2. •Instruction cycle has a number of smaller units
1.Fetch
2. Indirect
3. Execute
4.Interrupt
• Micro-operations
Each part of the cycle has a number of smaller
steps called micro-operations
• Micro-operations are Fundamental atomic
operations of the processor

4. It includes four 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

5. Clock
• Repetitive sequence of pulses
• Useful for measuring duration of micro-ops
• Different control signals at different times within instruction
cycle
• Need a counter with different control signals for t1, t2 etc.
The notation represents successive time units.
• First time unit:
Move contents of PC to MAR.
• Second time unit:
Move contents of memory location from MAR to MBR.
Increment by I the contents of the PC.
• Third time unit:
Move contents of MBR to IR.

6. • The fetch cycle actually consists of
3 steps
&
4 micro ops
• Each micro-op consists of moving data in or out of a Register
AS
• Address of next instruction is in PC
t1: MAR <- (PC)
• Data from data bus copied into MBR
• PC incremented
t2: MBR <- (memory)
PC <- (PC) +I
• Data (instruction) moved from MBR to IR
t3: IR <- (MBR)

7. • Proper
sequence must be followed
i. MAR <- (PC) must precede MBR <-(memory)
cycle
ii. Conflicts must be avoided
iii. Must not read & write same register in same
iv. MBR <- (memory) & IR <- (MBR) must not be in
same cycle
v. Also PC <- (PC) +1 involves addition
• Might need to Use ALU
• May need additional micro operations.

8. • If an instruction specifies an indirect addressing
• Then indirect cycles precedes the execution cycle.
i-Instruction fetched
ii-Fetch source operands
• It includes the following micro-operations:
i. The address field of the instruction is transferred to the MAR
t1: MAR <- (IR address) - address field of IR
ii. This is then used to fetch the address of the operand.
t2: MBR <- (memory)
iii. The address field of the IR is updated from the MBR, so that it
now contains a direct rather than an indirect address.
t3: IR address <- (MBR address)
• Now MBR contains direct address of operand
IR is updated with direct address of operand
IR is now in same state as if direct addressing .

9. • Fetch, Indirect and Interrupt cycles are simple
and predictable
• Execute cycle is different for each instruction
• There are A number of different sequences of microoperations
•Example: ADD R1,X
IR contains the ADD instruction
i. The address portion of the IR is loaded into the MAR
t1: MAR <- (IR address)
ii. The referenced memory location is read
t2: MBR <- (memory)
iii. Finally, the contents of R1 and MBR are added by ALU.
t3: R1 <- R1 + (MBR)

10. • Additional micro-operations may be required
i. To extract the register reference from the IR
ii. To stage the ALU inputs or outputs in some intermediate registers.
• More complex example:
ISZ X (increment and skip if zero):
The content of location X is incremented by 1.If the result is 0,the next instruction
is skipped . A possible sequence of micro-operations is
t1: MAR ← (IR(address))
t2: MBR ← Memory
t3: MBR ← (MBR) + 1
t4: Memory ← (MBR)
• This test and action can be implemented as one micro-operation:
Conditional action is “the PC is incremented if MBR=0”
If ((MBR) = 0) then (PC ← (PC) + I)
• Also that this micro-operation can be performed during the same time unit during
which the updated value in MBR is stored back to memory

11. • At end of execute cycle, processor tests interrupt signal.
i. If set, an interrupt cycle occurs
ii. Varies greatly from one machine to another
• How It Performs:
i. The contents of the PC are transferred to the MBR , so that they can be saved
for return from the interrupt
t1: MBR <-(PC)
saved
ii. MAR is loaded with the address At which the contents of the PC are to be
t2: MAR <- save-address
PC is loaded with the address of the start of the interrupt-processing routine.
PC <- routine-address
iii. Store the MBR, which contains the old value of the PC into memory.
t3: memory <- (MBR)
• This is a minimum level of Complexity.
• Most processors may provide multiple types of address
i. So there may be additional micro-ops to get addresses
ii. Note that saving context is done by interrupt handler routine, not micro-ops.

12. • Each phase is decomposed into a sequence of micro
operations
i. Fetch, indirect, interrupt and Execute cycles
ii. • Tie these above sequences together into the
instruction cycle
• Assume new 2-bit register:
The instruction cycle code (ICC) designates which
part of cycle is in processor
• 00: Fetch
• 01: Indirect
• 10: Execute
• 11: Interrupt

14. • Decomposing Functioning :
It decomposes functioning of the processor into
elementary operations, called micro-operations.
• Exactly what it is that the control unit doing?
• Functional Requirements:
i. Define the basic elements of the processor
ii. Describe the micro-operations that the processor
performs
iii. Determine the functions control unit must
perform in order to execute the micro-ops

15. • Basic Elements of processor
i.Registers
ii.Internal data pahs
iii.External data paths
iv.ALU
v.Control Unit
• Types of Micro-operation
i. Transfer data between registers
ii. Transfer data from register to external interface
iii. Transfer data from external interface to register
iv. Perform arithmetic or logical operations using
registers

16. • Control Unit performs two basic tasks
i. Sequencing
Causing the CPU to step through a series of micro
operations
ii. Execution
Causing the performance of each micro operations.
• How it Performs?
Ans:
• Key to operation is the use of control signals

17. Signal:
A pulse or frequency
of electricity or light that
represents a control command
• External specifications of
the control unit:
i. Inputs : allow it to
determine the state of the
system
ii. Outputs : allow it to
control the behavior of the
system.
• Internally it requires logic
to perform its sequencing and
execution functions.

18. • Clock
i. This is how the control unit “keeps time.”
ii. The control unit causes one micro-operation or more to be performed
for each clock pulse.
iii. processor cycle time, or the clock cycle time.
• Instruction register:
i. The op code and addressing mode of the current instruction are used
to
determine which micro-operations to perform during the execute cycle.
• Flags:
i. To determine the status of the processor
ii. To see the outcome of previous ALU operations.
For example
The increment-and-skip-if-zero (ISZ) instruction, the control unit will increment
the PC if the zero flag is set.
• Control signals from control bus:
The control bus portion of the system bus provides signals to the control unit.

19. • Control signals within the processor: These are two
types:
i. That cause data to be moved from one register to
another
ii. To activate specific ALU functions.
• Control signals to control bus: These are also of two
types :
i. Control signals to memory,
ii. Control signals to the I/O modules.
• Three types of control signals are used:
i. those that activate an ALU function,
ii. those that activate a data path
iii. those that are signals on the external system bus