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Data flow for the TIM/TOM function is via the
adder and B register, with the accumulator or A
register always left undisturbed. The address is 14
bits, allowing data to be read into or written from
the first 16K of core.
The word count (N) permits up to 63 words to be
read or written in a block. This is equal to 252,
189, 126, or 63 characters depending on the packing
mode used. The C and N fields are adjacent so a
carry-out of the C field will cause the N field to
increment, automatically counting both characters
and words.
The TIM (input) function builds a word in mem-
ory character by character from an input device,
using the following cycle:
1. Contents of the memory location go to the
B register.
2. The B register is circularly shifted left by
the number of bits in one character position
(6, 8, 12 bits).
It
is not shifted if P
=
11
(1 character /word.
3. A character is OR'ed into the least signifi-
cant bits of the B register from the input
device.
4. The contents of the B register are trans-
ferred back to memory.
This cycle is repeated for each character in the
word. The B register is cleared between steps 1
and 2 before the first character is transferred.
It
is
not cleared for the remaining characters in the word.
When more than one character is packed into a word
(P
=
2, 3, or 4), the first character i.s in the most
significant bits of the word and the last character is
in the least significant bits. When only one char-
acter is in the word and it is less than 24 bits, it is
right-justified in the word (positioned in the least
significant bits).
When the N-field reaches 77 8 , a second and
separate (echo signal) interrupt occurs calling for
program intervention. Any further attempt to exer-
cise the TIM/TOM function on the same channel
without updating the control word will result in a
no operation.
The TOM (output) function operates similarly.
The TIM and TOM functions are initiated by an
operate (OPR) instruction referencing the appropri-
ate device or subsystem number. The A register
must contain a special "medium enable" code at this
time. Thereafter, A is not involved in any way.
These functions are stopped by a "data termination
code, handled the same way.
Using the A Register. The GE/PAC 4020
computer also has a communications channel con-
nected with its A register, used mainly to drive the
digital input scanner, which reads the states of con-
tact closures in groups of 24. This subsystem is so
fast that no waiting - and hence no buffers or
interrupts - are necessary to use it. The contact
sensing programs are generally written to read a
group (with an IN command), process its information
and store it if necessary.
Since the TIM or TOM operation modifies the
control word, the driver program must re-initialize
it.
Depending on where a subsystem's controller or
a device's buffer is located physically with regard to
the central processor, I/O operations will require
either 8. 5 (IN or OUT command) or 12. 7 s.
TIM/TOM functions are 26. 5 or 30. 7 s. Most
process I/O subsystems require the extra time.
This mode is also useful when dealing with
special I/O situations requiring such functions as
selecting a device, activating it, operating it, or
aborting its operation (also usable with direct-
access controllers). GEN II commands perform
these functions.
The Input/Output Buffer. The input/output
buffer controls the operations of peripheral devices,
allowing the central processor to continue other
operations. High throughput rates are achieved by
individual device character buffering and interrupts.
The basic characteristics of the I/O buff er are:
Offers a maximum of 16 channels, of which 7
may be used for input.
Operates peripherals in a record-oriented mode,
for more accuracy and longer life.
Accommodates the American Standard Code for
Information Interchange (ASCII).
Permits concurrent operation of all attached de-
vices at their rated speeds through individual
one-character buffers for each device.
The I/O buffer is the connecting link between the
central processor and input/output devices: such as
tape readers, tape punches, card readers, card
punches, teletypewriters, output typewriters, I/O
typewriters, line printers (300 LPM), and the Data-
Edit Display (Local operation up to 1200 bps.)
Data-Edit Display (Local operation up to 1200
bps)
15
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