Instruction Times; Access Motion Time - IBM 7074 General Information Manual

IBM 1410 and 7000 Series Systems Sharing a 7631
File Control
At completion of any seek operation, an attention sig-
nal is sent to both using systems with the following
restrictions:
1. A prevent seek complete instruction (M/L X F
4 XXXXX W) is used to turn on
a
latch in the
IBM
1411
Processing Unit, which blocks seek complete signals
from reaching the 1410 system. This latch is turned oH
(allowing seek complete signals to be serviced) when
the 1410 issues a seek command.
2. Reception of a seek complete by the 1410 is
further conditioned by previous use of the 7631. If the
7631 is disconnected from both systems or is connected
to the 1410, seek complete signals are sent to the 1410.
If the 7631 is connected to the 7000 series system, seek
complete signals are not sent to the 1410 until the 7000
system releases the 7631.
Subject to these restrictions, operation of the 1410/
7000 shared disk storage system is essentially the same
as described for the 7000/7000 shared disk storage sys-
tems, except for the 1410 method of determining
whether an interrupt was caused by a seek command
which the 1410 initiated. This method consists of a
series of Y ( I ) d tests to determine the nature of the
interrupt. Having found that the interrupt was caused
by a seek complete, each access mechanism that is
seeking because of a 1410 command must then be
tested by issuing an input-output no-operation instruc-
tion to set status indicators for the access mechanism
and then an R (1)$ to test for a busy condition.
Programming Shared Disk Storage
In programming a shared disk storage system, one of
two methods may be used:
1. System A issues one or more seek orders and re-
tains control of the 7631 (by not issuing a release com-
mand) until all seek commands have been completed.
2. Two or more seeks issued by either or both sys-
tems may be in simultaneous operation. Seek execution
times may thus be eHectively overlapped. Operation is
as follows:
18
a. System A issues one or more seek commands
and then a release.
b. System B issues one or more seek commands
and then a release.
c. Control of the 7631 is now logically discon-
nected from both systems. Operation from this
point depends on which systems are sharing
the 7631. If one of the systems is a 1410, the
priority processing feature, which allows inter-
rupts to be processed by the system, must be
available.
Instruction Times
Ten digit orders require 148 microseconds in 6-bit
mode and 190 microseconds in 8-bit mode. Two-digit
orders demand 55 microseconds in 6-bit mode and 75
microseconds in 8-bit mode. A sense command re-
quires 123 microseconds in 6-bit mode and 158 micro-
seconds in 8-bit mode. Read and write commands
vary in time duration depending on the length of
record (for single record operations) and rotational
coincidence of starting points. A cylinder operation in-
volving all 40 tracks within a cylinder will take about
1.5 seconds.
Access time to position access mechanism is 50 to
180 milliseconds and average rotational delay is 17
milliseconds.
Access
Motion Time
The access mechanism requires time to move from one
cylinder to another. The time required depends on how
far the arm moves within certain machine-defined
limits. To calculate how much time will be required,
consider the 250 cylinders of a module organized into
five areas of 50 cylinders per area (Figure 12). If each
area of cylinders is further divided into six sections,
access motion time for anyone access is:
1. 50 milliseconds, to move the access arm within a
section of anyone area.
2. 120 milliseconds, to move the access arm from one
section to another section of an area.
3. 180 milliseconds, to move the access arm from
( one area to another area (crossing an area boundary).
For example, to move the access arm from track 0000
to 1960 requires 120 milliseconds of access motion
time. To move the access arm from track 1960 to 2000
requires 180 milliseconds.
Access time from one track to another can be deter-
mined by the use of Figure 13. The point of intersec-
tion of two lines on a coded area of the figure, one
drawn horizontally from a
FROM
track address and one
drawn vertically from a
TO
address, indicates access
time in milliseconds.