Bulk Memory Subsystems - GE PAC 4020 System Manual

BULK MEMORY SUBSYSTEMS
DRUM
General Characteristics
The magnetic drum auxiliary memory provides
non-destructive bulk storage. The drum capacity
varies from 16, 384 words to 262, 144 words in in-
crements of 8, 192 words. The drum is composed of
tracks, each containing 512 words. Each word con-
sisting of 24 information bits, one parity bit and a
three bit interword gap. The drum moduie contains
the necessary read-write amplifiers, control logic and
heads to enable reading and writing on the contin-
uously rotating magnetic surface.
Checking and Protection
An odd parity bit is written with each word as it
is transferred to the drum. Parity check is made
on each word as it is transferred to or from the
drum; a parity error signal is set if a parity error
is detected. Parity is received from core and
checked by hardware.
The write protect pinboard located in the drum
cabinet provides selective drum memory protection
in blocks of 4096 words. Figure 5 indicates details.
-
000 100 200 300 400 500 600 700 PIN STORAGE
@
@ @ @
® ®
@
@
0000
010 110 210 310 410 510 610 710
@ @ ® ® @ ® ® ®
0000
020 120 220 320 420 520 720
@ @ ® @ @ ® ® ®
0000
030 130 230 330 430 730
·@
@
@ @ @ ® @ @
0000
040 140 240 340 740
@ @ @ @ @ @ @ @
0000
050 150 250 750
@ @ @ ®
@ @
@
0000
060 160 760
® ® @ @
®
®
®
0000
070
770
@
~n in~d pr~ts;
@
@ @
0000
Octal address of first word of pro-
Pin removed unprotects.
tected block of 4096 words is 560000.
Figure
S
Drum Write Protect Pinboard
Overheat protection is provided by a single-
throw double-pole temperature sensing switch which
is incorporated in the ctrum cabinet. One contact
is available to control a remote overtemperature
indicating device and the other will trip the shutdown
function in the computer power sequencer.
Control Word Formats
The location of the program or data on the drum
is given by a block of three consecutive drum con-
trol words which are shown in Figure 6. These con-
trol words are located in core and contain the
starting drum address, number of words to transfer
(length of program or data) and starting core address.
Bit 23 of the first drum control word is either set or
reset depending on the direction of transfer.
To activate a drum transfer, place the location
Y
of the first drum control word in core memory
location zero. Then call for an output to drum with
an OUT instruction. The drum controller will per-
form a drum operation as specified by the control
words.
The time required to effect the transfer includes
drum access time plus actual transfer time. For
60 Hz operation:
Access time variable -0 to 16. 67 ms, average
8.3 ms
Transfer of 256 words per revolution
Transfer time 0. 0650 ms per word
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