IBM 1620 1 Manual page 49

Program Instructions
The description of program instructions has been ar-
ranged in the following manner:
Seek instruction
Instructions using groups marks to establish cor-
rect record length:
Sector Mode
Read Disk/WLRc
Write Disk/WLRc
Check Disk/wLRc
Disk operations with records of less than or in mul-
tiples of 100 characters:
Track Mode
Read Disk Track/WLRc
Write Disk Track/wLRc
Check Disk Track/wLRc
Instructions not using group marks:
Sector Mode
Read Disk
Write Disk
Check Disk
Track Mode
Read Disk Track
Write Disk Track
Check Disk Track
In the formulas for execution time of disk storage
instructions, the following symbols are used:
S
==
Number of sectors read or written
T
==
Time in milliseconds
A 2 ms read/write head delay occurs from the time
an
I/O
instruction is executed until data can be trans-
ferred. In addition, it is assumed that one-half of a
rotation (20 ms) occurs before reading or writing
begins because this is the average distance the read/
write head is from the sector addressed in any given
number of operations.
A summary of disk storage program instructions is
contained in Table 4.
Seek
SK or 34 - Q
ll
of 1
The execution of the Seek instruction causes the ac-
cess mechanism, in the drive unit addressed, to be
positioned at the cylinder designated by the sector
address in the disk control field. When the operation
is initiated, the access mechanism retracts to the
"home" position, which places the read/write heads
near the periphery of the disks. It then moves to
the cylinder specified and remains in that location
until another Seek instruction is given. The access
mechanism need not be repositioned for subsequent
read/write instructions with the same cylinder.
Execution Time. Average seek time is 250 ms;
maximum time is 392 ms. The 1620 CPU is inter-
locked only until the sector address is transferred to
the disk control unit (360 /l-sec). Internal 1620 proc-
essing that does not involve disk storage can then
continue and overlap the seek operation.
When the 1311 is attached to the 1710 Control Sys-
tem, a Seek Complete interrupt is available as part of
the Input/Output Interrupts special feature. The
Seek Complete interrupt signals the
CPU
that the
Seek instruction has been completed and that data
can be transferred to or from disk storage. This
allows the
CPU
to optimize available processing time
during the seek operation. The Seek Complete in-
dicator (42) enables a program test of this interrupt.
Read Disk/WLRC
. RDGN or
36 -
Q
l1
of
0
Execution of this instruction causes a read/write
head to be selected, as specified by the sector address
in the disk control field. The selected head then
scans the sector for addresses recorded on the disk
track until a matching address is found. When a sec-
tor address is found in disk storage that matches the
sector address in the disk control field, reading be-
gins and continues for the number of sectors speci-
fied. As each character is read, it is stored in the
core storage location specified and in sequentially
higher-numbered positions. It should be noted that if
the data transferred exceeds storage capacity, reading
will continue in location 00000 and in sequentially
higher positions.
If a matching address is not located within one
complete revolution of the disks (index point sensed
twice), the operation is terminated and the Address
Check indicator (36) is turned on.
The sector address in the 1620 register OR-1 is
incremented by one each time a sector is read, and
the sector address for each succeeding sector that is
read is compared to ensure correct sequential pro-
gression. If an address fails to compare, the opera-
tion is terminated and the Address Check indicator
( 36) is turned on. The address (plus one) that
failed to match is in OR-1 and the number of sec-
tors yet to be read, excluding the one whose address
failed to match, is in PR-3.
If the number of sectors read goes beyond one
disk surface, a shift to the read/write head for the
next disk surface is made automatically and reading
continues without loss of time. If the end of the
cylinder is reached however, and the sector count
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