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Coded information which causes a compu-
ter to perform a specific task (such as
Add or Subtract) is called an instruc-
tion.
A series of instructions used to
solve a problem on a computer is called
a program.
A program is sometimes
referred to as a stored program because
of the fact that it is kept in main
storage when it is executed.
The
instructions of the stored program are
read out of main storage, one at a
time:
Each instruction is decoded in
the control section of the Central Proc-
essing Unit (CPU).
After being decoded in the control
section of the CPU, the instruction is
then executed in the Arithmetic Logic
Unit (ALU) section of the CPU.
For
every instruction, there are two periods
of time.
The time during which the
instruction is read out (fetched) from
main storage and interpreted is known as
I-time.
The operation specified by the
instruction is performed during E-time
(or execution time).
Data is the name
generally given to information .read out
of main storage during E-time. Instruc-
tions are information read out of main
storage during I-time.
An instruction
may be treated as data and changed if it
is read out during E-time.
The instructions of a stored program
are generally read out and executed in a
sequential manner.
The sequential man-
ner of instruction fetching and execu-
tion can be changed by instructions
known as branch instructions.
In the System/360there is no clear
division between I-time and E-time.
That is, before the instruction has been
completely read out and analyzed by the
control section, some part of the execu-
tion may have already been started.
But
for purpose of our discussion, we can
think of I-time as being separate from
E-time.
Instructions are generally thought of
as having two parts.
One part of the
instruction is used to tell the computer
what to do (such as Add or Branch) •
The
portion of the instruction that tells
the computer what to do is known as the
Cp Code.
The othe.r portion of the
instruction generally tells the computer
where the data is located.
For this
reason it is called the address portion.
The address portion of an instruction
may contain other information besides
1-44
data addresses.
In a branch
instruction, it would give the address
of the next instruction to be executed.
In some instructions, the data to be
operated on may be contained in the
address portion.
Let's continue now
with the study of the System/360 and its
Program Status Word (PSW).
INSTRUCTION ADDRESS FIELD
•
•
•
The Program Status Word (PSW) is a
double-word contalning 8 bytes, or
64 bits of control and status infor-
mation.
The PSW is maintained as part of the
internal machine circuitry_
The address of the next sequential
instruction to be fetched from main
storage is contained in bits 40-63
(24
bits) of the PSW.
In the System/360 there is a doubleword
of information used to indicate the
status of the program as well as to
control the program.
This doubleword is
called the Program Status Word (PSW).
The PSW includes status information such
as:
1.
The location of the next instruc-
tion.
2.
Whether an arithmetic operation has
resulted in a positive or negative
answer.
Possibly the operation
ended with a zero balance or an
overflow.
The PSW is a doubleword and contains
8
bytes of information.
Like all dou-
blewords, the bits of the PSW are num-
bered 0 to 63 from. left to right.
0-----------------63
r---------------------,
IPROGRAM STATUS WORD
I
l _____________________
J
For right now let us examine only one
portion of the PSW.
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