2. Control Unit
⢠The operations of Input unit, output unit, ALU are co-ordinated by the
control unit.
⢠The control unit is the Nerve centre that sends control signals to other
units and senses their states.
⢠Data transfers between the processor and the memory are also
controlled by the control unit through timing signals.
4. Terms
⢠Clock: This is how the control unit âkeeps time.â The control unit causes one micro-operation to be performed
for each clock pulse. This is sometimes referred to as the processor cycle time, or the clock cycle time
⢠Instruction register: The opcode and addressing mode of the current instruction are used to determine which
micro-operations to perform during the execute cycle.
⢠Flags: These are needed by the control unit to determine the status of the processor and the outcome of
previous ALU operations. For example, for 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. The outputs are as follows:
⢠Control signals within the processor: These are two types: those that cause data to be moved from one
register to another, and those that activate specific ALU functions.
⢠Control signals to control bus: These are also of two types: control signals to memory, and control signals to
the I/O module
⢠Three types of control signals are used: those that activate an ALU function; those that activate a data path;
and those that are signals on the external system bus or other external interface. All of these signals are
ultimately applied directly as binary inputs to individual logic gates.
5. Hardwired Control Unit
⢠In Hardwired Control Unit, control logic is implemented with logic gates,
flip flops, encoders, decoders and other digital circuits i.e Control unit is
implemented with Hardware
⢠The control hardware can be viewed as a state machine that changes
from one state to another in every clock cycle, depending on the inputs.
⢠The sequence of the operation carried out by this machine is determined
by the wiring of the logic elements and hence named âhardwiredâ.
⢠After the execution of each instruction, another signal: the end signal is
generated which resets the state of control step counter & makes it
ready for the next instruction
7. Hardwired Control Unit
⢠Hardwired control unit gets set of inputs from IR,clock ,system busand transform them
into control signals.
⢠In the case of the flags and control bus signals, each individual bit typically has some
meaning (e.g., overflow). The other two inputs, however, are not directly useful to the
control unit.
⢠Instruction decoder decodes the instruction loaded in IR
⢠Sequence counter/Time generator provides separate line for each step or time
slot(T1,T2,T3)
⢠Hardwired control unit are faster and simpler in design,but they are also less flexible
and more difficult to update.
⢠RISC(Reduced Instruction Set Computer) are implemented with hardwired control unit.
8. Advantage & disadvantage
Advantage
ďźThey are fast
ďźRISC are implemented using Hardwired control unit.
Disadvantage
ďźInstruction set and control logic are tied together by special circuits
that are complex and difficult to design and modify
ďźIf someone later wants to extend instruction set ,physical
components must be changed
ďźExpensive as new chips need to designed and old need to replaced
10. Hardwired control Unit of basic Computer
⢠A Hard-wired Control consists of two decoders, a sequence counter, and a number of logic
gates.
⢠An instruction fetched from the memory unit is placed in the instruction register (IR).
⢠The component of an instruction register includes; I bit, the operation code, and bits 0 through
11.
⢠The operation code in bits 12 -14 are decoded by 3 x 8 decoder.
⢠Eight outputs of the decoder D0 to D7 goes to control logic gates to perform specific operation
⢠Last 15th
bit is transferred to a flip-flop designated by the symbol I.
⢠Bits 0 through 11 are applied to the control logic gates.
⢠4 bit Sequence counter (SC) can count from 0 through 15.
⢠SC output is 16 timing pulses T0 to T15 generated using 4x16 decoder
⢠SC is incremented by INR and clear by CLR
11. Microprogrammed Control Unit
⢠A microprogrammed control unit is a type of control unit that uses a
set of stored instructions known as a microprogram to generate
control signals.
⢠When compared to hardwired control units, which are designed
using combinational logic circuits, a microprogrammed control unit
utilizes programming approach
⢠Control signals are stored in control memory
⢠We can say control memory contains microprogram which is
sequence of micro instructions that specify microoperations
12. Microprogrammed Control Unit
⢠Control signals are stored in binary as control word in Control memory
⢠Each Control word contains microinstruction
⢠Micro instruction specifies one or more micro-operations
⢠Sequence of microinstructions is micro program
⢠Execution of micro instruction generates control signals
⢠Control memory is Read Only Memory(ROM)chip where control
information is permanently stored.
15. Microprogrammed control Unit
To execute an instruction,
1. Decoder /Next address generator determines address of microinstruction to be read from
memory
2. Control Address Register contains address of next microinstruction to be read
3. Control Buffer Register contains microinstruction read from memory for execution
4. When address is available in CAR , sequencing logic unit issues a READ command to the
control memory.
5. The word whose address is specified in the control address register is read into the control
buffer register.
6. The content of the control buffer register i.e microinstruction generates control signals and
next-address information for the sequencing logic unit.
7. The sequencing logic unit loads a new address into the control address register based on the
next-address information from the control buffer register and the ALU flags.
16. Advantage
⢠Design of micro programmed control unit is simple(less complex) because microprograms
are implemented using software routines
⢠Cheaper and less error prone to implement.
⢠More flexible because design,modification, correction and enhance ment is easily possible
⢠New instruction set can be easily implemented by rewriting or modifying contents of
control memory
⢠A hardwired control unit must contain complex logic for sequencing through the many
micro-operations of the instruction cycle. On the other hand, the decoders and
sequencing logic unit of a microprogrammed control unit are very simple pieces of logic.
⢠CISC(Complex Instruction Set computer) is implemented using Microprogrammmed
control
17. Disadvantage
⢠Control Memory access can slow down the overall execution of
instructions compared to the faster-hardwired control units.
⢠A large amount of control memory is needed to store the
microprogram, which increases the memory requirements.
⢠While flexible, the process of fetching and decoding microinstructions
can increase the overall complexity of execution when compared to
hardwired control units.
