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5. Indirect Addressing (Special Feature)
A flag bit over the units position of an instruc-
tion address (P or Q) indicates an indirect ad-
dress.
Record Mark (=f=). The record mark is a nondecimal
machine digit coded C-8-2. It is used primarily in
input/ output operations and in record transmission
within the 1620; it cannot be used as a significant
digit in an arithmetic or compare operation.
Group Mark ($). The group mark, coded C-8-4-2-1,
is used in disk storage operations to verify the correct
length of records written on or read from disk storage.
Numerical Blank. The numerical blank ( coded
C-8-4) is used for format control of blank columns in
card punching, and cannot be used in arithmetic or
compare operations. The
I/O
instructions, Read Nu-
merically and Write Numerically, further detail the
use of the numerical blank.
Field
A field is composed of related digits that are treated
as a unit of information (temperature, flow rate, etc.).
The digits of a field are consecutively addressed. A
field is addressed by its rightmost (low-order) digit
which occupies the highest-numbered core storage
position of the field. Fields are processed from right
to left into successively lower-numbered core storage
positions until a digit with a flag bit is sensed. The
shortest admissible field consists of two digits: the
addressed digit which mayor may not contain a flag
(negative or positive) and the high-order digit con-
taining the flag bit or field mark.
I - - F i e l d - - I
X X
. • • •
X
t
O;,ec,;o" P<oce ...
d
!
Flag Bit
Addressed Digit
(End of Field)
(Low-Order Position of Field)
Record
A record consists of a field or fields of related data
normally grouped for input/ output operations and
internal data transmission. A record is addressed at
the leftmost (high-order) digit which occupies the
lowest-numbered core storage position of the record.
Records are processed serially from left to right into
successively higher-numbered core storage positions.
4
Output and internal record transmission are termi-
nated when a record mark is sensed, except for card
output which is terminated only after 80 columns are
transferred to 1622 buffer storage.
A record is entered into core storage, starting at
the addressed digit and continuing from left to right
into successively higher-numbered core storage posi-
tions, until terminated by an end-of-record signal
from the input unit. The end-of-record signal from
paper tape causes a record mark to be placed in core
storage as the rightmost digit of the record. When
input is from the typewriter, the Record Mark key
must be depressed to place a record mark in core
storage. When input is from punched cards, a record
mark is automatically placed in core storage only
when 0, 8, 2 are punched in a card column.
Record Mark
Record Mark
~
_d _
XX+X • • • XX • • • XX
X
=+=
X
X
--Field-- - - F i e l d - --Field--
-
- - - I I - - - - - - - R e c o r d - - - - - - - - I
•
Arrows Indicate Direction of Processing
Magnetic Core Storage
A core storage module, which is 20,000 addressable
positions of magnetic core storage, is located in the
1620. Two additional modules are available in the
IBM
1623, Models 1 or 2, to increase the total core
storage capacity of the 1620 System to 40,000 or
60,000 positions. Data and instructions in core storage
are not affected by the manual turning on or off of
power if care is taken to ensure that the 1620 System
is in the manual mode before power is turned off.
Core Array
Each core storage module (20,000 positions) is made
up of 12 core planes as shown in Figure 3. Each
core plane contains all cores for. a specific bit value.
The core planes are labeled C, F, 8, 4, 2, and 1. The
even-address planes are· the top six planes, and the
odd-address planes are the bottom six planes. An even
address has an even number as its units digit, while
an odd address has an odd-numbered units digit.
The magnetic condition of the cores at any address
determines which digit is stored at that address. A
magnetic core is in either the on condition, or the off
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