ACT apricot - Page 104

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FLOPPY

DISK

INTERFACE

At

the

start

of

disk

write

and

formatting

operations,

the

FDC signifies

to

the

lOP

that

the

first byte is required, via

the

DRQ

output.

If

during

formatting,

the

lOP

fails

to

supply

the

first

byte

before

the

FDC

detects

the

Index Pulse,

the

Lost

Data

(LD)

bit

is

set

within

the

Status

Register,

INTRQ

is

activated

and

the

formatting

operation

is

terminated.

If

during

disk

writes,

the

lOP

fails

to

supply

the

first

byte

before

the

start

of

the

data field,

the

same

process

occurs;

the

Lost

Data

bit

is set,

INTRQ

is

activated

and

the

operation is

terminated.

During

disk read operations,

the

FDC

checks

the

two-

byte

CRC

code

at

the

end

of

the

sector

data field

to

ensure

the

validity of

the

data.

If

a

CRC

error is present,

the

CRC

error

bit

in

the

Status

Register

is

set,

INTRQ

is

activated

and

the

read

operation

is

terminated

(even if

the

operation is

a

multiple

sector read). .

In

addition

to

the

transfer

commands,

the

INTRQ

output

is also

activated

at

the

end

of

the

command

sequence

for

positioning

the

read/write

head

of

the

drives.

The

success of

the

operation is again signified by

the

status

bits

within

the

Status

Register.

The

first

operation

performed

by

the

FDC

on

receiving

the

Restore

command

is a

check

of

the

Track

0

input

(TROD).

If

the

TROD

input

is

set

low

(indicating

that

the

head

is already

positioned

over

the

first track),

and

the

verification

option

is

not

selected,

the

FDC clears

the

Track

Register

to

zero

and

activates

the

INTRQ

output.

If

the

TROD

input

is high,

the

FDC

issues

step

pulses

until

the

TROD

input

is

set

low,

which

then

produces

the

same

effect as described above, providing

the

verification

option

is

not

selected (i.e.

INTRQ

activated,

Track

Register

cleared).

If

the

TROD

input

is

not

set

low

within

255

step

pulses,

the

operation

is aborted,

INTRQ

is activated,

and

the

Seek Error

bit

with

the

Status

Register is set.

If

the

verification

option

is selected

with

any

of

the

head

positioning

commands,

the

FDC

moves

the

head

to

the

specified

position

and

then

reads

the

first

encountered

ID

field

on

the

disk.

The

track

number

from

the

ID field is

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