William Stallings Computer Organization and Architecture

1
William Stallings Computer Organization and
Architecture

• Chapter 17
• Microprogrammed Control

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Topics

• Basic Concepts
• Microinstruction Sequencing
• Microinstruction Execution
• Applications of Microprogramming

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Microprogrammed Control

• Use sequences of instructions (see earlier notes)
  to control complex operations
• Called microprogramming or firmware

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Implementation (1)

• All the control unit does is to 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
  microinstruction, depending on conditions

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Implementation (2)

• Control words are put in a memory (control
  memory), with each word having a unique address
• Bits are added to specify conditions
• Put everything together
• ? Micro-instruction

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Implementation (3)

• 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
• corresponding 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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Microprogram 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 microinstruction
  address is specified

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Microinstruction Types

• Horizontal microprogramming
• Each microinstruction specifies many different
  micro-operations to be performed in parallel
• Vertical microprogramming
• Each microinstruction specifies single (or few)
  micro-operations to be performed

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Horizontal Microprogramming

• Wide memory word
• 1 bit for each CPU control line
• 1 bit for each system bus control line
• Condition field indicates branch or not
• Address field for branch target
• High degree of parallel operations possible
• Little encoding of control information

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Horizontal Microprogrammed Diagram
Internal CPU Control Signals
Micro-instruction Address
Jump Condition
System Bus Control Signals
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Horizontal Microprogramming

• Interpretation of a microinstruction
• Turn on/off all control lines indicated by a 1/0
  bit
• If condition indicated by condition bits is false
  then
• execute next instruction in sequence
• else
• execute microinstruction indicated in address
  field

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Vertical Microprogramming

• Width is narrow
• A code is used for each action to be performed
• E.g., MAR ? (PC)
• 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
• i.e., decoder translates the code into individual
  control signals

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Vertical Micro-Programming Diagram
Micro-instruction Address
Function Codes
Jump Condition
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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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Microprogrammed Control Unit

• Key elements
• Control memory
• Store microinstructions
• Control Address Register (CAR)
• Contains address of next microinstruction to be
  read
• Control Buffer Register (CBR)
• Buffer where a read microinstruction is
  transferred to
• Left-hand portion connects to control lines
• ? reading a microinstruction ? executing a
  microinstruction
• Sequencing unit
• Loads CAR and issues a READ

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Control Unit
(A)
(B)
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Control Memory
Jump to Indirect or Execute
Fetch cycle routine
Jump to Execute
Indirect Cycle routine
Jump to Fetch
Interrupt cycle routine
Jump to Op code routine
Execute cycle begin
Jump to Fetch or Interrupt
AND routine
Jump to Fetch or Interrupt
ADD routine

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Control Unit Function

• Decoder (A) translates opcode of IR into a
  control memory address
• 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
• Decoder (B) is used for vertical microinstructions

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Advantages and Disadvantages

• Microprogrammed v.s. hardwired
• Simplifies design of control unit
• Cheaper
• Less error-prone
• Slower
• CISC microprogramming
• RISC hardwired

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Tasks Done By Microprogrammed Control Unit

• Microinstruction sequencing
• Get next microinstruction from memory
• Microinstruction execution
• Generate control signals needed to execute the
  microinstruction
• Must consider both together

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Microinstruction Sequencing

• Two concerns in design considerations
• 1. Size of microinstructions (size? ? cost? )
• 2. Next address generation time (faster ?
  better)
• Next address
• Determined by instruction register
• Once per instruction cycle, after instruction is
  fetched
• Next sequential address
• Common in most designed
• Branches
• Both conditional and unconditional

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Sequencing Techniques

• Based on current microinstruction, condition
  flags, and contents of IR, control memory address
  must be generated
• Categories based on format of address information
• Two address fields
• Single address field
• Variable format

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Address Generation Techniques (Optional)

• Explicit Implicit
• Two-field Mapping (from opcode)
• Unconditional Branch Addition (base offset)
• Conditional branch Residual control
  (e.g., subroutine)

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Microinstruction Execution

• The microinstruction cycle is the basic event on
  a microprogrammed CPU
• Each cycle is made up of two events
• Fetch
• Determined by generation of microinstruction
  address
• Execute

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Execute

• Effect is to generate control signals
• Some control points internal to processor
• Rest go to external control bus or other
  interface
• Example of microinstruction encoding
• Figure 17.12

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Control Unit Organization
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Applications of Microprogramming (1)

• Realization of computers
• Systematic technique for control unit
  implementation
• Emulation
• Use of a microprogram on one machine to execute
  programs originally written for another
• Most common use aiding users in migrating from
  one (older) computer to another

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Applications of Microprogramming (2)

• OS support
• Implement primitives to replace important
  portions of OS software
• ? simplify OS implementation
• improve OS performance
• Realization of special-purpose devices
• e.g., data communication board
• High-level language support
• Micro-diagnostics
• monitoring, detection, isolation, error repair
• User tailoring

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Required Reading

• Stallings chapter 17