Table of Contents
Advertisement
3. PROGRAM INTERFACE
DISK PACK ORGANIZATION
Each disk pack (see Figures 1 and 2) comprises 11 disks with
20 usable recording surfcices. (The outside surfaces of the
top and bottom disks are not usable.) Each surface has its
own read/write head mounted in a comb assembly on a mov-
able arm; the 20 heads are aligned in the same vertical
plane and move as a unit through the stack, although only
one head is reading or writing at any time.
Head numbers
range from 0 through 19.
Each recording surface is logically divided into 11 sectors
numbered 0 through 10, and 203 (411 for drive "B") con-
centric circles or tracks on which data are recorded.
The
tracks begin (are numbered) at the periphery of a disk and
progress inward toward the center. Each vertically-aligned
set of 20 tracks in the stack is referred to as a cylinder.
Thus for drive "A" there are 203 cylinders in a disk pack,
numbered 000 through 202; for drive "B" there are 411 cyl-
inders, numbered 000 through 410.
Cylinder 000, for
example, comprises track 000 on surface 0, track 000 on
surface 1, and so on through track 000 on surface 19.
The range of values in the fields of the disk address are
summarized as follows:
Field
Range for "A" Drive
Range for "B" Drive
Cylinder
0 - 20210
0-41010
Head
0 - 1910
0 - 1910
Sector
0 - 1010
0 - 1010
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.
HEADERS
Before data can be written on the disk, special records
cal led headers that identify the addresses of al
I
data groups
(sectors) must be written. The header data received from.
the IOP (see "Header Write (X'09')", later in this chapter)
comprises a flaw byte; head, sector, and cylinderaddresses;
and alternate head and cylinder addresses.
Headers are used in locating the desired data group (sector)
and in address verification when successive data groups are
written or read. Failure to acquire a given address results
in a verification error. When the program detects a flawed
track, it must write a flaw mark (X'FF') in the flaw byte in
al I 11 s·ector headers of the track.
A defective or flawed
track is one in which an error has been consistently detected
on successive write and then read operations.
When the
control
I
er detects a flaw mark in a header, it terminates
any read or write operaHon by indicating "unusual end".
Note that the current sector address is not incremented for
any unusual termination while the header is being read.
Therefore the alternate head and cylinder addresses are
normally obtained from the "flawed header" by issuing
a Header Read order (see "Header Read (X'OA ')", later in
this chapter) directly after the flaw is encountered. A new
Seek order is not required in this case.
DATA ACCESS
To select a device for any operation, the I/O instruction
must contain an address specifying the device controller
address and the device address.
Both the Model 7260
and the Model 7265 are multidevice controllers; conse-
quently the controller address lies in the range 816 through
Fl6• and device addresses in the range 016 through E16·
Note that device address Fl6 is for the controller and can
be used for any I/O instruction intended for the controller
alone (SIO, HIO, TIO, and TDV).
To address a particular data group (sector) for reading or
writing, the program must first issue a Seek order to the
device to select the desired cylinder (move the heads
into place), head, and sector.
After reading or writing
a sector, the controller automatically increments the
sector address to the next sector in sequence. Upan com-
pleting the last sector of a track, the controller sets the
sector address to zero and increments the head address to
the next head in sequence (in the cylinder). Thus a single
I/O order can cause a number of sequential sectors or
tracks to be read or written.
When the end of a cylinder
is reached, the program must issue a new Seek order to the
controller to select the next cylinder, since the controller
does not automatically increment to the next cylinder.
CHECK CHARACTERS
The controller computes and writes a two-byte check char-
acter at the end of each header and at the end of each
sector data field.
When a header is read, the controller
recomputes and compares the check characters with those
Program Interface
7
Advertisement
Table of Contents
