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even-numbered tracks are on the top surface, and the odd-
numbered tracks are on the bottom. Thus, address 0, 2, ••• ,
406 are on the top while addresses 1, 3, ••• ,407 are on
the bottom.
SECTORS
Each track is divided into 16 sectors. The index and sector
marks are physical slots on the disk surface. The index
mark, which is often called the origin, is sensed by the
index mark transducer; this same transducer senses the sec-
tor marks. The sector addresses are 0, 1, ••• ,15 and are
identical on both surfaces.
CYLINDERS
Each vertically aligned set of two tracks on the disk is re-
ferred to as a cylinder. Since a disk contains 408 tracks,
and since each set of two tracks constitutes a cylinder,
each disk has 204 cylinders. Tracks 0 and 1 are cylinder OJ
tracks 2 and 3 are cylinder 1; .•. ; and tracks 406 and 407
are cylinder 203. When loaded, the two read/write heads
are positioned at one of the cylinders.
DATA ORGANIZATION
Data is stored in groups of 360 bytes (see Figure 7). Each
data group is preceded by a header consisting of a gap and
a five-byte preamble. Similarly, each data group is fol-
lowed by a trailer consisting of a parity byte, a one-byte
postamble, a gap, the cylinder address, and a final gap.
During disk operations, the controller computes and checks
the parity and checks the cylinder address as described
below.
PARITY
During a write operation, the controller sums each byte re-
i
ceived to form a parity check byte. It always writes this
byte in the trailer portion of the sector, even when less
than 360 bytes are transmitted. During a subsequent read
operation, the controller sums the data as it is r.ead and
compares the resulting sum with the prestored parity byte.
Failure to compare results in an error condition.
CYLINDER ADDRESS
When a new disk is initialized, the controller writes the
sector marks and also writes the cylinder address in the
trailer of each sector.
Thereafter, the controller checks
this cylinder address each time the read/write heads are
moved. After modifying its address register to reflect the
track and sector addresses from the Seek order, the con-
troller moves the read/write heads accordingly. It then
converts the track address from the Seek orderand compares
it with the prestored cylinder address to ensure that the
heads are positioned correctly.
If the two addresses match, the contToller permits an I/O
operation on the next sector. However, if they do nor
match, the controller compares the converted track address
with the cylinder address of the next sector. Should the
comparison fail a second time, the
contr~ller
restores the
heads to cylinder 0 and reseeks the
speci~ied
track, untili
stopped by a software timeout routine.
I
.
!
Whenever the second comparison is successful, the control-
ler reseeks the originally specified address at the next sec-
tor mark. The controller always checks the cylinder address
twice since the first error could have resulted from a head
that had not yet settled completely.
DATA PRESENTATION
The basic addressable unit of information is a sector of
360 bytes.
Data are presented in eight-bit bytes to a disk
by a disk I/O program, andeachbyteiswritten bit-serially
on the selected sector. Similarly, data are read serially,
one byte at a time, from the disk and assembled in buffer
registers for presentation to the disk I/O program.
DATA ACCESS
Each data group has a unique address consisting of the
controller, device, track, and sector numbers.
The trock
and sector numbers are those listed above under "Disk
Organization
ll
i
the other numbers are defined here. Each
of the eight possible disks accommodated by a controller
is uniquely addressable because the two disks on the
7252 drive are considered to be two devices.
To select a disk for an operation, either the A register (16-
bit-word computers) or the I/O instruction (32-bit-word
computers) must contain the controller and device addresses.
(For 32-bit-word computers, the I/O instruction also in-
cludes the lOP address.) Since the 7250 is a multidevice
controller, the controHer address lies in the range 816
through F 16, and the device address in the range 0 through 7.
The address for a cartridge is always an even number; the
address for the fixed disk is always odd.
Access time is 12 msec, track to adjacent track, and 38 msec
average.
Optimum access times can be achievecl when
several consecutive sectors are read or written after a Seek
order is issued. Rotational delay may also be reduced by
first sensing the current position of the read/write heads
and then transferring data beginning immediately at the
next sector.
TRACK AND SECTOR INCREMENTING
The controller includes an address register in order to main-
tain the current track and sector addresses.
This register
is initially loaded witha Seek order and the read/write heads
are positioned at the specified track and sector. There-
after the controller automatically increments the address
Disk Organizati
on
11
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