The document covers basic computer organization and design, focusing on instruction codes, the structure of operation codes, and their functions in processing. It elaborates on control units, differentiating between hardwired and micro-programmed control units, discussing their advantages and disadvantages. Additionally, it outlines instruction cycles, memory-reference instructions, and the design of control units with references to key literature.

1. Module II
Basic Computer Organizations and
Design
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2. INSTRUCTION CODES
• A program is a set of instructions that specify
the operations, operands, and the sequence
by which processing has to occur.
• A computer instruction is a binary code that
specifies a sequence of micro-operations for
the computer. Instruction codes together with
data are stored in memory.
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Computer Organizations and Design
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3. • An instruction code is a group of bits that
instruct the computer to perform a specific
operation.
• The operation code of an instruction is a
group of bits that define such operations as
add, subtract, multiply, shift, and
complement.
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Computer Organizations and Design
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4. • An operation code is sometimes called a
macro-operation because it specifies a set of
micro-operations.
• An instruction code must specify not only the
operation but also the registers or the
memory words where the operands are to be
found, as well as the register or memory word
where the result is to be stored.
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Computer Organizations and Design
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5. Stored Program Organization
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Computer Organizations and Design
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6. Direct and indirect address
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Computer Organizations and Design
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7. Computer Registers
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Computer Organizations and Design
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8. Common Bus System
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Computer Organizations and Design
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9. Computer Instructions (Basic
computer instruction formats)
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Computer Organizations and Design
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10. Instruction Set Completeness
1. Arithmetic, logical, and shift instructions
2. Instructions for moving information to and from
memory and processor registers
3. Program control instructions together with
instructions that check status conditions
4. Input and output instructions
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Computer Organizations and Design
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11. Timing and Control
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12. Instruction Cycle
1. Fetch an instruction from memory.
2. Decode the instruction.
3. Read the effective address from memory if
the instruction has an indirect address.
4. Execute the instruction.
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Computer Organizations and Design
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13. Flowchart for instruction cycle
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14. Memory-Reference Instructions
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Computer Organizations and Design
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15. Input-Output and Interrupt
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Computer Organizations and Design
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16. Program Interrupt
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17. VD/ASET/COA/B.Tech./Module II/Basic
Computer Organizations and Design
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18. Design of Accumulator Logic
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Computer Organizations and Design
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19. VD/ASET/COA/B.Tech./Module II/Basic
Computer Organizations and Design
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20. VD/ASET/COA/B.Tech./Module II/Basic
Computer Organizations and Design
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21. Hardwired and Microprogrammed
control
 The control unit directs the entire computer system to carry
out stored program instructions.
 The control unit must communicate with both the
arithmetic logic unit (ALU) and main memory.
 The control unit instructs the arithmetic logic unit that
which logical or arithmetic operation is to be performed.
 The control unit co-ordinates the activities of the other two
units as well as all peripherals and auxiliary storage devices
linked to the computer.
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Computer Organizations and Design
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22. DESIGN OF CONTROL UNIT
Hardwired Control Unit
Micro-programmed Control Unit
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Computer Organizations and Design
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23. Hardwired Control Organization
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Computer Organizations and Design
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24. ADVANTAGES OF HARDWIRED CONTROL
UNIT
Hardwired Control Unit is fast because
control signals are generated by
combinational circuits.
The delay in generation of control signals
depends upon the number of gates.
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Computer Organizations and Design
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25. DISADVANTAGES OF HARDWIRED
CONTROL UNIT
More is the control signals required by CPU; more
complex will be the design of control unit.
Modifications in control signal are very difficult.
That means it requires rearranging of wires in the
hardware circuit.
It is difficult to correct mistake in original design
or adding new feature in existing design of
control unit.
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Computer Organizations and Design
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26. MICRO-PROGRAMMED CONTROL
UNIT
A micro-programmed control unit is implemented
using programming approach. A sequence of
micro-operations are carried out by executing a
program consisting of micro-instructions.
Micro-program, consisting of micro-instructions is
stored in the control memory of the control unit.
Execution of a micro-instruction is responsible for
generation of a set of control signals.
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27.  A micro-instruction consists of:
(i) One or more micro-operations to be executed.
(ii)Address of next microinstruction to be executed.
 Micro-Programs: Microprogramming is the concept for
generating control signals using programs. These programs
are called micro-programs.
 Micro-Instructions: The instructions that make micro-
program are called micro-instructions.
 Control Memory: Micro-programs are stored in the read
only memory (ROM). That memory is called control
memory
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Definitions

28. CONTROL MEMORY
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Computer Organizations and Design
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29. ADDRESS SEQUENCING
The address sequencing capabilities required in a control
memory are:
 Incrementing of the control address register.
 Unconditional branch or conditional branch, depending
on status bit conditions.
 A mapping process from the bits of the instruction to
an address for control memory.
 A facility for subroutine call and return.
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30. SELECTION OF ADDRESS FOR CONTROL MEMORY
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31. DESIGN OF CONTROL UNIT
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32. ADVANTAGES OF MICRO-PROGRAMMED
CONTROL UNIT
 The design of micro-program control unit is less complex because micro-
programs are implemented using software routines.
 The micro-programmed control unit is more flexible because design
modifications, correction and enhancement is easily possible.
 The new or modified instruction set of CPU can be easily implemented by
simply rewriting or modifying the contents of control memory.
 The fault can be easily diagnosed in the micro-program control unit using
diagnostics tools by maintaining the contents of flags, registers and
counters.
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Computer Organizations and Design
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33. DISADVANTAGES OF MICRO-
PROGRAMMED CONTROL UNIT
 The micro-program control unit is slower than
hardwired control unit. That means to execute an
instruction in micro-program control unit requires
more time.
 The micro-program control unit is expensive than
hardwired control unit in case of limited hardware
resources.
 The design duration of micro-program control unit is
more than hardwired control unit for smaller CPU.
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34. COMPARISON BETWEEN HARDWIRED AND
MICRO-PROGRAMMED CONTROL UNIT
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35. Source/Reference
• M. Morris Mano, Computer System Architecture,
Third Edition, Prentice Hall, 1993.
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Computer Organizations and Design
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