PC87311A datasheet(37/78 Pages) NSC

37 / 78 page

40 FDC Command Set Description (Continued)

posed to comparing against the internal present track regis-

ter for that drive The Relative Seek parameters are defined

as follows

DIR

ReadWrite Head Step Direction Control

0 e

Step Head Out

1 e

Step Head In

RTN

Relative Track Number This value will determine how

many incremental tracks to step the head in or out from the

current track number

The controller will issue RTN number of step pulses and

update the Present Track Register for the selected drive

The one exception to this is if the TRK0 disk input goes

active which indicates that the drive readwrite head is at

the outermost track In this case the step pulses for the

Relative Seek are terminated and the PTR value is set ac-

cording to the actual number of step pulses issued The

arithmetic is done modulo 255 The DRx BUSY bit in the

MSR is set for the selected drive The controller will simu-

late the Motor On time before issuing the step pulses After

the Motor On time the controller will enter the Idle Phase

The execution of the actual step pulses occurs in the Idle

Phase of the controller

After the step operation is complete the controller will gen-

erate an interrupt There is no Result Phase Relative Seeks

on more than one drive at a time should not be issued for

the same reason as explained in the Seek command No

other command except the Sense Interrupt command

should be issued while a Relative Seek command is in prog-

ress

4215 Scan Commands

The Scan command allows data read from the disk to be

compared against data sent from the mP There are three

Scan commands to choose from

Scan Equal

Disk Data e mP Data

Scan Low or Equal

Disk Data s mP Data

Scan High or Equal

Disk Data t mP Data

Each sector is interpreted with the most significant bytes

first If the Wildcard mode is enabled in the Mode command

an FF (hex) from either the disk or the mP is used as a don’t

care byte that will always match equal After each sector is

read if the desired condition has not been met the next

sector is read The next sector is defined as the current

sector number plus the Sector Step Size specified The

Scan command will continue until the scan condition has

been met or the EOT has been reached or if TC is assert-

Read errors on the disk will have the same error conditions

as the Read Data command If the SK bit is set sectors with

deleted data marks are ignored If all sectors read are

skipped the command will terminate with D3 of ST2 set

(Scan Equal Hit) The Result Phase of the command is

shown in Table 4-10

TABLE 4-10 Scan Command Termination Values

Status

Command

Conditions

Scan Equal

Disk e mP

Disk i mP

Scan Low

Disk e mP

or Equal

Disk k mP

Disk l mP

Scan High

Disk e mP

or Equal

Disk l mP

Disk k mP

4216 Seek Command

The Seek command issues step pulses to the selected drive

in or out until the desired track number is reached During

the Execution Phase of the Seek command the track num-

ber to seek to is compared with the present track number

The controller will determine how many step pulses to issue

and the DIR disk interface output will indicate which direc-

tion the RW head should move The DRx BUSY bit is set in

the MSR for the appropriate drive The controller will wait

the Motor On time before issuing the first step pulse

After the Motor On time the controller will enter the Idle

Phase The execution of the actual step pulses occurs in the

Drive Polling phase of the controller The step pulse rate is

determined by the value programmed in the Specify com-

mand An interrupt will be generated one step pulse period

after the last step pulse is issued There is no Result Phase

A Sense Interrupt command should be issued to determine

the cause of the interrupt

While the internal microengine is capable of multiple seek

on 2 or more drives at the same time software should en-

sure that only one drive is seeking or recalibrating at a time

This is because the drives are actually selected via the

DOR which can only select one drive at a time No other

command except a Sense Interrupt command should be is-

sued while a Seek command is in progress

If the extended track range mode is enabled with the ETR

bit in the Mode command a fourth command byte should be

written in the Command Phase to indicate the four most

significant bits of the desired track number Otherwise only

three command bytes should be written

4217 Sense Drive Status Command

The Sense Drive Status command returns the status of the

selected disk drive in ST3 This command does not gener-

ate an interrupt

4218 Sense Interrupt Command

The Sense Interrupt command is used to determine the

cause of interrupt when the interrupt is a result of the

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