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3. PROGRAM INTERFACE
DISK PACK ORGANIZATION
Each disk pack (see Figure 1) contains 11 disks with 20
usable recording surfaces. (The outside surfaces of the top
and bottom disks are not usable.) Each surface has its own
movable read/write head but the 20 heads are all aligned
in the same vertical plane and move as a unit through the
stack, though only one head is reading or writing at any
one time.
Head numbers range from 0 through 19. Each·
recording surface is logically divided into six sectors num-
bered from 0 through 5 and into 203 concentric rings or
tracks on which data is recorded. The tracks begin at the
periphery of a disk and progress inward toward the center.
Each set of 20 tracks aligned vertically in the stack is re-
ferred to as a cylinder. Thus, there are 203 cylinders in a
disk pack, numbered from 000 through 202. Cylinder 000,
for example, consists of track 000 on surface 0, track 000
on surface l, and so on through track 000 on surface 19.
The addressing is summarized, as follows:
Field
Range
Sector
0-510
Head
0-1910
· Cylinder
0-20210
DATA ORGANIZATION
Data is stored in groups of 1024 bytes. Each data group
has a unique address composed of its device, cylinder,
head, and sector number.
Each data group is preceded by
a header containing the cylinder, head, and sector number
for the group. The header is used for address identification
and verification.
DATA ACCESS
To select a disk pack unit, the
1/0
instruction must con-
tain an address specifying both the device controller and
the device. Since Model 7240 is a multiunit device con-
troller, its addresses range from 8 through F. The device
addresses range from 0 through 7.
To address a particular data group for reading or writing, a
Seek order must be given to the device to select the de-
sired cylinder (move the heads into place), .head, and
sector.
While reading or writing the sector (in a Read,
Write, Check-Write or Header Read/Write operation), the
controller automatically increments the sector number to
the next one in sequence. Upon completing the last sector
of a track, the controller sets the sector number to zero
and increments the head number to the next one in sequence.
Thus a-single order may cause a number of sequential sectors
and then tracks of a cylinder to be read or written.
On
4
Program Interface
reaching the end of a cylinder, a new Seek order must be
issued to the controller to select the next cylinder, since it
does not automatically increment to the next one.
ERROR DETECTION
A two-byte check charocter is written at the end of each
header and at the end of each sector data field. The check
characters are computed and inserted by the controller.
When reading a header, the check characters are again
computed and compared with those read. Any difference
causes an "unusual end" termination. Upon reading data,
the check characters are also recomputed and compared with
those read. In this case, if there is a difference, the con-
troller signals transmission error but not "unusual end".
HEADERS
Prior to writing data on the disk, headers must be written
identifying the addresses of all data groups (sectors). The
headers are used for locating the desired data group and for
address verification when successive data groups are written
or read. Failure to acquire a given address within one rev-
olution results in a verification error. When the program
detects a defective track, it should write flaw marks in all
sector headers of the track.
A defective track is one in
which an error has been detected on each of three suc-
cessive "write and then read" operations.
An alternate
head and cylinder number is then obtained from the "flawed"
header.
The program will normally use the alternate head
and cylinder number as the new Seek address.
DEVICE ORDERS
After completing an SIO instruction, or during command
chaining, the controller makes a 1-byte service cal
I
to the
IOP to obtain the order for the next operation. In all data
transmission operations, the data address is incremented to
the next sector even if it is a nonexistent address, e.g.,
head 20.
The following list shows order decoding. The reactions to
these orders are described in subsequent paragraphs.
Binary Representation
Bit Positions
Hexadecimal
Order
0 l 2 3 4 5 6 7
Code
Seek
MO 0 0 0 0
1
X'03'
Sense
0 0 0 0 0 l 0 0
X'04'
Write
0 0 0 0 0 0 0 1
X'Ol'
Header Write
0 0 0 0 l 0 0 l
X'09'
Check-Write
0 0 0 0 0 l 0 l
X'05'
Read l
0 0 0 l 0 0 l 0
X'l2'
Read 2
0 0 0 0 0 0 l 0
X'02'
Header Read
0 0 0 0 l 0 l 0
X'OA'
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