1. Chapter 16 Micro-programmed Control
EEL 4709C
Alan Garcia
Arturo Linares
Henry Mitzler

2. 16.1 Microprogrammed Control Basic Concepts

3. 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

6. 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

7. 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

8. 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.

9. 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

10. Horizontal Micro-programming
Wide memory word
High degree of parallel operations possible
Little encoding of control information

11. Typical Microinstruction Formats

12. 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

13. Organization of Control Memory

14. Control Unit

15. 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

16. 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

17. Functioning of Microprogrammed Control Unit

18. 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

19. Wilkes's Microprogrammed Control Unit

20. 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

21. 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.

22. Interesting Facts
The Nintendo 64’s graphics and audio co-processor (MIPS 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.

23. Tasks Done By Microprogrammed Control Unit
Microinstruction sequencing
Microinstruction execution
Must consider both together

24. 17.2 Microinstruction Sequence

25. Microinstruction Sequencing
Design Considerations:
Size of microinstruction
Address-generation time
Determined by instruction registers
Next sequential branch
Branch

26. Microinstruction Sequencing
Sequencing Techniques:
Two address fields
Single address field
Address-generation time
Determined by instruction registers
Next sequential branch
Branch

27. Branch Control Logic: Two Address Fields
Microinstruction Sequencing
Branch Control Logic:
Two Address Fields

28. Branch Control Logic: Single Address Field
Microinstruction Sequencing
Branch Control Logic:
Single Address Field

29. Branch Control Logic: Variable Format
Microinstruction Sequencing
Branch Control Logic:
Variable Format

30. Microinstruction Sequencing
Microinstruction Address Generation
Techniques:
Explicit Implicit
Two-Field Mapping
Unconditional Branch Addition
Conditional Branch Residual Control

31. 17.3 Microinstruction Execution

32. 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.

33. Control Unit Organization
Sequencing Logic Module
Generates the address of the next microinstruction
Control Logic Module
Generates control signals

34. 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

35. 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

36. 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.

37. Aspects of Encoding Functional Encoding Resource 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.

38. References
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39. References

40. 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).

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

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