Computer organisation..
Micro program control unit vs hard wired control unit.
To execute an instructions c.p.u. designs place an important role it is a process of creating a cpu component of computer hardware. The main objective of cpu is to execute list of instruction like addition etc. Cpu design focus on :–
1 data path. Ex ALU and pipeline
2 control unit that is the logic which controls the. Data path
3 memory component such as registers files. Cache memory
4 clock circuit
5 transceiver circuit
There are two possible designs of control unit available :–
1 Hard wired
2 Micro programmed
1. HARD WIRED :– in this unit the control unit uses fixed logic circuit to interpret the instructions and generate the control signals from them the fixed logic circuit block includes combinational and sequential circuit that generates the required control output for decoding and encoding functions by separating the decoding and encoding function we can draw more detailed block diagram for hard wired control unit as shown in fig.
The instructions decoder decodes the instructions loded in the instructions register if instructions register is of 8 bit then the instructions decoder generates 256 BITS OF ADDRESS 1 for each instructions
According to the op code in the ir register only one line among the all input lines of decoder goes high this is said to 1 and all other lines set to 0 the step decoder provides the separate signal line for each step in a control sequence the encoder gets the input from instructions decoder, step counter , external input and conditions codes it uses all these input to generate after execution of each instructions end signal is generated which reset the control of step counter and make it ready of the generations the control step for next generation .
The factors affecting the design of hard wired control unit :–
1 the amount of hardware.
2 the speed of operations.
3 the cost.
There are 4 techniques for the design of hard wired control unit :–
1. state table method:- it is a classical method of sequential design ot manages the hardware required in control unit and constructs a straight transition table. Every generation of state has a set of control signal.
2. Delay element method:- In this control signal follow a proper signal and specific time delay between two groups of control signal is added to ensure synchronization these are controlled by a common clock signal.
3. Sequence counter method;- it uses a step counter for timing purpose.
4. PLA(programmable logic array):- it uses PLA for generating control signal.
Advantages of hard wired control unit :–
1. Hard wired control unit is fast because control signal is generated by the sequential circuit based on present state and other input conditions.
2. The delay in generation of control signal that means one method can generate control signal faster than the classical method.
Disadvantages :-
1. More is the control signal required by the cpu more will be complex will be the design of control unit.
2. Hard wired control unit is a regit ( not flexible) approch.this means modification in this is very difficult that means it requires re arranging of wires in hardware circuit.
3. It is very difficult to add new features in the existing design of control unit.
MICRO PROGRAMMED CONTROL UNIT
In this program micro signal are stored in special memory called control memory it is implemented using programming approach every instruction in the processor is executed by a sequence of 1 or more sets of micro operations. Each micro operations is associated with a specific set of control lines which when activated causes that micro operations to take place. Micro programming is a method of control unit design in which the control signal selection and sequencing stored in a memory called control memory. The control signal to be activated at any time are specified by the micro signal which is fetched by the control memory in the similar way as the instructions from the main memory each micro instructions also specifies the next micro instructions to be used there by providing the necessary information for sequencing a sequence if one or more micro instructions designed to control specific operations such as addition, multiplication etc. micro programs for all instructions are stored in control memory the address. Were these instructions stored in control memory micro programs sequencer and micro program controller It generates the instructions according to the instruction stored in instructions register so,it consist of micro program sequencer .control address register, micro instructions register and control memory..
The components of control unit work as follows :–
1. Control address register:– it holds the address of next micro instructions to be read. Every time a new instructions is loaded into the instructions register the output of the block sequencer is loaded to the control address register
2. when the address is available in the control address register the sequencer issues the read command to control memory
3. After issue of read command the control word from the location of that address is read into micro instruction register.
4. The control address register is automatically incremented by the clock so that next instructions can be read by control memory.
5. The content of micro instruction register is sent to the instruction decoder which generates control signal and send it to the various parts of processor in the correct sequence.
ADVANTAGES:–
1. It simplifies the design of control unit so it is cheaper and less prone to error.
2. The control function or operation are implemented in software rather than hardware.
3. The design process is systematic.
4. It is more flexible than hard wired control and it can be changed to implement new system specification.
5. Complex operations such as floating point arithmetic can be done efficiently.
Disadvantages :–
1. it is slower than the hardware control unit because it takes time to access the micro instructions from the control memory and also in compling the micro program.
2. The flexibility is achieved at the extra hardware cost of control memory and it access circuit.
QUE. — Difference between micro program control unit and hard wired?
ANS:– Hard wired:-1. Hard wired control unit is fast
Micro program unit is slow speed
2. H.W. control function it is implemented in hardware
MICRO PROGRAM :– It is implemented in software.
3. HARD WIRED:–Flexibility it is not flexible or regit. Because to make new changes in the system specification or to add new instructions we have to change the complete hardware.circuit
M.PC.U.:– It is more flexible than hard wire as changes can be implemented by using new instructions.
4.M.P.C.U. Ability to handle large or complex instruction.
HARD WIRED:— IT is difficult to handle complex instructions.
5. Design process hard wired somewhat complicated
It is orderly and systematic.
6. H.W.C.U. Applications it is mostly 7sed in R.I.S.T. microprocessoR
it is used in C.I.S.T. microprocessor.
8. Instruction set size. There are usually under 100 instructions.
There are usually over 100 instructions.
