PC floppy to Amiga back to PC

[amigaksi]

>by trekiej on 2009/3/19 13:19:02

>He has 8088 to 286 era machines.

I recall having an AT&T 12Mhz 80286 Machine when I was in college and did add a 1.44MB floppy drive and it worked as long as you used an updated DOS and the correct format parameters.  Something like format A:/F:1440/s/v.  That machine came originally with 1.2MB 5.25" floppy drive.   As long as your 1.44MB floppy disk is using less than 720k space, you should be able to write a conversion program to dump the 1.44MB disk contents to a 720K floppy disk assuming you have a high density drive.  It would be easier to do on PC though.  Unless this is one of those King's Quest I type games which directly read sectors from disks and don't really have a file system.

By the way, I wasn't aware but that 80286 machine was sold for $5 with memory expansion cards, VGA card, both floppy drives, 20MB hard drive, Sound Blaster, and some good DOS games.  Good meaning they directly wrote to the VGA memory during vertical retrace and directly wrote to the DMA data going to the sound card.  I paid $4999 for this machine with it's upgrades.  So $4999-$5 = $4995/$4999 so this lucky guy got a 99.9% discount.

[amigaksi]

>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.