IBM 709 General Information Manual page 34

,
Figure 70.
IBM
766 Data Synchronizer
5. Use minimum time in transferring data between
input-output units and storage. One core stor-
age cycle is used for each word processed.
6. Synchronize the stored program with the input-
output operations when desired. Thus synchro-
nization is optional to the programmer.
To these six refinements, add a seventh and totally
new ability to:
7. Operate six different input-output devices and
compute simultaneously.
The 709 system programs involving input-output
and processing are considered as two separate pro-
grams although all instructions are stored in core
storage. These two programs are called the process-
ing program and the data channel program.
The processing program is responsible for all arith-
metic, logical, and some testing operations. It controls
all operations that occur within the central process-
ing unit, plus the starting of input-output operations.
The data channel program is responsible for the
actual movement of data, counting of words, and
some automatic testing.
The two programs run concurrently and the central
processing unit's program is interrupted only when a
data channel requires data from, or has data for, core
storage.
The operation of a data channel is started by cen-
tral processing unit instructions. Once started, how-
ever, the data channel operates independently of the
main program. The data channel is said to contain
or "stack" its own instructions. For example:
INSTRUCTION
1. Rewind tape unit 1.
2. Backspace tape unit 2. (These instructions are executed
immediately in the central processing unit.)
3. Write tape unit 3.
1. The rewind instruction starts tape unit I re-
winding. (This may take as long as I-Y2 minutes.)
2. Two machine cycles after the rewind instruc-
tion is decoded in the central processing unit, the
next instruction is decoded. Tape unit 2 then begins
its backspace operation. (The time to complete the
backspace is a variable depending on the record size.)
3. Again, after two machine cycles, tape unit 3
starts the writing operation. To summarize, the three
instructions are executed by the central processing
unit in six machine cycles. The central processing unit
is then free to process other data until it is asked for
a storage reference cycle by the data channel.
Instructions concerned with a data channel are
called "commands" instead of instructions in order
to keep their understanding apart from the central
processing unit instructions.
Figure 7 I shows a simple program in which the cen-
tral processing unit is processing data while the data
channel is reading information from a tape. The
data channel program, made up of commands, and the
instructions for the central processing unit are both
stored in core storage. A single command may trans-
mit a large block of words between core storage and
an
1-0
unit. Normally, many instructions in the main
program may be executed during the time taken to
execute a single command in the data channel.
Main Program. The first main program instruction
selects the proper tape attached to data channel
A.
The second instruction would then load the first
command (Ioep) into that data channel. The remain-
ing instructions would then start processing some other
central processing unit program.
Main Program Instructions
RTDA ------ read tape decimal, channel A.
RCHA------ Reset and load channel A command.
central processing unit instruction!
II II II II
" "
" "
Data Channel Program Instructions
10CP ------ 1-0 count and proceed.
10CPN----- 1-0 count and proceed, but do not transmit.
10RP------- 1-0 record control and proceed.
10CD------ 1-0 count and disconnect.
Figure 71.
CPU
and Data Channel Programs
Input-Output Components 33