IBM 709 General Information Manual page 39

Figure 81. Writing on a
Drum
Figure 82. Reading from a
Drum
To read the information, the magnetized surface is
passed under the recording head. This time, the mag-
netized area on the drum surface generates small volt-
age pulses in the head as shown in Figure 82, exactly
the reverse of the writing process. These voltage
pulses are amplified and are then available for use
in the computer.
The 36 recording heads spread across the surface
of one drum accommodate the word size of the com-
puter (Figure 83) .
Recordina---~-""~
Heeds
Drum
Figure 83. Magnetic Drum Recording Heads
A magnetic drum unit has a storage capacity ot
8,192 words, each word consisting of 36 bits. The
drum unit contains two distinct physical
drum~
each
with a capacity of 4,096 words. Each of the physical
drums contains two logical drums with a capacity of
2,048 words each. A logical drum is selected by giving
the appropriate address.
The 2,048 locations on each logical drum can be
individually addressed by integers in the range 0000-
2047 (0000-3777 octal). A record or block of words
Figure 84. Drum Sectors and Addresses
!l8 IBM 709-7090
Figure 85.
IBM
733 Magnetic Drum
is normally stored in sequentially numbered locations.
The program must indicate the drum address where
the first word is to be written or read. The number
of copy instructions executed then determines the
number of words in the record.
Figure 84 illustrates the physical arrangement of
words on a logical drum. The addresses are num-
bered octally. Observe that, when reading or writing
a continuous record, the computer refers to every
eighth word of a drum for consecutive addresses. Each
logical drum has 256 sectors (a group of eight loca-
tions) and therefore must make eight complete revo-
lutions for all 2,048 words to be read or written as
a continuous record.
In summary, the drum is a high-volume, moderate-
speed, long-term data storage medium for use with
both the 704 and the 709 systems. It is particularly
useful for the storage of large blocks of information
such as subroutines, rate tables, or supplementary data
needed for the solution of a problem. The data stored
on the drum are not destroyed when power is re-