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FLOPPY DISK INTERFACE

The

Verify Flag bi t

determines

whether

the

FDC verifies

the

track

position

after positioning

the

head, by reading

the

sector ID fields

on

the

disk. Verification is performed,

when

the

Verify Flag

bit

is

set

to logic high,

and

requires

the

head

to be loaded prior to

the

command.

The

verification operation checks

the

track

number

in

the

sector ID field

with

the

number

contained

in

the

Track

Register,

and

also checks

the

ID field

CRC

character.

Failure to

match

the

track

number

or

failure to find a

matching

track

with

a valid CRC,

within

five revolutions of

the

disk, causes

the

Seek Error

bit

to be

set

in

the

Status

Register.

If

the

verification operation detects a CRC error

within

any

of

the

ID fields checked,

the

CRC

error

bit

is

set

within

the

Status

Register.

The

Step

commands

contain

a

Track

Update

Flag,

which

determines

whether

the

Track

Register is

updated

every

time

the

head

moves

to

an

adjacent track.

The

Track

Register is

updated

if

the

Track

Update

Flag is

set

to logic

high.

Restore Command.

The

Restore

command

is used to

position

the

head

over

Track

0 of

the

disk.

The

FDC will

issue

up

to

255

step

pulses

at

the

rate

specified by

the

stepping

rate

bits,

in

an

attempt

to

locate

the

first track

on

the

disk. Failure to

locate

Track

0 (by

monitoring

the

state

of

the

TROO

input)

within

255

step

pulses, causes

the

command

to

terminate,

and

the

Seek Error

bit

to

be

set

within

the

Status

Register.

If

the

Verify Flag

bit

is set,

verification of

the

track

position is carried

out

as detailed

above.

Seek Command. Prior

to

issuing

the

Seek command,

the

Data

Register

has

to

be loaded

with

the

desired

track

number

and

the

Track

Register

is

presumed

to

contain

the

current

position

of

the

head.

On

receiving

the

command,

the

FDC

sets

the

Dirc

output

to

move

the

head

in

the

direction of

the

desired track. Step pulses are

then

issued

at

the

rate

specified

by

the

stepping

rate

bits,

and

the

Track

Register

updated

on

each

pulse,

until

the

number

in

the

Track

Register coincides

with

the

number

in

the

Data

Table of Contents

Other manuals for ACT apricot

Service Manual305 pages