1. CHAPTER 16 MICROPROGRAMMED CONTROL DR. H. WATSON FRANCISCO VASQUEZ
FOR:
DR. H. WATSON
BY:
FRANCISCO VASQUEZ
SHINA AHMAD
CARLOS FERNANDEZ
NINGYUAN WANG
MIGUEL RONDON
CARLOS LAGUERRE
MICHAEL BOZA
FLORIDA INTERNATIONAL UNIVERSITY – ENGINEERING CENTER
SUMMER 2011

2. Chapter # 16: Microprogrammed Control
Topics:
Basic Concepts
Microinstructions
Microprogrammed Control Unit
Wilkes Control
Advantages and Disadvantage
Microinstruction Sequencing
Design Considerations
Sequencing Techniques
Address Generation
LSI–11 Microinstruction Sequencing
Microinstruction Execution
A Taxonomy of Microinstructions
Microinstruction Encoding
LSI–11 Microinstruction Execution
IBM 3033 Microinstruction Execution
TI 8800
Microinstruction Format
Microsequencer
Registered ALU

3. Chapter # 16: Microprogrammed Control
Registers diagram:

4. Chapter # 16: Microprogrammed Control
Basic Elements of a Processor:
ALU
Registers
Internal data paths
External data paths
Control Unit

5. Chapter # 16: Microprogrammed Control
Functions of Control Unit:
Sequencing
Causes the CPU to step through a series of micro-operations
Execution
The Control Unit causes each micro-operation to be performed (Using control signals)

6. Chapter # 16: Microprogrammed Control
Data Paths and Control Signals:

7. Chapter # 16: Microprogrammed Control
Types of Micro-operations:
Transfer data between registers
Transfer data from register to external (Memory, I/O)
Transfer data from external to register
Perform arithmetic or logical operations

8. Chapter # 16: Microprogrammed Control
A microprogram has a sequence of instructions in a microprogramming
language.
This are very simple instructions that specify micro-operations.

9. Chapter # 16: Microprogrammed Control
As in a hardwired control unit, the control signal generated by a microinstruction are used to cause register transfers and ALU operations .
Use sequences of instructions to control complex operations.
Called micro-programming or Firmware.

10. Chapter # 16: Microprogrammed Control
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. Chapter # 16: Microprogrammed Control
Typical Microinstruction Formats:

12. Chapter # 16: Microprogrammed Control
Organization of Control Memory:

13. Chapter # 16: Microprogrammed Control
Control Unit:

14. Chapter # 16: Microprogrammed Control
Functioning of Microprogrammed Control Unit:

15. Chapter # 16: Microprogrammed Control
Processor block diagram example:

16. Chapter # 16: Microprogrammed Control
Wilkes Control - Diode Matrix:
A diode memory is just a collection of diodes connected in a matrix.

17. Chapter # 16: Microprogrammed Control
Wilkes’s Microprogrammed Control Unit:

18. Chapter # 16: Microprogrammed Control
Wilkes Control – key points:
Developed by Maurice Wilkes in the early 1950s
Matrix partially filled with diodes
During cycle, one row is activated
Generates signals where diode present
First part of row generates control
Second part generates address for next cycle

19. Chapter # 16: Microprogrammed Control
Advantages and disadvantages:
Hardwired:
It is difficult to design and test such piece of hardware.
The design is relatively inflexible. For example to add a new instruction.
It is relatively faster.
Microprogrammed:
Simplifies design of Control Unit
Cheaper
Less error-prone
Slower

20. MICROINSTRUCTION SEQUENCING
1- Design Considerations
2- Sequencing Techniques
3- Address Generation
4- LSI-11 Microinstruction Sequencing

21. Design Considerations
Determined by Instruction Register
Next sequential address
Branch.

22. Sequencing Techniques
Two address fields
Single address field
Variable format

23. Control address register
Two Address Fields
Control address register
Address Decoder
Control memory
Control
Address 1
Address 2
Flags
Branch
logic
Multiplexer
Address Selection
Instruction Register

