IBM 1130 User Manual page 443

Section
Subsections Page
45
10
I
00
01
DISK STORAGE
Models 2 and 3 of the 1131 CPU contain a disk stor-
age drive as an integral part of the console unit.
In
addition, these models may contain
up
to four addi-
tional disk drives, mounted in separate enclosures
(the IBM 2310 Disk Storage).
Each disk drive will hold one IBM 2315 Disk
Cartridge. Because the cartridges are removable,
the user may have an unlimited amount of data on
them; only one, however, may be mounted in a disk
drive at anyone time.
The 2315 cartridge consists of a single metal
plate, coated on both sides with magnetic material
and enclosed in a plastic container. When mounted
in an activated disk drive, the metal plate is driven
through a clutch mechanism at 1500 revolutions per
minute. The recording plate never leaves its con-
tainer, as it does in the case of some other disk
devices.
Each cartridge is divided into 200 cylinders, in
concentric circles, with each cylinder further di-
vided into eight sectors - four on the top surface
and four on the bottom. Since each of the 1600
sectors contains 320 words, each disk cartridge can
hold 512,000 words.
Data is read or written on the disk by two read-
write heads attached to a movable arm. One setting
of the arm gives the 1130 access to one cylinder, or
eight sectors. One head reads (or writes) the top
four sectors; the other, the bottom four sectors.
The two heads cannot move independently, since they
are fixed to the same arm.
Because one setting of the arm gives access to
only one cylinder, the arm must be moved in order
to read or write on a different cylinder. For ex-
ample, to read from cylinder 10 and then write on
cylinder 15, the arm must move, or "seek", from
cylinder 10 to cylinder 15. Since the arm moves in
steps of one ortwo cylinders, this would require two
2-cylinder moves (from 10 to 12, and from 12 to
14) and one I-cylinder move (from 14 to 15).
Each move, whether one or two cylinders in
length, takes 15 milliseconds (0. 015 seconds). A
five-cylinder "seek", as shown above, would take
45 milliseconds (15+15+15). A six-cylinder seek
would take the same length of time.
Because this can be a relatively lengthy opera-
tion (compared with other 1130 functions), you
should attempt to minimize the need for disk arm
movement. Many hints on how to do this are given
later in the manual (Sections 55, 60, 65, 70, 80,
85, and 90).
Having reached the desired cylinder, the arm
takes another 25 milliseconds to stabilize. After
the stabilization period, data may be read or written;
because the disk is rotating, however, it will be
.quite unusual for the desired sector to be passing
under the read/write head at the precise time you
want it. You will have to wait an average of half a
revolution (20 milliseconds) for the sector to reach
the heads, and then 10 more milliseconds for it to
actually be read or written.
Figure 45.2 gives some examples of how long it
takes to move n cylinders, then read one sector.
Average
Move This
Seek
Stabilization Rotational
Read
Many Cylinders
Time
Time Delay Time
or Write
Total
None 0 0
20
10
30
1 or 2 15 25
20
10
70
3 or 4 30 25
20 10 85
5 or 6 45 25
20 10 100
199 or 200 1500 25
20
1'0
1555
(maximum)
Figure 45.2.