Computer Organization Chapter 7 Quiz

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Questions and Answers

What are the two main types of control unit implementations?

Hardwired and Microprogrammed

What does "CAR" stand for?

Control Address Register

What is a control signal?

A group of bits used to select paths in multiplexers, decoders, and arithmetic logic units.

What is a control variable?

A binary variable that specifies microoperations. Signup and view all the answers

What is a control word?

A string of 1's and 0's representing control variables. Signup and view all the answers

What is a microinstruction?

A control word stored in control memory. Signup and view all the answers

What is a microprogram?

A sequence of microinstructions. Signup and view all the answers

Which of these options best describes the purpose of control memory?

Provides a table for mapping instruction codes to microinstructions Signup and view all the answers

Microprogrammed control units are faster than hardwired control units.

False Signup and view all the answers

What is a microprogram routine?

A group of microinstructions stored in control memory that performs a specific task. Signup and view all the answers

What is the process called by which an instruction code is transformed into an address in control memory where the routine is located?

Mapping Signup and view all the answers

What is a subroutine?

A sequence of microinstructions used by other routines to accomplish a particular task. Signup and view all the answers

What are the main types of microprogram sequencer functions?

Incrementing CAR, loading an address, loading an initial address Signup and view all the answers

What is conditional branching?

Branching from one routine to another based on the values of status bits. Signup and view all the answers

What are status bits?

Bits that hold information about the state of the processor and its operation. Signup and view all the answers

An unconditional branch always jumps to the next instruction in the microprogram.

False Signup and view all the answers

What is the purpose of mapping in microprogramming?

To convert an instruction code into an address in control memory. Signup and view all the answers

Study Notes

Computer Organization (COME321) - Fall 2024

Course taught by Dr. Eng. Ghada Abozaid
Course offered by the Electrical Engineering Department
Course offered during Fall 2024

Chapter 7: Microprogrammed Control

Examines Control Unit Implementation
Two primary methods discussed are: Hardwired and Microprogrammed

Hardwired Control Unit Implementation

Uses combinational logic circuits to generate control signals directly from the instruction code and the sequence counter
Complex circuitry
Provides faster execution times

Microprogrammed Control Unit Implementation

Uses a separate memory (control memory) to store microinstructions, which are sequences of control signals
Easier to design, modify, and debug
Control memory stores microinstructions
Microinstructions are used to execute microoperations
Control data register (CDR) or pipeline register holds the microinstruction read from control memory
Enables executing microoperations while generating the next microinstruction

Microprogrammed Control Unit

Control Signals: Group of bits to select paths in multiplexers, decoders and arithmetic logic units
Control Variables (Binary Variables): Specify microoperations; certain microoperations initiate while others are idle
Control Word (string of 1's and 0's): Represents control variables

Control Memory

Read-only memory (ROM)
Stores microinstructions (control words)
Each instruction in the computer initiates a sequence of microinstructions from the associated microprogram in control memory
Microinstructions in control memory generate microoperations to complete the computer instruction (Fetch, evaluate effective address, execute, return control)

Microprogrammed Control Organization

Control memory: Contains microprograms (sets of microinstructions) - Microinstruction contains bits that initiate microoperations and determine the address sequence for control memory
Control Address Register (CAR): Specifies the address of the microinstruction
Next address generator (microprogram sequencer): Determines address sequence for control memory
Typical Microprogram sequencer functions include incrementing the CAR (by one), loading the address from the control memory to the CAR and Loading the initial address into the CAR to begin control operations
Control data register (CDR): Holds microinstruction
Allows execution of microoperations specified by the control word simultaneously with generating the next microinstruction

Microprogram Routines

Routine: Group of microinstructions stored in control memory
Each computer instruction has its own microprogram routine to generate microoperations
Routine to determine effective address: (Conditional or branch on status bit)
Microoperations that execute the retrieved instruction
Mapping: Transformation of instruction code bits to control memory address of the routine

Address Sequencing

Address sequencing capabilities in control memory are required
Incrementing the CAR
Unconditional or conditional branching, influenced by status bit conditions
Mapping from instruction bits to control memory address
Facilities for subroutine calls and returns

Conditional Branching

Status bits (carry-out of adder, sign bit of number, mode bits of instruction)
Decisions based on status bit conditions
Actions initiated based on conditions (1 or 0)
Unconditional branches (set status bit)

Mapping of Instructions

Each computer instruction has its own microprogram; stored in a specific location in control memory
Transformation from instruction code bits to the address in control memory where the routine is located is called mapping

Microprogram Example

Computer instruction format
Four computer instructions (ADD, BRANCH, STORE, EXCHANGE) explained
Microinstruction format (F1, F2, and F3 fields explained with descriptions)

Microinstruction Fields (F1, F2, F3)

Describes various microoperations within their fields, such as adding, subtracting, reading, storing, shifting, and branching.

Microinstruction Fields (CD, BR, AD)

CD (Condition): Defines conditions for branching, including "Always," "Indirect Address (bit)," "Sign of AC," "Zero value in AC"
BR (Branch): Defines the set of branches like JMP, CALL, RET or MAP
AD (Address): Address field

Symbolic Microinstruction

Sample format for symbolic representation
The role of labels, micro-ops, CD, BR, AD explained

Fetch Routine

Includes reading an instruction from memory, decoding, and updating the program counter.
Symbolic Microprogram examples and the binary format for fetch routine.

Design of Control Unit

A visual representation of the control unit's architecture and its components

Microprogram Sequencer

A detailed visual representation of the microprogram sequencer including external input(s), multiplexers, incrementer, control memory and the relationship between them

Input Logic for Microprogram Sequencer

Detailed logic and functions for different input fields and combinations
Different conditions that affect address lookup(e.g., subroutine call, return logic)

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Description

Test your knowledge on Chapter 7: Microprogrammed Control in Computer Organization (COME321). This quiz covers the implementation of both hardwired and microprogrammed control units, their characteristics, and related concepts. Enhance your understanding of control unit designs and their operational efficiencies.