24. Single Address Field

25. Variable Format Multiplexer Branch logic Flags Instruction Register
Address Decoder
Control Memory
Control Buffer Register
Branch control field
Control address register
Entire Field +1
Address field
Gate and function logic
Multiplexer
Branch
logic
Address Selection
Flags
Instruction Register

26. Description and Advantages
Introduced by M. V. Wilkes in 1951
Storage of Micro-Instruction
Sequences
Hardwire Vs Micro-Instruction
Advantages & Disadvantages

27. Hardwire Control

28. Microinstruction Address Generation Techniques
Explicit
Implicit
Two-Field
Mapping
Unconditional branch
Addition
Conditional branch
Residual control

29. Explicit Memory Address Generation
Address Location
Two Field Approach
Conditional Branch Instruction

30. Implicit Memory Address Generation
Control Memory
Mapping Approach
Adding Approach

31. Microinstruction Execution
A Taxonomy of Microinstruction
&
Microinstruction Encoding
Ninyuan Wang

32. Microinstruction Cycle
The basic event on a microprogrammed processor.
Two parts: fetch and execute.
Fetch: determined by the generation of a microinstruction address.

33. Execute
The effect of the execution of a microinstruction is to generate control signals.
Some of these signals control points internal to the processor.
The remaining signals go to the external control bus or other external interface.
As an incidental function, the address of the next microinstruction is determined.

35. A Taxonomy of Microinstructions
Microinstructions can be classified in a variety of ways. Distinctions that are commonly made in the literature include the following:
Vertical/horizontal
Packed/unpacked
Hard/soft microprogramming
Direct/indirect encoding

36. How to Encoding
K different internal and external control signals to be driven by the control units.
In Wilkes’s scheme:
K bits of the microinstruction would be dedicated to this purpose.
2 𝐾 possible combinations of control signals to be generated during any instruction cycle.

37. Not all be used – can do better
Two sources cannot be gated to same destination.
A register cannot be both source and destination.
Only one pattern of control signals can be presented to ALU at a time.
Only one pattern of control signals can be presented to external control bus at a time.
Require 𝑄< 2 𝐾 which can be encoded with 𝑙𝑜𝑔 2 𝑄<𝐾 bits.

38. In practice, this form of encoding is not used, for two reasons:
It is as difficult to program as pure decoded (Wilkes) scheme.
It is requires complex and therefore slow control logic module.
Instead, some compromises are made. These are of two kinds:
More bits than necessary used to encode the possible combination.
Some combinations that are physically allowable are not possible to encode.

39. Microinstruction Encoding
In practice, microprogrammed control units are not designed using a pure unencoded or horizontal microinstruction format.
The basic technique for encoding is illustrated in Figure 16.11a.

41. The microinstruction is organized as a set of fields.
Each field contains a code, which, upon decoding, activates one or more control signals.
The design of an encoded microinstruction format can now be stated in simple terms:

42. Organize the format into independent fields.
Each 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

43. Machine Structure

44. Machine Structure cont.

45. Processor Detail

46. PSW

47. LSI-11 First member of the PDP-11 family.
Offered as a single board processor.
Board consists of 3 LSI chips, internal bus, and interfacing logic.

48. Q-Bus board LSI 11/2 CPU

49. Q bus
The Q-bus was one of several bus technologies used with PDP computer systems.
Over time, the physical address range of the Q-bus was expanded from 16 to 18 and then 22 bits. Block transfer modes were also added to the Q-bus.

50. LSI-11 cont.
The three LSI are the data, control, and control store chips.

51. Data Chip
Data chip contains an 8 bit ALU, twenty six 8 bit registers, and CCR storage.
Registers include PSW, MAR and MBR.
Sixteen of the twenty six 8 bit registers are used to implement the eight 16 bit PDP-11 general purpose registers.
ALU needs 2 passes to implement the 16 bit arithmetic.

52. Control store chip 22 bit wide control memory.
More than one control store chip could be used.
Logic for sequencing and executing instructions are stored in this chip.
Registers include CAR, CDR, and a copy of the IR.

