1. Computer Organization and Architecture + Networks
Lecture 10
Control Unit

2. Overview
So far in the course, we have looked at the processor design and assumed that the control portion worked
In this section, we will
Survey the control unit operation
Highlights design methods using
Hardwired control
Microprogrammed control

3. Control Unit Function We have already seen that
Computer executes a program.
Programs are executed as a sequence of instructions
Each instruction contains a series of steps that make up the instruction cycle  fetch, decode, execute, interrupt cycles
Each of these steps/cycles has a number of fundamental operations/a smaller series of steps called microeperations

4. Control Unit Function
Thus, the control unit operation can be defined by
Defining the elements of the CPU
Defining the microoperationsthe CPU performs
Determining the functions the control unit must perform to cause the execution of the microoperations in the desired time sequence
Sequencing
Execution
Control unit causes the processor to step through a series of microoperations in the proper sequence based on the program being executed
Control unit causes each microoperation to be performed
How the control unit performed the 2 functions? – through control signals

5. Control Unit Function General layout of a control unit is shown below
Clock: one micro-instruction per clock cycle
Instruction register: opcode for current instruction, determines which micro-instructions are performed
Flags: status of the processor and indication of results of previous operations
From control bus: control signal from bus eg. interrupts
Within CPU: cause data movement and to activate specific functions
Via control bus: control signal to memory and control signal to I/O modules

6. Control Unit Function
Each step of the instruction cycle can be decomposed into microoperation primitives that are performed in precise time sequence
Instruction fetch
t1: MAR  (PC)
t2: MBR  memory
PC  PC+1
t3: IR  (MBR)
And add instruction - Add R1, X
t1: MAR  (IR(address))
t3: R1  (R1) + (MBR)
Each microoperation is initiated and controlled based on the used of control signals/lines coming from the control unit
cause data to move from one register to another and activate specific ALU functions
Memory address register
Memory buffer register
Program counter
Instruction register

7. Data paths and control signals
Microoperations
Control unit design is then the collection and the implementation of all of the needed control signals
Simple processor with single AC
Data path between element
Termination of control signals, Ci
The control unit receives inputs from the clock, the IR and flags
With each clock cycle, the control unit reads all of its inputs and emits a set of control signals – which go to 3 separates destinations
Data paths
ALU
System bus
Data paths and control signals

8. Microoperations

9. Control unit with decoded inputs
Despite the 4 key inputs
Flags & control bus signals – bit with meaning
IR and CLK – not directly useful
Control unit use the opcode and perform different actions for different instructions  control unit with decoded inputs – complex to than general to account for variable-length opcodes
Timing generator in order the control unit to emit different control signals at different time units within a single instruction cycle
Control unit with decoded inputs

10. Two approaches of implementation
Hardwired implementation
Control unit is viewed as a sequential logic circuit
Used to generate fixed sequences of control signals
Implemented using any of a variety of “standard” digital logic techniques
Principle advantages
Higher speed operation
Smaller implementations (components counts)
Modifications to the design can be hard to do
Favored approach in RISC style designs

11. Microprogrammed control unit
Control signal values for each microoperation are stored in a memory device – the control store
Reading the contents of the control store in a prescribed order is equivalent to sequencing through the microoperations
Since the “program” of microoperations and their control signal values are stored in memory, microprogrammed units
Are more systematic with a well defined format
Can be easily modified during the design process
Require more components to implement
Tend to be slower than hardwired units (due to having to perform memory read operations)

12. Hardwired Implementation
Three general design approaches
Traditional “state-table” method from a digital logic course
Can produce the minimum component design
Complex design that may be hard to modify
Clocked delay element
Straight-forward layout based on flowchart of the instruction implementation
Requires more delay elements (flip-flops) than are really needed
Sequence counter approach
Polyphase clock signals are derived from the master clock using a standard counter-decoder approach
These signals are applied to the combinational portion of the circuit
Example: load register B (control signal C12)
C12 = T4I3 + T5I6Z + T6I24P

13. Wilkes’ microprogrammed control unit
Microprogramming
The concept of programming was developed by Maurice Wilkes (1951), using diode matrices for the memory element
Wilkes’ microprogrammed control unit

14. Microprogramming
Modern microprogrammed control units have replaced the diode matrices with standard memory components
The control unit operates by performing consecutive control storage reads to generate the next set of control function outputs
Performing the series of control memory accesses is, in effect, executing a program for each instruction in the machine’s instruction set – hence the term microprogramming
The control unit appears to be a complete computer within the larger computer – with all of its problems
How do you specify the next microinstruction to be executed?
How are branches handled?

15. Simple microprogrammed control unit
The option for next address
Address field
Instruction register code
Next sequential address
Branch control logic: single address field
Mux – serves as destination for a address field + instruction register
Based on address-selection input, the Mux transmits either the opcode of address field to the control address register
CAR – decoded to produce the next microinstruction address.
Address-selection – provided by a branch logic module whose input consists of control unit flags + bits from the control portion of the microinstruction
Simple microprogrammed control unit

16. Branch control logic: two address fields – two address field in each microinstruction
Similar process – Mux can transmits one of the two address + opcode
More complex unit

17. For the typically large microprocessor systems today, there are
Many instructions and associated register-level hardware
Many control point to be manipulated
This can result in a control memory that
Contains a large number of words – corresponding to the number of instructions to be executed
Has a wide word width – due to the large number of control points to be manipulated

18. Length is based on 3 factors
Most modifications to the basic design of a microprogrammed control unit have been concerned with the word length
Length is based on 3 factors
Maximum number of simultaneous microoperations that must be supported
The control information is represented or encoded
The way in which the next microinstruction address is specified
Designer must choose the parallel “power” of each instruction
Each microinstruction specifies a single (or few) microoperations to be performed (vertical microprogramming)
Each microinstruction specifies many different microoperations to be performed in parallel (horizontal microprogramming)

19. Vertical microprogramming
Width is narrow: n control signals can be encoded into log2 n 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
Horizontal microprogramming
Wide memory word
High degree of parallel operations are possible
Little to no encoding of control information
Compromise
Divide control signals into disjoint groups
Implement each group as a separate field in the memory word
Supports reasonable levels of parallelism without too much complexity

20. Summary
This section has overviewed the operation and the design of the control unit
Hardwired and microprogramming techniques discussed
Basic operation of each
Advantages and disadvantages of each