GE PAC 4020 System Manual page 20

Operation
Peripheral inputs and outputs are accomplished
by TIM and TOM functions. The information flows
by character between the arithmetic unit and the
buffer. The data will consist of 7 to 12 bits, de-
pending on the device to be used. All devices con-
nected to the I/O buff er operate in a record format or
are programmed in a record format. For example,
the card reader always reads at least one entire card
(unit record), and the paper tape reader continues
reading until the "end of record" character is de-
tected or until the TIM table fills up. The overall
I/O capability of the computer is 50-60,000 char-
acters per second.
Normally each channel will function using two
interrupts, one to indicate "data ready", and the
other "end of record" or "table full (or empty)".
Even parity is checked or added by the periph-
eral buffer for all devices except card readers, card
punches, and typewriters, which do not use parity.
The size of the buffer register depends on the
device. The card readers and card punches use
12-bit registers. All other devices have 8-bit
registers.
The bits are transmitted in parallel from the
arithmetic unit to the I/O buffer, and from the I/O
buffer to the device, except for teletypewriters and
the Data-Edit, which are bit serial. Also for the
teletypewriter, the I/O buffer generates one bit to
indicate a start of a character and two bits to indi-
cate the end of a character.
Test lines enable the program to determine the
status of any given channel. A "channel busy" line
tells the program when the addressed channel is in
use. A "data exchange ready" line tells the program
when the addressed device wants data or when it has
data ready to input. An error line informs the
program of an error condition in the addressed
device.
Computer-to-computer communication between
I/O buffers is possible. An output control module
and an input control module provide a 12-bit-
parallel, medium-speed, computer-to-computer
communication link. Communications are carried
out via the TIM/TOM mode at tr an sf er rates up to
approximately 50-60,000 characters per second.
The I/O buffer has been designed to handle
character-oriented I/O devices which send and re-
ceive data coded in ASCII. Devices which do not
respond to ASCII may be used, but program trans-
lation is required.
Programming all I/O functions on the GE/PAC
4020 computer (except the DIC) can be done most
efficiently and easily through calling sequences to
RTOS. The programmer must specify only where to
find the data and format descriptive words, which
device is desired, and what to do if that device is
bad or out of service. RTOS then handles all the
I/O mechanics including stacking requests from one
program or several for a particular device, recog-
nizing and remembering peripheral failure and out of
service conditions, performing code conversions, and
using working alternative devices as required.
RTOS thus maximizes peripheral utilization,
minimizes the possibility of loss of data, and elimi-
nates the possibilities of peripheral conflicts and
system "hang ups" due to failed devices.
Program Load. The Program Load permits an
operator to "bootstrap" a loader subroutine into
GE/PAC core memory from paper tape or cards,
using the TIM feature.
Program load uses the two highest priority
interrupt locations 000 and 001 (program interrupt
locations 2008 and 2018) corresponding to a "pri-
mary" input device and a "secondary" input device.
GE/PAC bootstrap input devices are:
Teletype ASR35
100 cps paper-tape reader
200 cps paper-tape reader
300 cpm card reader
The detailed procedure is:
Place the bootstrap card (tape) into the
reader and ready so that: the card is in "load posi-
tion"; or the tape is positioned so that a leader
frame is under the read heads.
Place the computer in manual.
Push the program load button
Initialize the computer by pushing the on
button. (This sets P
=
0 and clears interrupts.)
Latch save P switch.
Store a "LDA 0" instruction in location 0.
Store a TIM control word in the chosen
readers' API location (200
8
or 201
8
).
23
18 13
27 00000
Where P
=
2 if cards
P
=
0
if
paper tape
17