Table of Contents
Advertisement
register until it is changed, ,with another Seek order. Track
and sector incrementing occurs as soon as an I/O opera-
tion begins. For instance, the controller would start a write
operation at the current contents of the address register
and, as soon as the operation begins, increment the address
register. Identical incrementing occurs when data are read.
EXAMPLES OF AUTOMA TIC INCREMENTING
Assume that a Seek order specifies track 0, sector O. The
controller will load these in the address register and will
position the read/write heads.
Next, assume that a Write
order specifies a byte count of 500. As soon as the con-
troller begins writing in track 0, sector 0, it increments the
address register. Since a sector contains a maximum of 360
bytes, the controller writes the remaining bytes in track 0,
sector 1, and increments the address register as soon as it
begins writing in sector
1.
At the end of this operation,
the address register contains track 0, sector 2.
Because the controller increments addresses, a data record
may extend from sector to sector and from track to track.
Now assume that a Seek order specifies track 0, sector 0
and thata subsequent Write order includes sufficient data for
several tracks. The controller first writes sector 0 of track 0,
then sector 1 of track 0, and so on, unti I data have been
wri tten on sector 15 of track O. The current addresses are
track 1, sector O. The contr'oller then writes all sectors of
track 1, followed by all sectors of track 2, and so on, until
all data have been written.
To summarize this example, the controller writes all sectors
(0, 1, .•• , 15) of track 0 on the top surface, then writes
all sectors of track 1 on the bottom surface, and could con-
tinue to alternate from the top surface (upper read/write
head) to the bottom surface (lower read/write head) until
a II sectors of all tracks contain data.
ERROR CONDITIONS
An error condition occurs if the disk drive attempts to seek
an address
~
408. An error condition also occurs if the
disk I/O program issues a Seek order to load the address
register with the address of a nonexistent sector.
SYSTEM STATES
Three possible conditions - not operational, ready, or
busy - may exist for the addressed controller and device
(see Table 3).
The controller may be not operational,
ready, or busy and, similarly, a device may be not opera-
tional, ready, or busy. The controller is busy if one of its
devices is busy; it is ready if all of its operational devices
are ready.
The condi tions discussed here are directly related to the
section entitled "Status Response" in Chapter 3.
The possible state transitions, as well as the events causing
these transitions, are summarized in Table 4. The initial
12
System States
Table 3. Conditions·
Condition
I nterpretat ion
Not operati ona
I
The power is turned off, or the con-
troller or device is not operational.
I
Ready
The controller can accept an SIO
instruction if the addressed device
is avai lable and if the system is free
of pendi ng interrupts. To become
ready, the device must be opera-
tional and the execution of on order
must not be in progress or pendi ng.
Busy
The controller has accepted on
510
i nstructi on. It wi
II not accept a
new order until the current order is
completed and no interrupt is
pending.
Table 4. State Transitions
~
State
Present
Not
State
Operati ona I Ready
Busy
Not
Power is
Not possible.
Operational
turned on.
Ready
Power is
510
instruc-
turned off.
tion has been
accepted.
Busy
Power is
HIO instruc-
turned off.
tion received,
I/O Reset
signal re-
ceived, or
operation as-
sociated with
the last order
is completed.
state of the system depends on the power. Absence of power
removes the device from the system, and the address wi II
not be recognized. If the power is turned on, the address
wi
II
be recognized when an I/O instruction is issued.
OPERATIONAL DEVICES
A device is operational when the READY indicator on the
operator control panel is lighted.
The yellow light in-
dicates that
1.
The cartridge is correctly loaded, the bottom cover is
inverted and placed over the cartridge, and the hold-
ing clamps are closed.
Advertisement
Table of Contents
