Chapter 16 Micro-programmed Control

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Chapter 16Micro-programmed Control

• EEL 4709C
• Alan Garcia
• Arturo Linares
• Henry Mitzler

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16.1 Microprogrammed Control Basic Concepts
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Micro-programmed Control

• Use sequences of instructions to control complex
  operations
• An alternative to a hardwired control unit
• Called micro-programming, microcode, or firmware

4
Control Unit Organization
5
Implementation (1)

• All the control unit does is generate a set of
  control signals
• Each control signal is on or off
• Each control signal is represented 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

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Implementation (2)

• Todays 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

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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

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Micro-instruction Types

• Each micro-instruction specifies single (or few)
  micro-operations to be performed
• (vertical micro-programming)
• Vertical instructions must be decoded to produce
  control signals.
• Slower than horizontal microprogramming, but take
  up less memory (ROM) space.
• Each micro-instruction specifies many different
  micro-operations to be performed in parallel
• (horizontal micro-programming)
• Requires more memory, but does requires little if
  any decoding.

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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

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Horizontal Micro-programming

• Wide memory word
• High degree of parallel operations possible
• Little encoding of control information

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Typical Microinstruction Formats
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Compromise

• Divide control signals into disjoint groups
• Implement each group as separate field in memory
  word
• Supports reasonable levels of parallelism without
  too much complexity

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Organization ofControl Memory
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Control Unit
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Control Unit Function

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

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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

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Functioning of Microprogrammed Control Unit
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Wilkes Control

• Microprogram was coined by Wilkes in the early
  1950s.
• 1951 Wilkes first proposed microprogrammed
  control.
• 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

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Wilkes's Microprogrammed Control Unit
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Advantages and Disadvantages of Microprogramming

• Simplifies design of control unit
• Cheaper
• Less error-prone
• Slower than a pure hard-wired implementation
• RISC processors typically use hardwired control
  units because of their simpler instruction
  formats.
• Bugs and Errata can be corrected by microcode
  update
• E.g. Intel Core 2 Duo bugs and possible
  security vulnerabilities resolved via microcode
  update

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Advantages and Disadvantages of Microprogramming

• Instructions can be easily be added with
  microprogramming compared with hardwired control
  units.
• Specifying the architecture and instruction set
  becomes a software programming issue rather than
  hardware design problem.

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Interesting Facts

• The Nintendo 64s graphics and audio co-processor
  (MIPS 4000 based) utilized programmable microcode
• Allowed for tuning of the processor for extra
  speed and quality e.g. to run at 640x480.
• Playstation 2s vector processor units were
  microprogrammable via microcode.
• 3D Geometry and floating point arithmetic
• Microcode allows for programmers to fine-tune the
  processors at a below assembly code level, allows
  for more optimization.

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Tasks Done By Microprogrammed Control Unit

• Microinstruction sequencing
• Microinstruction execution
• Must consider both together

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17.2 Microinstruction Sequence
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Microinstruction Sequencing

• Design Considerations
• Size of microinstruction
• Address-generation time
• Determined by instruction registers
• Next sequential branch
• Branch

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Microinstruction Sequencing

• Sequencing Techniques
• Two address fields
• Single address field
• Address-generation time
• Determined by instruction registers
• Next sequential branch
• Branch

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Microinstruction Sequencing

• Branch Control Logic
• Two Address Fields

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Microinstruction Sequencing

• Branch Control Logic
• Single Address Field

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Microinstruction Sequencing

• Branch Control Logic
• Variable Format

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Microinstruction Sequencing

• Microinstruction Address Generation
• Techniques
• Explicit Implicit
• Two-Field Mapping
• Unconditional Branch Addition
• Conditional Branch Residual Control

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17.3 Microinstruction Execution
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The microinstruction cycle is the basic event on
a micro programmed processor.

• Each cycle is made up of two events
• Fetch
• Determined by generation of microinstruction
  address
• Execute
• To generate control signals.

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Control Unit Organization

• Sequencing Logic Module
• Generates the address of the next
  microinstruction
• Control Logic Module
• Generates control signals

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Microinstruction Classification

• Vertical microprogramming
• Each microinstruction specifies a single (or
  few) micro-operations to be performed
• Width is narrow n control signals can be encoded
  into log2n control bits
• Limited ability to express parallelism
• Considerable encoding of control information
  requires external memory word decoder to identify
  the exact control line being manipulated

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Microinstruction Classification

• Horizontal microprogramming
• Each microinstruction specifies many different
  micro-operations to be performed in parallel.
• Wide memory word
• High degree of parallel operations are possible
• Little to no encoding of control information

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Microinstruction Encoding

• Microinstruction is organized as a set of fields.
  
• Each field is depicted as a set of actions.
• Only one of the actions specified for a given
  field could occur at a time.

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Aspects of Encoding

• Functional Encoding
• Identifies functions within the machine and
  designates fields by function type.
• Resource Encoding
• Devotes one field to each source( e.g., I/O,
  memory, ALU)
• Direct or Indirect Encoding
• The difference is that with indirect encoding,
  one field is used to determine the interpretation
  of another field.

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References

• http//fourier.eng.hmc.edu/e85/lectures/processor/
  node11.html
• http//www.cs.binghamton.edu/reckert/hardwire3new
  .html
• http//en.wikipedia.org/wiki/Microcode
• http//en.wikipedia.org/wiki/Hardwired_control
• http//en.wikipedia.org/wiki/Control_unit
• http//en.wikipedia.org/wiki/Core_2_duo

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References

• http//en.wikipedia.org/wiki/Erratum

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Summary

• Microprogramming alternative to hard-wired
  control units, provides more flexibility in
  designing the processor and control unit.
• Disadvantages not as fast as a hard-wired
  implementation, RISC processors typically use
  hardwired control units.
• Microprogramming allows for errors microcode to
  be corrected via patches (with writable memory
  i.e. EEPROM) (as in C2D)
• Horizontal microcode represents each control
  signal as a bit with little or no encoding used.
• Vertical microcode encodes mutually exclusive
  control signals in different fields, which must
  then be decoded to produce the control signals.
  (Which requires more time because of additional
  combinational circuits).

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Review Questions

1. What is the difference between horizontal and
   vertical microinstructions?
2. Why is micro programmed control used instead of
   hardwired control for the control unit.
3. What are the advantages and disadvantages of
   micro programmed control compared with a
   hardwired control unit implementation?
4. What are the main tasks performed by a micro
   programmed control unit?

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Review Questions

1. Why are vertical microinstructions slower than
   horizontal microinstructions?
2. What is the preferred sequencing technique?
3. What are the differences between explicit
   techniques and implicit techniques?
4. What range of values does the length of vertical
   and horizontal microinstructions fall under?
5. What does the control memory contain?
6. What concerns are involved in the design of a
   microinstruction sequencing technique?