1. Micro-programmed Control

2. Typical Microinstruction Formats
Horizontal microinstruction
Vertical microinstruction
Such a microinstruction is interpreted as follows:
To execute this microinstruction, turn on all the control lines indicated by a 1 bit; leave off all control lines indicated by a 0 bit. The resulting control signals will cause one or more micro-operations to be performed.
2. If the condition indicated by the condition bits is false, execute the next microinstruction in sequence.
3. If the condition indicated by the condition bits is true, the next microinstruction to be executed is indicated in the address field.

3. Typical Microinstruction Formats

4. Organization of Control Memory
These control words or microinstructions could be arranged in a control memory.
The microinstructions in each routine are to be executed sequentially.
Each routine ends with a branch or jump instruction indicating where to go next.
There is a special execute cycle routine whose only purpose is to signify that one of the machine instruction routines (AND,ADD, and so on) is to be executed next, depending on the current op-code.

6. Control Unit Micro-architecture
The control unit functions as follows:
To execute an instruction, the sequencing logic unit issues a READ command to the control memory.
2. The word whose address is specified in the control address register is read into the control buffer register.
3. The content of the control buffer register generates control signals and next address information for the sequencing logic unit.
4. The sequencing logic unit loads a new address into the control address register based on the next-address information from the control buffer register and the ALU flags.

7. Depending on the value of the ALU flags and the control buffer register, one of three decisions is made:
• Get the next instruction: Add 1 to the control address register.
• Jump to a new routine based on a jump microinstruction: Load the address field of the control buffer register into the control address register.
• Jump to a machine instruction routine: Load the control address register based on the op-code in the IR.

9. Control Unit Organization

10. Implementation (1)
All the control unit does is 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 micro-instruction, depending on conditions

11. Today’s large microprocessor
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

12. 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

13. Micro-instruction Types
Each micro-instruction specifies single (or few) micro-operations to be performed
(vertical micro-programming)
Each micro-instruction specifies many different micro-operations to be performed in parallel
(horizontal micro-programming)

14. 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

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

16. 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

17. Organization of Control Memory

18. Control Unit

19. Control Unit Function Sequence login 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 login loads new address into control buffer register based on next address information from control buffer register and ALU flags

20. Depending on ALU flags and control buffer register
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

21. Functioning of Microprogrammed Control Unit

22. Matrix partially filled with diodes During cycle, one row activated
Wilkes Control
1951
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

23. Wilkes's Microprogrammed Control Unit

24. Advantages and Disadvantages of Microprogramming
Simplifies design of control unit
Cheaper
Less error-prone
Slower

25. Q. Explain Microinstruction Sequencing ?
2 basic Tasks Done By Micro-programmed Control Unit
Microinstruction sequencing
Microinstruction execution

26. Design Considerations
Size of microinstructions
Minimizing the size of the control memory reduces the cost of that component
Address generation time
To execute microinstructions as fast as possible

27. Sequencing Techniques
Based on current microinstruction, condition flags, contents of IR, control memory address must be generated.
Based on format of address information
Two address fields
Single address field
Variable format

28. Branch Control Logic: Two Address Fields

29. Branch Control Logic: Single Address Field

30. Branch Control Logic: Variable Format

31. Variable format
This approach provides the two entirely different microinstruction formats.
In 1st format, the some bits are used to activate control signals.
In the 2nd format, some bits used for the branch logic module, and the remaining bits provide the address
With the first format, the next address is either the next sequential address or an address derived from the instruction register.
With the second format, either a conditional or unconditional branch is being specified.
One disadvantage of this approach is that one entire cycle is consumed with each branch microinstruction.

32. Que. Explain Micro-instruction Execution
The cycle is the basic event
Each cycle is made up of two events
Fetch
Execute

33. Work Of Execution
Effect is to generate control signals
Some control points internal to processor
Rest go to external control bus or other interface

34. Control Unit Organization

35. A Taxonomy of Microinstructions
Vertical/horizontal
Packed/unpacked
Hard/soft microprogramming
Direct/indirect encoding

36. Improvements over Wilkes
Wilkes had each bit directly produced a control signal or directly produced one bit of next address
More complex address sequencing schemes,
using fewer microinstruction bits, are possible
Require more complex sequencing logic module
Control word bits can be saved by encoding and subsequently decoding control information

37. How to Encode K different internal and external control signals
Not all used
Two sources cannot be gated to same destination
Register cannot be source and destination
Only one pattern presented to ALU at a time
Only one pattern presented to external control bus at a time
Require Q < 2K which can be encoded with log2Q < K bits
Not done
As difficult to program as pure decoded (Wilkes) scheme
Requires complex slow control logic module
Compromises
More bits than necessary used
Some combinations that are physically allowable are not possible to encode

38. Specific Encoding Techniques
Microinstruction organized as set of fields
Each field contains code
Activates one or more control signals
Organize format into independent fields
Field depicts set of actions (pattern of control signals)
Actions from different fields can occur simultaneously
Alternative actions that can be specified by a field are mutually exclusive
Only one action specified for field could occur at a time

39. Microinstruction Encoding Direct Encoding

40. Microinstruction Encoding Indirect Encoding

41. Required Reading
Stallings chapter 16