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12.04. What is in the Enhanced Disk Parameter Table?




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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).

12.04. What is in the Enhanced Disk Parameter Table?

In a standard BIOS, the Fixed Disk Parameter Table (FDPT) contains information about the geometry of the harddisk(s). It more or less contains the same information as the drive type entry in the CMOS setup. A program that wants to use the harddisk on a low level, bypassing DOS, normally uses the BIOS' int13 functions to achieve this.

The Enhanced fixed Disk Parameter Table (EDPT) is an extension of the ordinary FDPT that makes use of undefined fields to provide information about the translation mode used. It uses a magic number (A0 in byte 3) and a checksum (in byte 15) to ensure that software cannot mistake random data for an EDPT. This practice is more or less standard across various flavors of translating BIOSes, with differing magic numbers.

On top of this, the Phoenix Enhanced BIOS standard specifies a number of extended int13 functions and a 16 byte FDPT extension. The latter contains detailed information about the current PIO or DMA type, block mode used, LBA or (E)CHS addressing, 32 bit transfer mode, media type (removable, CD-ROM), control port base and IRQ. In other words, it covers all new features of the ATA family and is flexible enough to accommodate more than four devices, nonstandard port addresses and IRQs. Proper support of all this is important to achieve any degree of Plug'n'Play functionality with ATA hardware. The WD EIDE BIOS lacks all these features.

 

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