William stallings computer organization and architecture 8 th edition
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William Stallings Computer Organization and Architecture 8 th Edition. Chapter 16 Micro-programmed Control. Presenters: Andres Borroto Juan Fernandez Laura Verdaguer. Control Unit Organization. Micro-programmed Control. Use sequences of instructions to control complex operations

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William stallings computer organization and architecture 8 th edition

William Stallings Computer Organization and Architecture8th Edition

Chapter 16

Micro-programmed Control

Presenters: Andres Borroto

Juan Fernandez

Laura Verdaguer

Control unit organization

Control Unit Organization

Micro programmed control

Micro-programmed Control

  • Use sequences of instructions to control complex operations

  • Called micro-programming or firmware

Implementation 1

Implementation (1)

  • All the control unit does is generate a set of control signals

  • Each control signal is on or off

  • Represent each control signal by a bit

  • Have a control word for each micro-operation

  • Have a sequence of control words for each machine code instruction

  • Add an address to specify the next micro-instruction, depending on conditions

Implementation 2

Implementation (2)

  • Today’s large microprocessor

    • Many instructions and associated register-level hardware

    • Many control points to be manipulated

  • This results in control memory that

    • Contains a large number of words

      • co-responding to the number of instructions to be executed

    • Has a wide word width

      • Due to the large number of control points to be manipulated

Micro program word length

Micro-program Word Length

  • Based on 3 factors

    • Maximum number of simultaneous micro-operations supported

    • The way control information is represented or encoded

    • The way in which the next micro-instruction address is specified

Micro instruction types

Micro-instruction Types

  • Each micro-instruction specifies single (or few) micro-operations to be performed

    • (vertical micro-programming)

  • Each micro-instruction specifies many different micro-operations to be performed in parallel

    • (horizontal micro-programming)

Vertical micro programming

Vertical Micro-programming

  • Width is narrow

  • n control signals encoded into log2 n bits

  • Limited ability to express parallelism

  • Considerable encoding of control information requires external memory word decoder to identify the exact control line being manipulated

Horizontal micro programming

Horizontal Micro-programming

  • Wide memory word

  • High degree of parallel operations possible

  • Little encoding of control information

Typical microinstruction formats

Typical Microinstruction Formats



  • Divide control signals into disjoint groups

  • Implement each group as separate field in memory word

  • Supports reasonable levels of parallelism without too much complexity

Organization of control memory

Organization ofControl Memory

Control unit

Control Unit

Control unit function

Control Unit Function

  • Sequence login unit issues read command

  • Word specified in control address register is read into control buffer register

  • Control buffer register contents generates control signals and next address information

  • Sequence login loads new address into control buffer register based on next address information from control buffer register and ALU flags

Next address decision

Next Address Decision

  • Depending on ALU flags and control buffer register

    • Get next instruction

      • Add 1 to control address register

    • Jump to new routine based on jump microinstruction

      • Load address field of control buffer register into control address register

    • Jump to machine instruction routine

      • Load control address register based on opcode in IR

Functioning of microprogrammed control unit

Functioning of Microprogrammed Control Unit

Wilkes control

Wilkes Control

  • 1951

  • Matrix partially filled with diodes

  • During cycle, one row activated

    • Generates signals where diode present

    • First part of row generates control

    • Second generates address for next cycle

Wilkes s microprogrammed control unit

Wilkes's Microprogrammed Control Unit

Advantages and disadvantages of microprogramming

Advantages and Disadvantages of Microprogramming

  • Simplifies design of control unit

    • Cheaper

    • Less error-prone

  • Slower

Tasks done by microprogrammed control unit

Tasks Done By Microprogrammed Control Unit

  • Microinstruction sequencing

  • Microinstruction execution

  • Must consider both together

Design considerations

Design Considerations

  • Size of microinstructions

  • Address generation time

    • Determined by instruction register

      • Once per cycle, after instruction is fetched

    • Next sequential address

      • Common in most designed

    • Branches

      • Both conditional and unconditional

Sequencing techniques

Sequencing Techniques

  • Based on current microinstruction, condition flags, contents of IR, control memory address must be generated

  • Based on format of address information

    • Two address fields

    • Single address field

    • Variable format

Branch control logic two address fields

Branch Control Logic: Two Address Fields

Branch control logic single address field

Branch ControlLogic: Single Address Field

Branch control logic variable format

Branch Control Logic: Variable Format

Address generation

Address Generation



  • The cycle is the basic event

  • Each cycle is made up of two events

    • Fetch

      • Determined by generation of microinstruction address

    • Execute



  • Effect is to generate control signals

  • Some control points internal to processor

  • Rest go to external control bus or other interface

Control unit organization1

Control Unit Organization

A taxonomy of microinstructions

A Taxonomy of Microinstructions

  • Packed/unpacked

  • Vertical/horizontal

  • Hard/soft microprogramming

  • Direct/indirect encoding

Improvements over wilkes

Improvements over Wilkes

  • Wilkes had each bit directly produced a control signal or directly produced one bit of next address

  • More complex address sequencing schemes,

  • using fewer microinstruction bits, are possible

  • Require more complex sequencing logic module

  • Control word bits can be saved by encoding and subsequently decoding control information

How to encode

How to Encode

  • K different internal and external control signals

  • Wilkes’s:

