Stored Programs
A look at how
programs are executed
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The Central Processing Unit
The CPU is the computer’s mechanism
for performing operations on data and
coordinating the sequence of these
operations.
 The CPU consists of two main parts

o The Arithmetic Logic Unit - contains
circuitry that performs operations
o The Control Unit - contains circuitry that
coordinates the machine’s activities.
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Registers

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Since the CPU is a separate device
from main memory, it needs to have
special memory cells to use as a
temporary storage place to hold the
data it will be working with. These
memory cells within the CPU are called
registers.
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Control Unit & ALU jobs
The control unit is responsible for
placing the data that’s needed in the
registers, telling the ALU which registers
are being used, and to turn on the
correct circuitry in the ALU for the
desired operation.
 The ALU uses the data in the indicated
registers to do the operation and places
the result in the indicated register.

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CPU - Main Memory Diagram
Registers
Arithmetic
Logic
Unit
Control
Unit
Central Processing Unit
Bus
Main Memory
The number of registers varies from machine to machine
The number is a multiple of 2, typically 16 or 32.
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Machine Instructions : Example 1
Add Two Numbers Stored in Memory
1) Get one value and put it in a register.
2) Get the other value and put it in a
register.
3) Activate the addition circuitry with
the registers used in steps 1 and 2
as inputs and another register
designated to hold the result.
4) Store the result in memory.
5) Stop.
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Machine Instructions Example 2
Division
1) LOAD a register with a value in memory
2) LOAD another register with another value in
memory
3) If this second value is zero, jump to step 6.
4) Divide the contents of the first register by the
contents of the second register and put the result in
the accumulator
5) STORE the contents of the accumulator in
memory
6) Stop.
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3 Categories of Machine Instructions

Data Transfer - Instructions that move
data around (#s 1, 3, & 4 in example 1).
o Data isn’t really moved, which implies that
the original location is left vacant.
o Data is actually copied from one place to
another. The original is undisturbed.
Arithmetic/Logic - Instructions that tell
the control unit to request an activity
within the ALU (Arithmetic operations or
Logical operations)
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
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3 Categories of Machine Instructions
(continued)

Control - Instructions that direct the
execution of a program rather than
manipulate data.
o Conditional Branch (Step 3 in example 2) branch to a different step if a certain
condition is true.
o Unconditional branches or jumps Example : Jump to step 25.
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The Stored Program
The idea that a program can be stored
in main memory is generally credited to
John Von Neumann.
 In fact, modern computers are said to
have Von Neumann architecture.
 Many programs can be stored in
memory. To execute the program we
want its first instruction is fetched from
memory.

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The Stored Program
Remember that whether the program is
written in a high-level language, like C,
or in assembly language, it is
transformed into machine language
before the computer can use it.
 Machine language is comprised of a
series of individual instructions that are
coded into 1’s and 0’s.

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Machine Language Instructions

Each machine language instruction is
made up of two parts
o The opcode - a bit pattern (of 1’s and 0’s)
that is a code for an operation, like LOAD,
ADD, JUMP or STORE
o The operands - bit patterns that provide
more information about the instruction.
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•Example: If the opcode was for LOAD, the the
operands would have to say what memory
location was to be read and the register to
place the value in.
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Program Execution
A computer follows a program stored in
its memory by getting the instructions
from memory and putting a copy of
them in the control unit as they are
needed.
 There are two special purpose registers
used for program execution

o Instruction register
o Program counter
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Instruction Register
& Program Counter
The Instruction Register (IR) contains
the instruction that is currently being
executed.
 The Program Counter (PC) contains the
address of the next instruction in
memory.

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Program Execution
The control unit performs its job by
continually repeating what is called the
machine cycle.
 The machine cycle consists of 3 steps

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o Fetch - gets the next instruction from
memory
o Decode - decodes the bit pattern in the
instruction register
o Execute - performs the action requested by
the instruction in the instruction register.
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Program vs Data


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Many programs can be stored in the
computer’s memory
We can start a program running by putting its
starting address in the program counter.
Both data and programs are stored in
memory, and remembering that they are both
just a collection of 1’s and 0’s, if the address
of data is put into the program counter, the
machine will attempt to execute data. The
likelihood is that it won’t get very far.
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Other ALU Operations
Arithmetic Instructions (already
mentioned)
 Logic Instructions

o AND, OR and EXCLUSIVE OR are the
logic instructions.
o They all take two operands and produce
one result.
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Logical Operations
For all logical operations, we consider a
1 to be true and a 0 to be false.
 Truth tables can be constructed for each
of the logical operations and the
contents of these truth tables can be
considered the definitions of the
operations.
 With logical operations, unlike addition,
each column’s result is independent of
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
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Truth Tables
A truth table is a table of values for an
operation. To construct a truth table for
an operation, put all possible values of
one variable across the top of the table
and all possible values of a second
variable down the left side of the table.
 The upper left corner of the table is
divided in two and should give the
names of the variables being used.

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Truth Table for AND
A
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B
0
1
0
0
0
1
0
1
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AND
We can see from the truth table for AND
that the only way we can get a result of
true is if both operands are true.
 So A AND B is true if and only if A is
true and B is also true.

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Truth Table for OR
A
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B
0
1
0
0
1
1
1
1
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OR
We can see from the truth table for OR
that the only way we can get a result of
false is if both operands are false.
 So A OR B is true, if A is true and B is
also true, or if only A is true, or if only B
is true. So either A or B needs to be
true or both can be true.

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