53. Men In Black? MIB Ties all components together. During fetch:
Control chip generates 11 bit address.
Control store chip uses this to produce a 22 bit microinstruction.
Low order 16 bits go to data chip, low order 18 bits to the control chip and high order 4 bits to bus control and processor board logic.

54. LSI-11 Microinstruction
Vertical, 22 bit wide microinstruction format.
High order 4 bits control special functions on processor board.
Translate bit enables the translation array to check for interrupts.
Load return register bit enables the next microinstruction address to be loaded from the return register.
The remaining 16 bits are used for encoded micro operations.

55. IBM 3033 Memory consists of 4k words.
Addresses FF contain 108 bit microinstructions.
Addresses FFF contain 126 bit microinstructions.
Horizontal format.

56. IBM 3033 cont ALU inputs are four dedicated registers. A, B, C, and D.
These registers are non-user-visible.
Microinstruction format contains fields to load these registers from user visible registers.
The ALU results are stored in user visible registers.

57. IBM 3033 MI Format

59. TI 8800 SDB

60. objective
The TI 8800 is a 32 bit programmable microprocessor chip equipped with:
Writeable control store
Implemented in RAM rather than ROM
Usage consist of:
Educational implementations
Useful in creating multifunctional prototypes
Etc….

61. Why microprogramming
creating logic controls through basic interconnections are difficult to:
Analyze
Design
Modify
Solution: Develop microinstructions to attain the control settings

62. microinstruction
Def.
An instruction that controls data flow and instruction-execution sequencing in a processor
Not visible or changeable by a programmer
Only run on its designated processor
Microcode differs from one machine to the next

63. 8800 Microinstruction format
The format for the 8800 comprise of 128 bits decoded into 30 functional fields.
The field are categorized in 5 groups:
Control of board
8847 floating-point and integer processor chip
8832 registered ALU
8818 microsequencer
WCS data field

66. TI 8800 SDB Structure Main components consist of: Microcode memory
Microsequencer
32-bit ALU
Floating-point and integer processor
Local data memory

68. Microsequencer
The 8818 Microsequencer fetch next microinstruction address then send it to the microcode memory from 5 different locations.

69. Controlling the Microsequencer
Controlled by a 12-bit microinstruction OSEL (1bit) SELDR (1bit) ZEROIN (1bit) RC2-RC0 (3bits) S2-S0 (3bits) MUX2-MUX0 (varies)
Example: Instruction is INC88181 = Decoded into OSEL = 0 SELDR = 0 ZEROIN = 0 R = 000 S = 111 MUX = 11

70. 8818 Microsequencer instructions

71. sn74act8847 floating-point and integer processor

72. Floating Point and integer processor
included are fields 7-16
there is a total of 32-bits involved

74. sn74act8832 32-bit registered ALU

76. 32-bit registered alu
included are fields 17-27
Can be configure to work as 4-8bit alu, 2-16bit alu, or 1-32bit alu
there is a total of 32-bits involved but with inputs and settings of the alu there are 39 bits
Field 27 provides opcode of operation to be performed by alu

77. cONT11 [17], welh, selryfymx, [24], r3, r2, r1, pass+ add
We want to add contents of register 1 to contents of register 2 and put the result on register 3
cONT11 [17], welh, selryfymx, [24], r3, r2, r1, pass+ add

78. cONT11 [17], welh, selryfymx, [24], r3, r2, r1, pass+ add
Basic NOP instruction
WELH
Field 17 changed to WELH (Write Enable Lo and Hi)
SELRYFYMX
Field 18 changed to SELRYFYMX (select feedback from ALU Y MUX output) R3
Field 24 changed to designate R3 as destination register R2
Field 25 changed to designate R2 as a source register R1
Field 26 changed to designate R1 as a source register
PASS+ ADD
Field 27 changed to specify ALU to ADD. PASS to denote ALU not to shift.

