Specifications

– Words 60 and 61 contain the total number of sectors available on

the drive. This number remains constant for the life of the drive, and

is unaffected by the current translation mode. This value may be

greater than the number of sectors implied by Words 1, 3, and 6.

There are no translation modes that allow Cylinder/Head/Sector

(CHS) access to more sectors than specified by words 60 and 61.

• In the Drive/Head register, bit 6 (LBA) is set to 0 for CHS addressing

and set to 1 for logical block addressing.

• The logical address is a 28-bit unsigned binary number, which is

placed into the command block as follows:

– bits 27–24 into the Drive/Head register bits 3–0

– bits 23–16 into the Cylinder High register

– bits 15–8 into the Cylinder Low register

– bits 7–0 into the Sector Number register

• In all valid translation modes, the following formula describes an

equivalent logical block address (LBA) for any CHS address:

LBA

SpC

SpT

– 1

where

, and

are the cylinder, head, and sector numbers,

respectively,

SpC

is the number of sectors per logical cylinder for the

current translation mode, and

SpT

is the number of sectors per logical

track for the current translation mode. For all translation modes, C=0,

H=0, S=1 is equivalent to LBA=0. It is not possible to compute an

equivalent CHS for all logical block addresses in all translation modes

because this formula only works in one direction. This is because CHS

addressing can’t access 1/256th of all of the possible sectors that

logical block addressing can access, since there is no sector 0 in CHS.

• The maximum allowed value for the LBA is one less than the number

returned in the Identify Drive data words 60 and 61.

5.2 ATA Command Types and Protocols

The ATA commands can be grouped into four different types, depending

on the protocols followed during command execution. These four types

are: PIO read commands, PIO write commands, DMA data transfer

commands and nondata-transfer commands. The following pages sum-

marize the execution of each of these command types.

26 ATA Interface Reference Manual, Rev. C