3.5" drive which had 286 Kbytes formatted. It used 70 tracks @ 16 sectors/track @ 256 bytes/sector. The extra 10 tracks were used for track sparing and a media revolution count. This format was the same as we used on the HP 82901 5.25" drive so any product that used the 5.25" drive could switch to the 3.5" with no changes in software. After you take out the system overhead the user has about 270K for data files and applications. Our newer products that use the double sided 3.5" drive default to 77 tracks @ 9 sectors/track @ 512 bytes per sector for a total of 710 Kbytes. The double sided drives can also read and write disks with the single sided format so they are upwards compatible. If you format a 3.5" disc with all 80 tracks with 5 sectors of 1024 bytes you can get 819 Kbytes which is more than you would get with a double sided Mac disk. I believe that the Mac gets about an extra 20% capacity by using varible speed but loses about that much by using GCR encoding. The net effect is that Mac provides about the same storage density as the standard IBM format. John Eaton !hplabs!hp-pcd!john