IBM 709 General Information Manual page 9

Number 1500
Figure 8. Core Storage Unit
notes the location of a particular item of data inside
the core storage unit called a "word" (Figure 8).
Each word is further subdivided into elements
called bits. Each bit has a value of either a 0 or a 1.
Thus. the bits (36 in each word) are the basic units
of information in the computer. Figure 8 also shows
S6 "planes" or "stories" in the core storage "building."
The operation of locating a word in core storage may
be compared to the operation of an elevator in an
office building. The elevator picks up passengers from
each floor; in core storage, one bit of information is
read or stored from each plane. In actual computer
operation, all 36 bits are read, stored, or operated
upon simultaneously, one from each plane.
As previously stated, one word contains 36 bits of
information. Core storage contains two types of
words:
1. A word upon which arithmetic or logical oper-
ations are to be performed is called a data-word.
2. A word interpreted by the computer as a code
to "order or instruct" it to perform a particular
operation is called an instruction-word.
8 IBM 709-7090
Figure 9. Instructions and Data in Core Storage
Both types of words are combinations of 36 zeros and
ones. An instruction is able to "direct" the computer
to perform some type of operation, e.g., read. write,
add, subtract, or test for zero. If a data word were
incorrectly used as an instruction, the computer might
perform an illegal operation depending upon the data
bit configuration. Data words form records. fields,
amounts, results, and so on. Figure 9 is a schematic
of core storage with both types of words contained in
separate locations.
Core storage units are available to provide the com-
puters with a capacity of 4096, 8192, or 32,768 word
locations of 36 information bits each. As a decimal
digit is expressed by three of these bits, a total of II
significant decimal digits may be expressed by each
word. In
BCD
coding (alphamerical characters), one
word contains six numerical or alphabetic characters.
Thus, the 32,768 words of core storage may contain
the binary equivalent to 360,448 decimal digits of stor-
age or the
BCD
representation of 196,608 characters.
The actual capacity of storage is thus directly related
to the type of coding used.
Figure 10 shows the three core storage units that
are available for the 709 and 7090 Data Processing
Systems. The
IBM
737 Core Storage, used with the 709
system, has 4,096 words of storage. The
IBM
738 and
the
IBM
7302 Core Storage have 32,768 words of stor-
age and are used with the 709 and the 7090 systems,
respectively.