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12.01. Why translation?




Description

This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).

12.01. Why translation?

Both the 'int13' software interface used by the BIOS to communicate with the outside and the Cylinder/Head/Sector (CHS) fields in the partition table reserve

o 10 bits for the cylinder field, for a total of up to 1024 cylinders;

o 8 bits for the head field, good for up to 256 heads;

o 6 bits for the sector field, which gives a maximum of 63 sectors since for historic reasons the sector field starts at sector 1, not 0.

The maximum disk capacity accessible through the traditional int13 interface is therefore 8GB (1024*256*63 sectors of 512 bytes). In some books, you may encounter references to 12-bit cylinder numbers; this extension (using the upper two bits of the sector field) was never widely implemented and isn't supported anywhere.

Now IDE disks have their own set of limitations; these disks, no matter if they're ATA/IDE or ATA-2/EIDE, use

o 16 bits for the cylinder field, giving 65536 cylinders;

o 4 bits for the head field, or only 16 heads at most;

o 8 bits for the sector field.

This is good for a maximum disk capacity of 128GB. However, combine this with the BIOS limitations and you suddenly can't see more than the first 1024 cylinders of the IDE disk, which makes for a limit of just 504MB or 528 million bytes. This is unacceptable today. In the long term, the BIOS limit of 8GB is just as unacceptable, but as a short term solution it is desirable to get the maximum out of the standard int13 interface with IDE drives. This is where translation comes in.

 

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