Advantages of Hardwired Control Unit over the Micro Program Control Unit

Hardwired Control Unit

Hardwired control is a control mechanism that generates control signals by using an appropriate finite state machine (FSM). Microprogrammed control is a control mechanism that generates control signals by reading a memory called a control storage (CS) that contains control signals. Although microprogrammed control seems to be advantageous to implement CISC machines, since CISC requires systematic development of sophisticated control signals, there is no intrinsic difference between these 2 types of control.

The pair of a “microinstruction-register” and a “control storage address register” can be regarded as a “state register” for hardwired control. Note that a control storage can be regarded as a combinational logic circuit. We can assign any 0,1 values to each output corresponding to each address, that can be regarded as the input for a combinational logic circuit. This is a truth table.

It is implemented as logic circuits. Hardwired control and micro programmed control. In the hardwired organization, the control logic is implemented with gates, flip-flops, decoders, and other digital cirn. In the micro programmed organization, the control information is stored in a control memory.

The control memory is programmed to initiate the required sequence of micro operations. A hardwired control, as the name implies, re¬quires changes in the wiring among the various components if the design has to be modified or changed. In the micro programmed control, any required changes or modifications can be done by updating the microprogram in control memory.

The block diagram of the control unit is shown. It consists of two decoders, a sequence counter, and a number of control logic gates. An instruction read from memory is placed in the instruction register (IR). The position of this register in the common bus system is indicated. It is divided into three parts: the I bit, the operation code, and bits 0 through 11. The operation code in bits 12 through 14 are decoded with a 3 x 8 decoder. The eight outputs of the decoder are designated by the symbols D0 through D7.

The subscripted decimal number is equivalent to the binary value of the corresponding opera¬tion code. Bit 15 of the instruction is transferred to a flip-flop designated by the symbol I. Bits 0 through 11 are applied to the control logic gates. The 4-bit sequence counter can count in binary from 0 through 15.

The outputs of the counter are decoded into 16 timing signals T0through, flip-flops, decoders etc.) in the hardware. This organization is very complicated if we have a large control unit.

In this organization, if the design has to be modified or changed, requires changes in the wiring among the various components. Thus the modification of all the combinational circuits may be very difficult.

Hardwired Control Unit is fast because control signals are generated by combinational circuits. The delay in generation of control signals depends upon the number of gates.

An instruction read from memory is placed in the instruction register (IR). The instruction register is divided into three parts: the I bit, operation code, and address part. First 12-bits (0-11) to specify an address, next 3-bits specify the operation code (opcode) field of the instruction and last left most bit specify the addressing mode I. I = 0 for direct address I = 1 for indirect address CPU is partitioned into Arithmetic Logic Unit (ALU) and Control Unit (CU).

The function of control unit is to generate relevant timing and control signals to all operations in the computer. It controls the flow of data between the processor and memory and peripherals.

The control unit directs the entire computer system to carry out stored program instructions. The control unit must communicate with both the arithmetic logic unit (ALU) and main memory.

The control unit instructs the arithmetic logic unit that which logical or arithmetic operation is to be performed. The control unit co-ordinates the activities of the other two units as well as all peripherals and auxiliary storage devices linked to the computer.

Hardwired control units are implemented through use of sequential logic units, featuring a finite number of gates that can generate specific results based on the instructions that were used to invoke those responses.Hardwired control units are generally faster than microprogrammed designs.

Their design uses a fixed architecture—it requires changes in the wiring if the instruction set is modified or changed. This architecture is preferred in reduced instruction set computers (RISC) as they use a simpler instruction set.

A controller that uses this approach can operate at high speed; however, it has little flexibility, and the complexity of the instruction set it can implement is limited.

The hardwired approach has become less popular as computers have evolved. Previously, control units for CPUs used ad-hoc logic, and they were difficult to design.

Major Advantages of Hardwired Control Unit

1. It uses flags, decoder, logic gates and other digital circuits.

2. As name implies it is a hardware control unit.

3. On the basis of input Signal output is generated.

4. Difficult to design,test and implement.

6. Inflexible to modify.

7. Faster mode of operation.

8. Expensive and high error.

9. Used in RISC processor.

10. Faster than micro- programmed control unit.

11. Can be optimized to produce fast mode of operation.

12. It has the advantage that it can be optimized to produce a fast mode of operation

CISC also can be implemented by using hardwired control:

In the above sense, microprogrammed control is not always necessary to implement CISC machines. Hardwired control also can be used for implementing sophisticated CISC machines. The bases of this opinion are as follows:

1. The same field configuration (state assignment) can be used for both of these two types of control. This is clear because of the above identification.

