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Stored Program Concept
The 1620
CPU
is a stored program computer, that is,
it stores and executes its instructions internally. The
computer can perform distinct operations such as
adding, subtracting, multiplying, comparing, branch-
ing, and so on. It is directed by an instruction placed
in core storage to perform a specific operation. The
programmer can select the most suitable operations,
from various computer operations, to solve a problem
or process data. A group of instructions representing
the operations to be performed is called a program.
Once the program is placed in core storage, thE)
computer can be directed to execute automatically
the instructions composing the program. The program
normally is executed in a sequential manner, that is,
the computer starts with the first instruction and pro-
gresses serially through the program, interpreting and
executing each instruction. However, this sequence
of operations can be altered by the use of instructions
that may direct the computer to an instruction located
somewhere other than the next sequential position.
Instruction
Characteristics
The 1620 uses a 12-digit machine language instruction
divided into three parts: a 2-digit operation ( Op )
code, a 5-digit P address, and a 5-digit
Q
address. An
instruction as it appears in core storage may be divid-
ed into 0, P, and Q subscripted numbers, as follows:
In contrast to a data field, which is addressed at
its rightmost (low-order) digit and read from right
to left, instructions are addressed at
0
o ,
the leftmost
(high-order) digit, and read from left to right.
Op Code
Upon initiation of an instruction, the Op code is
placed in a 2-digit Op register and is analyzed to
1620 Instructions
determine the operation to be performed. The address
of an instruction must always be even, i.e., the
0
0
digit of an operation code must be stored in an even-
numbered address so that the Op register can receive
both digits.
P Address
The P address specifies: (1) the location to which
data is transmitted, (2) the location to which the
program branches, (3) the location from which data
is transmitted (output instructions), or (4) the loca-
tion of the alphameric field in the Transfer Numerical
Strip and Transfer Numerical Fill special feature in-
structions.
QAddress
The
Q
address specifies: (1) the location from which
data is transmitted, (2) the indicator being interro-
gated, (3) the input/output device being used, or
( 4) the location of the numeric field in the Transfer
Numerical Strip and Transfer Numerical Fill special
feature instructions. Also, instruction modifier digits
are placed in the Q address for those instructions with
the same Op code number.
Instruction Execution Time
Each instruction or operation performed by the com-
puter is divided into two parts: I (Instruction) cycle
and E (Execution) cycle.
INSTRUCTION CYCLE
During the I -cycle, an instruction is read from core
storage and interpreted; control circuitry is established.
The I-cycle always takes eight 20-microsecond machine
cycles (160 microseconds).
EXECUTION CYCLE
The operation specified by the instruction is carried
out during the E-cycle. The number of machine cycles
necessary to execute an instruction depends on the
operation, size of the data fields, and signs of the
fields (in arithmetic operations). The last E-cyde of
an instruction is followed by the first I -cycle of the
next instruction.
The formula for computing the total execution
time follows the description of each instruction. Exe-
cution times for all instructions are summarized in
7
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