Chapter 16Micro-programmed
Control
EEL 4709C
Alan GarciaArturo LinaresHenry 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 microprocessorMany instructions and associated register-level hardware
Many control points to be manipulated
This results in control memory thatContains 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 factorsMaximum 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) 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.
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 ofControl 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 registerGet 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 activatedGenerates signals where diode presentFirst part of row generates controlSecond generates address for next cycle
Wilkes's Microprogrammed Control Unit
Advantages and Disadvantages of Microprogramming
Simplifies design of control unitCheaperLess error-prone
Slower than a pure hard-wired implementationRISC processors typically use hardwired control units because of their simpler instruction formats.
Bugs and Errata can be corrected by microcode updateE.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 microcodeAllowed 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 timeo Determined by instruction registerso Next sequential brancho Branch
Sequencing Techniques:
•Two address fields• Single address field• Address-generation time
o Determined by instruction registerso Next sequential brancho 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 ImplicitTwo-Field Mapping
Unconditional Branch Addition
Conditional Branch Residual Control
Microinstruction Sequencing
17.3 Microinstruction Execution
Each cycle is made up of two eventsFetch
o Determined by generation of microinstruction address
Executeo To generate control signals.
The microinstruction cycle is the basic event on a micro programmed processor.
Control Unit Organization
Sequencing Logic Module• Generates the address of the next microinstruction
Control Logic Module • Generates control signals
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
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 Classification
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.
Microinstruction 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.
Aspects of Encoding
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
References
http://en.wikipedia.org/wiki/Erratum
SummaryMicroprogramming – 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 Questions5. 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?
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