79. NOT NECESSARY TO DENOTE FIELDS WHEN
CONSECUTIVE FIELDS ARE USED
GROUP 1 MUST BE USED ALONGSIDE WITH GROUP; AT
NO TIME SHOULD GROUP BE USED WITH GROUPS 3-5
GROUP 1 AND 2 ARE FOR ARITHMETIC FUNCTIONS
GROUP 3-5 ARE FOR LOGICAL FUNCTIONS

82. WSC Data field

83. QUESTIONS NAME AN INPUT TO THE CONTROL UNIT
WHAT PICKS UP THE NEXT INSTRUCTION
FROM THE CONTROL STORE AND SENDS
IT TO MICROMEMORY

84. QUESTIONS NAME AN INPUT TO THE CONTROL UNIT CLOCK, ALU FLAGS, IR, CAR
WHAT PICKS UP THE NEXT INSTRUCTION
FROM THE CONTROL STORE AND SENDS
IT TO MICROMEMORY

85. QUESTIONS NAME AN INPUT TO THE CONTROL UNIT CLOCK, ALU FLAGS, IR, CAR
WHAT PICKS UP THE NEXT INSTRUCTION
FROM THE CONTROL STORE AND SENDS
IT TO MICROMEMORY
MICROSEQUENCER 8818

86. MORE QUESTIONS HOW MANY BITS IS THE MICROINSTRUCTION
FORMAT FOR THE TI8800 PROCESSOR
WHAT ARE THE TWO MAIN FUNCTIONS OF THE CONTROL UNIT
WHAT IS THE FORMAT OF THE LSI-11 MICROINSTRUCTION

87. MORE QUESTIONS HOW MANY BITS IS THE MICROINSTRUCTION
FORMAT FOR THE TI8800 PROCESSOR
128 BITS
WHAT ARE THE TWO MAIN FUNCTIONS OF THE CONTROL UNIT
WHAT IS THE FORMAT OF THE LSI-11 MICROINSTRUCTION

88. MORE QUESTIONS HOW MANY BITS IS THE MICROINSTRUCTION
FORMAT FOR THE TI8800 PROCESSOR
128 BITS
WHAT ARE THE TWO MAIN FUNCTIONS OF THE CONTROL UNIT
SEQUENCING AND EXECUTING
WHAT IS THE FORMAT OF THE LSI-11 MICROINSTRUCTION

89. MORE QUESTIONS HOW MANY BITS IS THE MICROINSTRUCTION
FORMAT FOR THE TI8800 PROCESSOR
128 BITS
WHAT ARE THE TWO MAIN FUNCTIONS OF THE CONTROL UNIT
SEQUENCING AND EXECUTING
WHAT IS THE FORMAT OF THE LSI-11 MICROINSTRUCTION
VERTICAL FORMAT

90. EVEN MORE QUESTIONS
WHAT IS THE DIFFERENCE BETWEEN VERTICAL AND HORIZONTAL
MICROINSTRUCTION
NAME AN ADVANTAGE AND DISADVANTAGE TO USE A MICROCONTROLLED
CONTROL UNIT

91. EVEN MORE QUESTIONS
WHAT IS THE DIFFERENCE BETWEEN VERTICAL AND HORIZONTAL
MICROINSTRUCTION
VERTICAL ENCODED- EASIER TO PROGRAM
HORIZONTAL DETAILED-FASTER DUE TO LESS ENCODING
NAME AN ADVANTAGE AND DISADVANTAGE TO USE A MICROCONTROLLED
CONTROL UNIT

92. EVEN MORE QUESTIONS
WHAT IS THE DIFFERENCE BETWEEN VERTICAL AND HORIZONTAL
MICROINSTRUCTION
VERTICAL ENCODED- EASIER TO PROGRAM
HORIZONTAL DETAILED-FASTER DUE TO LESS ENCODING
NAME AN ADVANTAGE AND DISADVANTAGE TO USE A MICROCONTROLLED
CONTROL UNIT
ADVANTAGE-FLEXIBILITY, CHEAPER, SIMPLER DESIGN, LESS ERROR PRONE
DISADVATAGES-SLOWER