Computer Organization
and Architecture + Networks
Lecture 10
Control Unit
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
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
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
• Sequencing
— Control unit causes the processor to step through a series of
microoperations in the proper sequence based on the program
being executed
• Execution
— Control unit causes each microoperation to be performed
• How the control unit performed the 2 functions? –
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 microinstructions 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
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:
t2:
t3:
MAR  (PC)
MBR  memory
PC  PC+1
IR  (MBR)
Memory address register
Memory buffer register
Program counter
Instruction register
— And add instruction - Add R1, X
t1:
t2:
t3:
MAR  (IR(address))
MBR  memory
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
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
Data paths and control signals
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
1.Data paths
2.ALU
3.System bus
Microoperations
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 variablelength 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
• 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
— 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)
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
Microprogramming
• The concept of programming was developed by Maurice
Wilkes (1951), using diode matrices for the memory
element
Wilkes’ microprogrammed control unit
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?
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.
Simple microprogrammed control unit
Address-selection – provided by a
branch logic module whose input
consists of control unit flags + bits
from the control portion of the
microinstruction
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
• 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
• 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)
• 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
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