4GB DISK LIMIT

2/20/99

4GB Disk Limit/missing space on 8GB drives: This is a problem with pre-1998 BIOSes from Phoenix, and probably from other vendors that may prevent partitioning of IDE hard drives in the 4 to 8 GB range using Fdisk. The problem comes about because the CHS translation algorithm used in BIOSes works by halving cylinders and doubling heads until cylinders are less than 1024. Autodetected disk parameters frequently start with 16 heads which is doubled to 32 at 504MB, 64 at 1.1GB, 128 at 2.1GB and finally to 256 heads at 4.2GB. Unfortunately, while 256 heads are acceptable to the drive; are passed properly through INT13; and are documented as being acceptable in MSDOS, in reality, neither DOS nor Windows 95 can handle more than 255 heads.

The workaround is to override the autodetected/recommended parameters and specify 15 heads and the largest integer number of cylinders less than 16/15ths of the autodetected/recommended parameters. BIOSes that are aware of the problem may do this automatically or may truncate 256 heads computed to 255. In either case, there will be a reduction in the space available below the theoretical 8.4GB that DOS can use of either 0.4% (if 255 heads used by the BIOS) or 6.25% (if 15*16=240 heads). Some of the algorithms in use lock sectors per track at 63 -- which can make a drive configured in one machine unreadable on another with an incompatible algorithm.

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