DISK DRIVE OPERATION
6.3Logical Interface
6.3.1General
6.3.1.1Bit Conventions
Bit names are shown in all upper case letters except where a lower case n precedes a bit name. This indicates that when nBIT=0 (bit is zero) the action is true, and when nBIT=1 (bit is one) the action is false. If there is no proceeding n, then when BIT=1 it is true, and when BIT=0 it is false.
A bit can be set to one or cleared to zero, and polarity influences whether it is to be interpreted as true or false:
True BIT=1 nBIT=0
False BIT=0 nBIT=1
6.3.1.2Environment
Data is transferred in parallel (16 bits) either to or from host memory to the device’s buffer under the direction of commands previously transferred from the host. The device performs all of the operations necessary to properly write data to, or read data from, the media. Data read from the media is stored in the device’s buffer pending transfer to the host memory, and data is transferred from the host memory to the device’s buffer to be written to the media.
The devices using this interface shall be programmed by the host computer to perform commands and return status to the host at command completion. When two devices are daisy chained on the interface, commands are written in parallel to both devices, and for all except the Execute Diagnostics command, only the selected device executes the command. On an Execute Diagnostics command addressed to Device 0, both devices shall execute the command, and Device 1 shall post its status to Device 0 via
Drives are selected by the DEV bit in the Drive/Head register (see 6.3.4.9), and by a jumper or switch on the device designating it as either Device 0 or Device 1. When DEV=0, Device 0 is selected. When DEV=1, Device 1 is selected. When a single device is attached to the interface, it shall be set as Device 0.
Throughout this document, device selection always refers to the state of the DEV bit, the position of the Device 0/Device 1 jumper or switch, or use of the CSEL pin.
A device can operate in either of two addressing modes, CHS or LBA, on a
This term defines the addressing mode of the device as being by physical sector address. The physical sector address is made up of three fields: the sector number, the head number and the cylinder number. Sectors are numbered from 1 to a device specific maximum value, which cannot exceed 255. Heads are numbered from 0 to a device specific maximum value, which cannot exceed 15. Cylinders are numbered from 0 to a device specific maximum value, which cannot exceed 65,535. Typically, sequential access to the media is accomplished by treating the sector number as the least significant portion, the head number as the mid portion, and the cylinder number as the most significant portion of the CHS address.
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