>by trekiej on 2009/3/19 15:38:10
>Anyone have an idea as to when a 720K FDD was installed in a pc?
>I have not a clue. I believe the drive was not around that much. The only other drive I can think of is a 5.25 FDD.
I first saw it in a PS/2 in my school on MCGA machines (circa 1987). MCGA had 320*200*256 and 640*480*16 modes with palette of 262144 (6-bits/primary) and were called like Model 30, 50 and so on. Later machines with PS/2 supported 1.44MB floppies with backward compatibility for 720K. But BIOS/floppy controller were flexible enough that you could put a 720K/1.44MB drive in a machine that supported 1.2MB HD drives and use a driver to access them via Int 13h like:
AH = 2 (function read sector)
AL = # of sectors to read
CH = Cylinder (track # 0..1023)
CL = sector # (1..63) with high 2 bits containing bit 8 and 9 of cylinder number (for hard drives)
DH = disk head (0..15)
DL = Drive letter (0=A, 1 = B, 80h = C hard drive, etc.)
ES:BX = Buffer ptr
The same function was used for hard drives so that's why you see that if you fully use the CX registers and 4-bit DH value, you get 20 bit value with 512 byte sectors gives the 512MB hard drive limit on old DOSes (prior to 4.00). Later with disk translation the upper 4-bits of DH were used so you got a 24-bit value with 512 byte sectors for a 33 bit value (8Gig hard drive limit) which still is the limit for Windows 3.x and DOSes.
720K disks are 9 sectors/track, 2 heads, 80 tracks whereas 1440K disks are 18 sectors/track, 2 heads, 80 tracks.
Amiga disk (880K) are 11 sectors/track, 2 heads, 80 tracks.
Atari ST is something like 9 or 10 sectors/track.