Stored Programs

1
Stored Programs

• A look at how
• programs are executed

2
The Central Processing Unit

• The CPU is the computers mechanism for
  performing operations on data and coordinating
  the sequence of these operations.
• The CPU consists of two main parts
• The Arithmetic Logic Unit - contains circuitry
  that performs operations
• The Control Unit - contains circuitry that
  coordinates the machines activities.

3
Registers

• 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.

4
Control Unit ALU jobs

• The control unit is responsible for placing the
  data thats 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.

5
CPU - Main Memory Diagram
Registers
Arithmetic Logic Unit
Control Unit
Bus
Central Processing Unit
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.

7
Machine Instructions Example 2Division

• 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).
• Data isnt really moved, which implies that the
  original location is left vacant.
• 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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3 Categories of Machine Instructions (continued)

• Control - Instructions that direct the execution
  of a program rather than manipulate data.
• Conditional Branch (Step 3 in example 2) - branch
  to a different step if a certain condition is
  true.
• Unconditional branches or jumps - Example
  Jump to step 25.

10
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.

11
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 1s
  and 0s.

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Machine Language Instructions

• Each machine language instruction is made up of
  two parts
• The opcode - a bit pattern (of 1s and 0s) that
  is a code for an operation, like LOAD, ADD, JUMP
  or STORE
• The operands - bit patterns that provide more
  information about the instruction.
• 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
• Instruction register
• 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.

15
Program Execution

• The control unit performs its job by continually
  repeating what is called the machine cycle.
• The machine cycle consists of 3 steps
• Fetch - gets the next instruction from memory
• Decode - decodes the bit pattern in the
  instruction register
• Execute - performs the action requested by the
  instruction in the instruction register.

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Program vs Data

• Many programs can be stored in the computers
  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 1s and 0s, if the address of data is put
  into the program counter, the machine will
  attempt to execute data. The likelihood is that
  it wont get very far.

17
Other ALU Operations

• Arithmetic Instructions (already mentioned)
• Logic Instructions
• AND, OR and EXCLUSIVE OR are the logic
  instructions.
• They all take two operands and produce one result.

18
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
  columns result is independent of the results of
  its neighboring columns.

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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
0
1
B
0
0
0
1
0
1
21
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.

22
Truth Table for OR
A
0
1
B
1
0
0
1
1
1
23
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.