IBM 1620 1 Manual page 48

DRIVE
CODE
X X
SECTOR ADDRESS
X X X X
Sector Address
00000-19999
20000-39999
40000-59999
60000-79999
SECTOR COUNT
X X X X
CORE ADDRESS
X X X X
Core Address
00000-19999
00000-39999
00000-59999
80000-99999r--_ _ _ _ _ _ -.
Figure 48. Disk Control Field
The disk address of the data indicates which storage
drive is to be used by the system for an instruction
(if
more than one is installed on the system).
The drive code enables reading and writing on any
block of disk addresses regardless of the disk drive on
which the pack is located, and when the occasion re-
quires, to have more than one disk pack contain-
ing the same disk addresses on-line at one time.
The selection of a disk storage drive by either a
drive code or a sector address is summarized in the
chart below.
Drive Code or Address Range Selects Drive
1
00000-19999 0
3 20000-39999
2
5
40000-59999
4
7
60000-79999 6
Single Disk Drive System. If only one disk storage
drive is attached to the system, the drive code has no
significance; the drive is addressed regardless of the
digit in the drive code or the range into which the
sector address falls. Also, in a single drive system, the
range of addresses is not limited to 00000-19999; any
range of disk storage addresses between 00000-99999
can be used.
Multiple Disk Drive Systems. If an address and/or
drive code used in a read or write operation addresses
a nonexistent disk drive, the computer is interlocked
The Disk light (under the Instruction and Execute
lights) and the Punch/Disk Interlock light turn on.
A seek operation to an invalid address or to a non-
existent drive does not result in an interlocked condi-
44
tion; in this case, an error is detected during the fol-
lowing read operation by the address compare feature.
The sector count specifies the number of sectors
read or written. At the beginning of the operation,
the sector count is transferred to a 1620 register
( PR-3 ), which is decremented by one before each
sector is processed. The operation is terminated
when the sector count reaches 000. At any time during
a disk operation, the number in PR-3 is one less than
the number of sectors yet to be processed, and the
sector count in the disk control field has the total num-
ber of sectors in the operation.
The core address specifies the location of the left-
most position of the data transferred to or from
disk storage. The core storage address of a disk storage
record must be an even number.
Disk Storage Data Protection
Two safeguards, controlled by programming, are in-
corporated into the 1311 Disk Storage Drive to pre-
vent incorrect reading and writing of disk storage data.
Read-Only Flag
A Hag in the high-order (leftmost) position of the
sector address, recorded on the disk, permits the
sector to be read only. An attempt to write in the
sector (with the exception of a write disk track opera-
tion) results in an address check and termination
of the operation. The Address Check indicator (36) is
turned on.
Wrong-Length Record Check
This function provides a check, in addition to the sec-
tor count, that ensures the transfer of the correct
length record and prevents the loss of data through
reading or writing beyond the intended point. In this
respect, the wrong-length record check also verifies
that the correct sector count was entered in the disk
control field.
To enable a record to be checked for the correct
length, a group mark must be placed in core storage
in the position following the end of the record. The
data transfer is made with a Read, Write, or Check
Disk instruction, and is modified to verify the length
of the record by checking the position of the group
mark and the sector count in PR-3. If the group
mark and a sector count of 000 do not coincide, the
operation terminates and the Wrong-Length
~ecord/
Read-Back Check indicator (37) is turned on.