CS304PC : Computer Organization and Architecture

1. CS304PC:Computer Organization
and Architecture (R18 II(I sem))
Department of computer science and engineering
(AI/ML)
Session 7
by
Asst.Prof.M.Gokilavani
VITS
1/9/2023 Department of CSE (AI/ML) 1

2. TEXT BOOK:
• 1. Computer System Architecture – M. Moris Mano, Third Edition,
Pearson/PHI.
REFERENCES:
• Computer Organization – Car Hamacher, Zvonks Vranesic, Safea
Zaky, Vth Edition, McGraw Hill.
• Computer Organization and Architecture – William Stallings Sixth
Edition, Pearson/PHI.
• Structured Computer Organization – Andrew S. Tanenbaum, 4th
Edition, PHI/Pearson.
1/9/2023 Department of CSE (AI/ML) 2

3. Unit II
Microprogrammed Control: Control memory, Address sequencing,
micro program example, Design of Control unit .
Central Processing Unit : General Register Organization, Stack
organization, Instruction formats, Addressing modes, Data Transfer and
Manipulation, Program Control.
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4. Topics covered in session 7
1/9/2023 Department of CSE (AI/ML) 4
• Microprogrammed Control
• Control memory
• Address sequencing
• Micro program example
• Design of Control unit

5. Control memory
 The function of the control unit in a digital computer is to initiate sequences of
microoperations.
 Microprogramming is a second alternative for designing the control unit of a
digital computer. The principle of microprogramming is an elegant and
systematic method for controlling the micro-operation sequences in a digital
computer.
 The control function that specifies a micro-operation is a binary variable.
 In a bus organized system, the control signals that specify micro-operations are
groups of bits that select the paths in multiplexers, decoders, and arithmetic units.
The control unit initiates a series of sequential steps of micro-operations. During
any given time, certain micro-operations are to be initiated, while others remain
idle.
 The control variables at any given time can be represented by a string of l's and
0's called a control word.

6. Control memory
• A sequence of microinstructions constitutes a microprogram. Since alterations of
the microprogram are not needed once the control unit is in operation, the control
memory can be a read-only memory (ROM).
• Two major types of Control Unit
• Hardwired Control :
• The control logic is implemented with gates, F/Fs, decoders, and
other digital circuits.
• + Fast operation, - Wiring change(if the design must be modified).
• Microprogrammed Control :
• The control information is stored in a control memory, and the
control memory is programmed to initiate the required sequence of
microoperations.
• + Any required change can be done by updating the microprogram in
control memory, - Slow operation .

7. Control memory
• Microinstruction : Control Word in Control Memory
• The micro instruction specifies one or more microoperations.
• Microprogram: A sequence of microinstruction
• Dynamic microprogramming : Control Memory = RAM
• RAM can be used for writing (to change a writable control memory)
• Microprogram is loaded initially from an auxiliary memory such as a
magnetic disk
• Static microprogramming : Control Memory = ROM
• Control words in ROM are made permanent during the hardware
production.

8. Microprogrammed control Organization
 A computer that employs a microprogrammed control unit will have
two separate memories:
 Main Memory: for storing user program (Machine
instruction/data)
 Control Memory: for storing microprogram (Microinstruction)
 Main memory is available to the user for storing the programs. The
contents of main memory may alter when the data are manipulated
and every time that the program is changed. The user's program in
main memory consists of machine instructions and data.

9. Microprogrammed control Organization

10. Microprogrammed control Organization
1. Control memory holds a fixed microprogram that cannot be altered by the
occasional user. The microprogram consists of microinstructions that specify
various internal control signals for execution of register micro-operations.
 Each machine instruction initiates a series of microinstructions in control
memory. These microinstructions generate the microoperations to
 fetch the instruction from main memory.
 to evaluate the effective address,
 to execute the operation specified by the instruction
 to return control to the fetch phase to repeat the cycle for the next
instruction.
 The general configuration of a microprogrammed control unit is demonstrated in
the block diagram of Figure.

11. Microprogrammed control Organization
2. Control Address Register: The control memory is assumed to be a ROM, within
which all control information is permanently stored.
 The control memory address register specifies the address of the
microinstruction, and the control data register holds the microinstruction read
from memory.
 The microinstruction contains a control word that specifies one or more micro-
operations for the data processor. Once these operations are executed, the control
must determine the next address.
3. Next Address generator(sequencer):
 The next address generator is sometimes called a microprogram sequencer, as
it determines the address sequence that is read from control memory.
 The address of the next microinstruction can be specified in several ways,
depending on the sequencer inputs.

12. Microprogrammed control Organization
4. Control data register:
 The control data register holds the present microinstruction while the next
address is computed and read from memory. The data register is sometimes
called a pipeline register. It allows the execution of the micro-operations
specified by the control word simultaneously with the generation of the next
microinstruction.

13. Advantage of microprogrammed control
 The main advantage of the microprogrammed control is the fact that once the
hardware configuration is established, there should be no need for further
hardware or wiring changes.
 If we want to establish a different control sequence for the system, all we need
to do is specify a different set of micro-instructions for control memory.
 The hardware configuration should not be changed for different operations;
the only thing that must be changed is the microprogram residing in control
memory.

14. Topics to be covered in next session 8
• Address sequencing
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Thank you!!!