Chapter 16
Micro-programmed Control
EEL 4709C
Alan Garcia
Arturo Linares
Henry Mitzler
16.1 Microprogrammed Control
Basic Concepts
Micro-programmed Control
 Use sequences of instructions to control complex
operations
 An alternative to a hardwired control unit
 Called micro-programming, microcode, or firmware
Control Unit Organization
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
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
 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
 Each micro-instruction specifies single (or few) microoperations 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 microoperations to be performed in parallel
 (horizontal micro-programming)
 Requires more memory, but does requires little if any
decoding.
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
 Wide memory word
 High degree of parallel operations possible
 Little encoding of control information
Typical Microinstruction Formats
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
Organization of
Control Memory
Control Unit
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
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
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
Wilkes's Microprogrammed
Control Unit
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
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.
Interesting Facts
 The Nintendo 64’s 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 2’s 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.
Tasks Done By Microprogrammed
Control Unit
 Microinstruction sequencing
 Microinstruction execution
 Must consider both together
17.2 Microinstruction Sequence
Microinstruction Sequencing
Design Considerations:
•Size of microinstruction
•Address-generation time
o Determined by instruction registers
o Next sequential branch
o Branch
Microinstruction Sequencing
Sequencing Techniques:
•Two address fields
• Single address field
• Address-generation time
o Determined by instruction registers
o Next sequential branch
o Branch
Microinstruction Sequencing
Branch Control Logic:
Two Address Fields
Microinstruction Sequencing
Branch Control Logic:
Single Address Field
Microinstruction Sequencing
Branch Control Logic:
Variable Format
Microinstruction Sequencing
Microinstruction Address Generation
Techniques:
Explicit
Implicit
Two-Field
Unconditional Branch
Conditional Branch
Mapping
Addition
Residual Control
17.3 Microinstruction Execution
The microinstruction cycle is the basic event on a
micro programmed processor.
Each cycle is made up of two events
 Fetch
o Determined by generation of microinstruction address
 Execute
o To generate control signals.
Control Unit Organization
Sequencing Logic Module
• Generates the address of the next
microinstruction
Control Logic Module
• Generates control signals
Microinstruction Classification
Vertical microprogramming

Each microinstruction specifies a single (or few) microoperations 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
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
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.
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.
References
 http://fourier.eng.hmc.edu/e85/lectures/processor/no
de11.html
 http://www.cs.binghamton.edu/~reckert/hardwire3ne
w.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
References
 http://en.wikipedia.org/wiki/Erratum
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).
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?
Review Questions
5. Why are vertical microinstructions slower than
horizontal microinstructions?
6. What is the preferred sequencing technique?
7. What are the differences between explicit techniques
and implicit techniques?
8. What range of values does the length of vertical and
horizontal microinstructions fall under?
9. What does the control memory contain?
10. What concerns are involved in the design of a
microinstruction sequencing technique?