    • K bits dedicated

    • 2K control signals during any instruction cycle

  • Not all used

    • Two sources cannot be gated to same destination

    • Register cannot be source and destination

    • Only one pattern presented to ALU at a time

    • Only one pattern presented to external control bus at a time

  • Require Q < 2K which can be encoded with log2Q < K bits

  • Not done

    • As difficult to program as pure decoded (Wilkes) scheme

    • Requires complex slow control logic module

  • Compromises

    • More bits than necessary used

    • Some combinations that are physically allowable are not possible to encode

Specific encoding techniques

Specific Encoding Techniques

  • Microinstruction organized as set of fields

  • Each field contains code

  • Activates one or more control signals

  • Organize format into independent fields

    • Field depicts set of actions (pattern of control signals)

    • Actions from different fields can occur simultaneously

  • Alternative actions that can be specified by a field are mutually exclusive

    • Only one action specified for field could occur at a time

Microinstruction encoding direct encoding

Microinstruction EncodingDirect Encoding

Microinstruction encoding indirect encoding

Microinstruction EncodingIndirect Encoding

Required reading

Required Reading

  • Stallings chapter 16

  • Supplemental Sites:

  • http://gyan.frcrce.ac.in/~surve/COA/ISA/ControlUnit.html

  • http://www.jkalman.com/coursesites/cs311website/cs311.html

  • http://www.cs.binghamton.edu/~reckert/220/hardwire3.htm

  • http://www.egyankosh.ac.in/bitstream/123456789/24753/1/Unit-4.pdf

  • http://www.tomshardware.com



  • Discussed the microprogrammed control unit. The key to such a unit is a microinstruction.

  • Microinstruction explained

  • Explained a basic structure of microprogrammed control unit.

Extra extra

Extra Extra….

  • May 14, 2009 Fujitsu has introduced Venus SPARC64 VIIIfx which is capable of 128 GFLOPs vs Intel's top of the line Nehalem-based Xeon 5500 which achieves roughly 76 GFLOPs.

  • The original report also states that Fujitsu’s chip has 2.5 times the “high speed operation” and one-third the power consumption of an Intel CPU 

  • It’ll be reserved specifically for industrial and scientific applications, such as for studying space, weather, and astronomy.

Review questions

Review Questions?

William stallings computer organization and architecture 8th edition

1) What is the difference between a hardwired implementation and a microprogrammed implementation of a control unit?

  • A hardwired control unit is a combinatorial circuit, in which input logic signals are transformed into a set of output logic signals that function as the control signals. In a microprogrammed control unit, the logic is specified by a microprogram. A microprogram consists of a sequence of instructions in a microprogramming language. These are very simple instructions that specify micro-operations.

2 how is horizontal microinstruction interpreted

2) How is horizontal microinstruction interpreted?

  • 1. To execute a microinstruction, turn on all the control lines indicated by a 1 bit; leave off all control lines indicated by a 0 bit. The resulting control signals will cause one or more micro-operations to be performed. 2. If the condition indicated by the condition bits is false, execute the next microinstruction in sequence. 3. If the condition indicated by the condition bits is true, the next microinstruction to be executed is indicated in the address field.

3 what is the purpose of a control memory

3) What is the purpose of a control memory?

  • The control memory contains the set of microinstructions that define the functionality of the control unit.

4 what is a typical sequence in the execution of a horizontal microinstruction

4) What is a typical sequence in the execution of a horizontal microinstruction?

  • The microinstructions in each routine are to be executed sequentially. Each routine ends with a branch or jump instruction indicating where to go next.

5 what is a the difference between horizontal and vertical microinstructions

5) What is a the difference between horizontal and vertical microinstructions?

  • In a horizontal microinstruction every bit in the control field attaches to a control line. In a vertical microinstruction, a code is used for each action to be performed and the decoder translates this code into individual control signals.

6 what are the basic tasks performed by a microprogrammed control unit

6) What are the basic tasks performed by a microprogrammed control unit?

  • Microinstruction sequencing: Get the next microinstruction from the control memory. Microinstruction execution: Generate the control signals needed to execute the microinstruction.

7 what is the difference between packed and unpacked microinstructions

7)What is the difference between packed and unpacked microinstructions?

  • The degree of packing relates to the degree of identification between a given control task and specific microinstruction bits. As the bits become more packed, a given number of bits contains more information. An unpacked microinstruction has no coding beyond assignment of individual functions to individual bits.

8 what is the difference between hard and soft microprogramming

8) What is the difference between hard and soft microprogramming?

  • Hard microprograms are generally fixed and committed to read-only memory. Soft microprograms are more changeable and are suggestive of user microprogramming.

9 what is the difference between functional and resource encoding

9) What is the difference between functional and resource encoding?

  • Two approaches can be taken to organizing the encoded microinstruction into fields: functional and resource. The functional encoding method identifies functions within the machine and designates fields by function type. For example, if various sources can be used for transferring data to the accumulator, one field can be designated for this purpose, with each code specifying a different source.

  • Resource encoding views the machine as consisting of a set of independent resources and devotes one field to each (e.g., I/O, memory, ALU).

10 list some common applications of microprogramming

10)List some common applications of microprogramming?

  • Realization of computers. Emulation. Operating system support. Realization of special-purpose devices. High-level language support. Micro diagnostics. User Tailoring.

William stallings computer organization and architecture 8th edition

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