William Stallings
Computer Organization and Architecture
8 th Edition
Chapter 15
Control Unit Operation

Micro-Operations
• A computer executes a program
• Fetch/execute cycle
• Each cycle has a number of steps
— see pipelining
• Called micro-operations
• Each step does very little
• Atomic operation of CPU

Constituent Elements of
Program Execution

Fetch - 4 Registers
• Memory Address Register (MAR)
— Connected to address bus
— Specifies address for read or write op
• Memory Buffer Register (MBR)
— Connected to data bus
— Holds data to write or last data read
• Program Counter (PC)
— Holds address of next instruction to be fetched
• Instruction Register (IR)
— Holds last instruction fetched

Fetch Sequence
• Address of next instruction is in PC
• Address (MAR) is placed on address bus
• Control unit issues READ command
• Result (data from memory) appears on data bus
• Data from data bus copied into MBR
• PC incremented by 1 (in parallel with data fetch from memory)
• Data (instruction) moved from MBR to IR
• MBR is now free for further data fetches

Fetch Sequence (symbolic)
• t1: MAR <- (PC)
• t2: MBR <- (memory)
• PC <- (PC) +1
• t3: IR <- (MBR)
• (tx = time unit/clock cycle)
• or
• t1: MAR <- (PC)
• t2: MBR <- (memory)
• t3: PC <- (PC) +1
• IR <- (MBR)

Rules for Clock Cycle Grouping
• Proper sequence must be followed
— MAR <- (PC) must precede MBR <- (memory)
• Conflicts must be avoided
— Must not read & write same register at same time
— MBR <- (memory) & IR <- (MBR) must not be in same cycle
• Also: PC <- (PC) +1 involves addition
— Use ALU
— May need additional micro-operations

Indirect Cycle
• MAR <- (IR address
) - address field of IR
• MBR <- (memory)
• IR address
<- (MBR address
)
• MBR contains an address
• IR is now in same state as if direct addressing had been used
• (What does this say about IR size?)

Interrupt Cycle
• t1: MBR <-(PC)
• t2: MAR <- save-address
• PC <- routine-address
• t3: memory <- (MBR)
• This is a minimum
— May be additional micro-ops to get addresses
— N.B. saving context is done by interrupt handler routine, not micro-ops

Execute Cycle (ADD)
• Different for each instruction
• e.g. ADD R1,X - add the contents of location X to Register 1 , result in R1
• t1: MAR <- (IR address
)
• t2: MBR <- (memory)
• t3: R1 <- R1 + (MBR)
• Note no overlap of micro-operations

Execute Cycle (ISZ)
• ISZ X - increment and skip if zero
— t1: MAR <- (IR address
)
— t2: MBR <- (memory)
— t3: MBR <- (MBR) + 1
— t4: memory <- (MBR)
— if (MBR) == 0 then PC <- (PC) + 1
• Notes:
— if is a single micro-operation
— Micro-operations done during t4

Execute Cycle (BSA)
• BSA X - Branch and save address
— Address of instruction following BSA is saved in X
— Execution continues from X+1
— t1: MAR <- (IR address
)
— MBR <- (PC)
— t2: PC <- (IR address
)
— memory <- (MBR)
— t3: PC <- (PC) + 1

Instruction Cycle
• Each phase decomposed into sequence of elementary micro-operations
• E.g. fetch, indirect, and interrupt cycles
• Execute cycle
— One sequence of micro-operations for each opcode
• Need to tie sequences together
• Assume new 2-bit register
— Instruction cycle code (ICC) designates which part of cycle processor is in
– 00: Fetch
– 01: Indirect
– 10: Execute
– 11: Interrupt

Flowchart for Instruction Cycle

Functional Requirements
• Define basic elements of processor
• Describe micro-operations processor performs
• Determine functions control unit must perform

Basic Elements of Processor
• ALU
• Registers
• Internal data pahs
• External data paths
• Control Unit