2. We can use any large FSM, that has horizontal microcode like state assignment, since the delay for the FSM does not matter at all so long as it is less than or equal to the delay for the data-path that includes adders, shifters and so on, since the FSM works in parallel with the data-path.

3. The horizontal microcode like state assignment has become very easy to be implemented because of the spread of the hardware description language (HDL). In Verilog HDL, g`defineh statements enable us to get perfect net-list for any large FSMs in a very short time by using appropriate logic synthesizers. “Parameter” statements also can be used for the state assignment in Verilog HDL.

CISCs and RISCs are two major different types of ordinary SISD machines. Since hardwired control has been historically faster, both of these two types of machines are implemented by using hardwired control in our microcomputer design educational environment City-1.

In 1996, that was the first year of City-1, an example description named CISC-1 with an FSM that uses a horizontal microcode like state assignment was given to all of junior students of the Department of Computer Engineering, Faculty of Information Sciences, Hiroshima City University. They succeeded.

Micro Programed Control Unit

The control memory is assumed to be a ROM, within which all control information is permanently stored. The control memory address register specifies the address of the microinstruction, and  the control data register holds the microinstruction read from memory.

The microinstruction contains a control word that specifies one or more micro-operations for the data processor. Once these operations are executed, the control must determine the next address.

The location of the next microinstruc¬tion may be the one next in sequence, or it may be located somewhere else in the control memory. For this reason it is necessary to use some bits of the present microinstruction to control the generation of the address of the next microinstruction.

The next address may also be a function of external input conditions. While the microoperations are being executed, the next address is computed in the next address generator circuit and then transferred into the control address register to read the next microinstruction.

Thus a microinstruc-tion contains bits for initiating microoperations in the data processor part and bits that determine the address sequence for the control memory.

The next address generator is sometimes called a microprogram sequencer, as it determines the address sequence that is read from control memory. The address of the next microinstruction can be specified in several ways, depending on the sequencer inputs.

Typical functions of a microprogram sequencer are incrementing the control address register by one, loading into the control address register an address from control memory, transferring an external address, or loading an initial address to start the control operations.

The control data register holds the present microinstruction while the next address is computed and read from memory. The data register is some-times called a pipeline register.

It allows the execution of the microoperations specified by the control word simultaneously with the generation of the next microinstruction. This configuration requires a two-phase clock, with one clock applied to the address register and the other to the data register.

The system can operate without the control data register by applying a single-phase clock to the address register. The control word and next-address information are taken directly from the control memory. It must be realized that a ROM operates as a combinational circuit, with the address value as the input and the corresponding word as the output.

The content of the specified word in ROM remains in the output wires as long as its address value remains in the address register. No read signal is needed as in a random-access memory. Each clock pulse will execute the microoperations specified by the control word and also transfer a new address to the control address register.

In the example that follows we assume a single-phase clock and therefore we do not use a control data register. In this way the address register is the only component in the control system that receives clock pulses. The other two components: the sequencer and the control memory are combinational circuits and do not need a clock.

The main advantage of the microprogrammed control is the fact that once the hardware configuration is established; there should be no need for further hardware or wiring changes.

If we want to establish a different control sequence for the system, all we need to do is specify a different set of microin¬structions for control memory. The hardware configuration should not be changed for different operations; the only thing that must be changed is the microprogram residing in control memory.

Disadvantage of micro-programmed control unit

1. Slower compared to hardwired control unit.

REFERENCES

Patterson, David; Hennessy, John (2012). Computer Organization and Design: The Hardware/Software Interface, 4th ed., revised. Morgan Kaufmann. ISBN 978-0-12-374750-1.

Von Neumann, John (1945), First Draft of a Report on the EDVAC (PDF), Moore School of Electrical Engineering, University of Pennsylvania

Englander, Irv (2009). The Architecture of Computer Hardware, Systems Software, & Networking, 4th Ed. Hoboken, NJ: John Wiley & Sons, Inc. p. 200. ISBN 978-0-471-71542-9.

Barkalov, Alexander (2009). Logic synthesis for FSM based control units / Alexander Barkalov and LarysaTitarenko. Berlin: Springer. ISBN 978-3-642-04308-6.

Wiśniewski, Remigiusz (2009). Synthesis of compositional microprogram control units for programmable devices. ZielonaGóra: University of ZielonaGóra. p. 153. ISBN 978-83-7481-293-1.