9 ROM/CACHE SIZE there is no memory used.(HARD WIRED)
The control memory is of size 2k to 10k.(M.P.C.U.)
10. Chip area hard wired uses least area on the IC
it uses more area on the IC.(M.P.C.U.)
REDUCED INSTRUCTIONS SET (R.I.S.C.)
The classification of instruction set of the processor inclides two types.
1 R.I.S.C.
2. C.I.S.C.
1.. R.I.S.C :- it is a computer that was used in early 1980s it can be considered as revolution in the Architecture of computers it is simply a computer with less number of vertical micro instructions in risc the user program are compiled into the sequence of these micro instructions and then directly executed by the hardware without any interpreter these is the secret behind the speed of risc machine due to elemination of interpretation. As the result of which simple things such as adding of two registers by a program can be completed in a single micro instructions. The risc instruction set consist of less than 100 instructions with a fixed instruction format of 32 bits. Only 3 to 5 simple addressing modes are used in these most of the instructions are executed in one cycle with hard wired control unit most of the instructions are register based. the main market for risc processor are the system requires low power,small size and low cost.
Characteristics of R.I.S.C. :–
1. It has few instruction types and less addressing modes
2. It has fixed and easily decoded instructions format
3. Most instructions are executed in one cycle with hard wired control unit.
4. Fast single cycle instructions execution.
5. Memory access is limited due to load and store instructions.
6. it uses complier but no interpreter.
7. All the operations are done within the register of cpu
8. It uses pipeline concept.
R.I.S.C
INSTRUCTION FORMAT :–
The basic instructions format used in risc processor contains
1. 7 bit op code
2. To 5 bit register name as destination. Source.
3. Address mode bit represented as I i =0 not immediate address. I=1 means immediate address
Condition mode bit represented as c
C=0 means do not set condition code, c=1 means set condition code.
There are basically 3 addressing mode
1 Register direct mode
2 register indirect mode
3 Index addressing mode.
Drawbacks of risc.
1. R.I.S.C. processor lacks some important instructions found in CISC.
2. THE presence of hard wired is another short coming because it is less flexible and more prone to error.
3. Increase in the length of 5he risc program results in more no. Of instruction or more memory.
4. It uses a large register file which can cause problem .
C.I.S.C. COMPLEX INSTRUCTION SET COMPUTER.
The computers at the earliest time were very simple in their design also the high level languages used by these computer were simple. But with the release of IBM360 series the concept of microprogramming starts the instruction set used in this processors were designed not only to do simple binary operations but they can do floating point and other complex operations as wella as As a result of the instructions set is highly complexed and so the micro program becomes complex also. With the concept of microprogramming the programming become easier. Because a programmer could remember a set of 30 to 100 instructions and a set of 3 to 10 addressing modes this is known as orthogonal instructions set and further called complex instructions set computer the slower speed of main memory in comparison cpu also encouraged the spread cisc computers.
The main characteristics of C.I.S.C. :–
1 It has a large no. Of instructions typically in 100s ranging from 100 to 350
2. Some instructions are for special task which are rarely used.
3. It has different types of instruction and data format of different length.
4. It has at wide variety of addressing modes ranging from 5 to 20. It has a small set 9f general purpose register ranging from 8 to 24
5. many high level language is directly implemented in the hardware.
6. the translation of program from high level to low level language is done by a compiler
7. the main goal cisc is to provide single machine instructor for each statement written in high level language.
This architecture is based on micro programmed control unit design.
Disadvantages Of CISC.
1. CISC processor are slower in nature in comparison of risc because of more and more instructions the machine language is more complicated.
2. More instructions in the instruction set means more time is spent in decoding the operations mode.
3. In CISC the analysis of address is very difficult because same micro instruction would have been repeated in the micro programs
4. Many instruction and addressing that have been added that have rarely used.
DIFFERENCE BETWEEN RISC AND CISC
ANS:-
1.. INSTRUCTION SET risc has small set of instruction and most of the instruction based on register
Cisc has large no. Of instructions
2.. instruction format :– risc has instruction has fixed format of 32 bit per instruction
Cisc instruction have variable format ranging from 6 to 64 bits.
3.. addressing mode risc has a limited addressing mode ranging from 3 to 5
Cisc has large no. Of addressing mode ranging from 5 to 20.
4.. general purpose register risc it has.a large no. Of general purpose register from 32 to 192.
Cisc has small set of general purpose register ranging from 8 to 24.
5.. cache design and usage:- risc uses spilt cache memory one for instructions and one for data
Cisc uses common cache memory for instructions and data.
6.. pipelining risc architecture that is it performs parallel processing and pipelined
Cisc is not pipelined at all.
7.. processor speed:- risc computer has a speed of 52 to 200mhz with no cycle for all instructions.
Cisc has a processor speed of 33 to 66 mhz .
8.. cpu control design risc uses hard wired control without any control memory
Cisc uses microprogrammed with control memory.
SOME IMPORTANT QUESTIONS ON THIS CHAPTER:–
1. What are the addressing modes used in the micro computer system?
2. what are the various types of instruction format used in a computer system?
3. what do you mean by stack organization?
4. what do you mean by general organisation register explain with the help of example?
5. what of you mean by cpu what are various components of cpu?
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