Types of Micro-operation
• Transfer data between registers
• Transfer data from register to external
• Transfer data from external to register
• Perform arithmetic or logical ops

Functions of Control Unit
• Sequencing
— Causing the CPU to step through a series of micro-operations
• Execution
— Causing the performance of each micro-op
• This is done using Control Signals

Control Signals
• Clock
— One micro-instruction (or set of parallel microinstructions) per clock cycle
• Instruction register
— Op-code for current instruction
— Determines which micro-instructions are performed
• Flags
— State of CPU
— Results of previous operations
• From control bus
— Interrupts
— Acknowledgements

Model of Control Unit

Control Signals - output
• Within CPU
— Cause data movement
— Activate specific functions
• Via control bus
— To memory
— To I/O modules

Example Control Signal Sequence -
Fetch
• MAR <- (PC)
— Control unit activates signal to open gates between PC and MAR
• MBR <- (memory)
— Open gates between MAR and address bus
— Memory read control signal
— Open gates between data bus and MBR

Data Paths and Control Signals

Internal Organization
• Usually a single internal bus
• Gates control movement of data onto and off the bus
• Control signals control data transfer to and from external systems bus
• Temporary registers needed for proper operation of ALU

CPU with
Internal
Bus

Hardwired Implementation (1)
• Control unit inputs
• Flags and control bus
— Each bit means something
• Instruction register
— Op-code causes different control signals for each different instruction
— Unique logic for each op-code
— Decoder takes encoded input and produces single output
— n binary inputs and 2 n outputs

Hardwired Implementation (2)
• Clock
— Repetitive sequence of pulses
— Useful for measuring duration of micro-ops
— Must be long enough to allow signal propagation
— Different control signals at different times within instruction cycle
— Need a counter with different control signals for t1, t2 etc.

Control Unit with Decoded Inputs

Hardwired Control Unit Logic
• For each control signal, to derive a
Boolean expression of that signal as a function of the inputs
• Let us consider a single control signal, C5, signal causes data to be read from the external data bus into the MBR
• Let us define two new control signals, P and Q, that have the following interpretation:
PQ = 00 Fetch Cycle
PQ = 11 Interrupt Cycle
PQ = 10 Execute Cycle
PQ = 01 Indirect Cycle

Hardwired Control Unit Logic
• Then C5 can be defined as:
C5 = P # Q # T2 + P # Q # T2
• That is, the control signal C5 will be asserted during the second time unit of both the fetch and indirect cycles.
• C5 is also needed during the execute cycle. For our simple example, let us assume that there are only three instructions that read from memory:
LDA,ADD, and AND. Now we can define
C5 as
— C5 = P # Q # T2 + P # Q # T2 + P # Q #
(LDA + ADD + AND) # T2

Hardwired Control Unit Logic
• This same process could be repeated for every control signal generated by the processor. The result would be a set of
Boolean equations that define the behavior of the control unit and hence of the processor.

Hardwired Control Unit Logic
• To tie everything together, the control unit must control the state of the instruction cycle. As was mentioned, at the end of each subcycle (fetch, indirect, execute, interrupt), the control unit issues a signal that causes the timing generator to reinitialize and issue T1. The control unit must also set the appropriate values of P and Q to define the next subcycle to be performed

Problems With Hard Wired Designs
• Complex sequencing & micro-operation logic
• Difficult to design and test
• Inflexible design
• Difficult to add new instructions

Chapter 16
Micro-programmed Control

Control Unit Organization

Micro-programmed Control
• Use sequences of instructions (see earlier notes) to control complex operations
• Called micro-programming or firmware

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 microoperation
• Have a sequence of control words for each machine code instruction
• Add an address to specify the next microinstruction, depending on conditions

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

Micro-program Word Length
• Based on 3 factors
— Maximum number of simultaneous microoperations supported
— The way control information is represented or encoded
— The way in which the next micro-instruction address is specified

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)

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

Typical Microinstruction Formats

Organization of
Control Memory

Control Unit

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

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

Functioning of Microprogrammed
Control Unit