Disc - GE PAC 4020 System Manual

One to 16, 384 words can be transferred by one
drum transfer instruction. Each word takes one
core memory cycle, or 1. 6 µs of central processor
time during drum transfer, slowing the central
processor by only 2. 5%.
Location 00
l23l15
y
0
J
f
23
22
17 0
O=Read Drum not
Starting
l=Write used Drum Address
23 13
0
number of
not used words to
transfer
23 15 0
not used Starting
Core Address
Figure 6
Drum Control Words
22
DISC
General Characteristics
The disc auxiliary memory provides non-
destructive bulk storage. Disc memory consists of
removable disc units and a controller which con-
tains the necessary hardware to communicate
directly to core memory. Each word consists of 24
information bits and one parity bit.
From one to four discs may be connected to a
single controller.
Checking and Protection
Odd parity is checked by hardware during disc
read and write operations. Error indications are
provided for illegal addressing, positioning error,
and limit error.
Write protection is provided for each disc drive.
When the write-protect function is activated, no
program can inadvertently write on that disc pack.
Control Word Formats
To activate a disc transfer, first place the
location Y of the first disc control word in core
memory location zero. Then call for an output to
disc utilizing an OUT instruction. This will cause
the disc controller to fetch the contents Y from core
memory location zero. The disc controller will
perform a disc operation as specified by these con-
trol words.
From
1
to 16K words can be transferred from
or to the disc for each programmed access. Trans-
fer is direct to core memory at a rate of
1.
2
million data bits per second. The central processor
operates at 92% of rated speed while transfer is
taking place.
23
LOCATION 00
First Operand Word
y
1 = write
DISC UNIT CYLINDER HEAD
SECTOR
0 = read
23 22 18 17 16 15 8 7
4 3 0
Second Operand Word
SECTOR/ WORD COUNT XFRD
13 6 5 0
STARTING CORE ADDRESS
15
0
Disc Control Words