Types of Processors, Processor speed, Registers

A processor’s instruction set is a determining factor in its architecture. On the basis of the instruction set, microprocessors are classified as—Reduced Instruction Set Computer (RISC), and Complex Instruction Set Computer (CISC). The x86 instruction set of the original Intel 8086 processor is of the CISC type. The PCs are based on the x86 instruction set.

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CISC Processors

o   CISC architecture hardware the processor with complex instructions, which are difficult to create otherwise using basic instructions. CISC combines the different instructions into one single CPU.

o   CISC has a large instruction set that includes simple and fast instructions for performing basic tasks, as well as complex instructions that correspond to statements in the high level language.

o   An increased number of instructions (200 to 300) results in a much more complex processor, requiring millions of transistors.

o   Instructions are of variable lengths, using 8, 16 or 32 bits for storage. This results in the processor’s time being spent in calculating where each instruction begins and ends.

o   With large number of application software programs being written for the processor, a new processor has to be backwards compatible to the older version of processors.

o   AMD and Cyrix are based on CISC.

RISC Processors 

o  RISC has simple, single-cycle instructions, which performs only basic instructions. RISC architecture does not have hardwired advanced functions. All high-level language support is done in the software.

o  RISC has fewer instructions and requires fewer transistors, which results in the reduced manufacturing cost of processor.
o   The instruction size is fixed (32 bits). The processor need not spend time in finding out where each instruction begins and ends.

o    RISC architecture has a reduced production cost compared to CISC processors.

o    The instructions, simple in nature, are executed in just one clock cycle, which speeds up the program execution when compared to CISC processors.

o  RISC processors can handle multiple instructions simultaneously by processing them in parallel.

o   Apple Mac G3 and PowerPC are based on RISC.

Epic Processors

The Explicitly Parallel Instruction computing (EPIC) technology breaks through the sequential nature of conventional processor architectures, by allowing the software to communicate explicitly to the processor, when operations can be done in parallel. For this, it uses tighter original code, explicitly describe it to the processor. Processors based on EPIC architecture are simpler and more powerful than traditional CISC or RISC processors. These processors are mainly targeted to next generation, 64 bit, high-end server and workstation market (Not for personal computer markets).

Processors like Athlon XP and Pentium IV use a hybrid of both technologies.

Pipelining

It improves instruction execution speed by putting the execution steps into parallel. A CPU can receive a single instruction, begin executing it, and receive another instruction before it has completed the first. This allows for more instructions to be performed, about, one instruction per clock cycle.

Parallel Processing

It is the simultaneous execution of instructions from the same program on different processors. A program is divided into multiple processes that are handled in parallel in order to reduce execution time.

Processor Speed

The operations of the ALU and control unit are performed as incredible speed. These operations are usually synchronized by a built-in electronic clock (Known as system clock), which emits millions of regularly spaced electric pulses per second (known as clock cycles). Commands are interpreted, and then executed at proper intervals, and then executed at proper perform one operation, such as moving a byte of data from one memory location to another. Normally, several clock cycles are required to fetch, decode, and execute a single program instruction. The shorter the clock cycle, the faster the processor.  Hence, the speed with which an instruction is executed is directly related to the computer’s built-in clock speed, which is the number of pulses, produced per second. Over the years, clock speeds of processors have increased steadily. Most of today’s popular personal computers have cloch speeds in the 500-to 2000- MHz range. However, processors are now being built with ever faster ratings. With all other parameters being equal (although they never are ), a CPU operating at 500 MHz can process data five times as fast as one operating at 100 MHz.

Registers

As the instructions are interpreted and executed by the CPU, there is a movement of information between the various units of the computer system. In order to handle this process satisfactorily, and to speed up the rate of information transfer, the computer uses a number of special memory units, called registers. These registers are used to hold information on a temporary basis, are part of the CPU (not main memory).

The length of a register equals the number of bits it can store. Hence, a register can store 8 bits is normally referred to as an 8-bit register. Most CPUs sold today, have 32-bit or 64-bit registers. The size of the registers is sometimes called the word size. The bigger the word size, the faster the computer can process a set of data. Register are used to quickly accept, store, and transfer data and instructions that are being used immediately by the CPU, there are various types of Registers those are used for various purpose. Among of the some Mostly used Registers named as AC or Accumulator, Data Register or DR, the AR or Address Register, program counter (PC), Memory Data Register (MDR ,Index register, Memory Buffer Register.

These Registers are used for performing the various Operations. While we are working on the System then these Registers are used by the CPU for Performing the Operations. When We Gives Some Input to the System then the Input will be Stored into the Registers and When the System will gives us the
Results after Processing then the Result will also be from the Registers. So that they are used by the CPU
for Processing the Data which is given by the User. Registers Perform:-

1) Fetch: The Fetch Operation is used for taking the instructions those are given by the user and the Instructions those are stored into the main memory will be fetch by using Registers.

2) Decode: The Decode Operation is used for interpreting the Instructions means the Instructions are
decoded means the CPU will find out which Operation is to be performed on the Instructions.
3) Execute: The Execute Operation is performed by the CPU. And Results those are produced by the CPU
are then Stored into the Memory and after that they are displayed on the user Screen.

Types of Registers are as Followings

MAR stand for Memory Address Register

This register holds the memory addresses of data and instructions. This register is used to access data and instructions from memory during the execution phase of an instruction. Suppose CPU wants to store some data in the memory or to read the data from the memory. It places the address of the-required memory location in the MAR.

Program Counter

The program counter (PC), commonly called the instruction pointer (IP) in Intel x86 microprocessors,
and sometimes called the instruction address register, or just part of the instruction sequencer in
some computers, is a processor register. It is a 16 bit special function register in the 8085
microprocessor. It keeps track of the next memory address of the instruction that is to be executed once the execution of the current instruction is completed. In other words, it holds the address of the memory location of the next instruction when the current instruction is executed by the microprocessor.

Accumulator Register

This Register is used for storing the Results those are produced by the System. When the CPU will generate Some Results after the Processing then all the Results will be Stored into the AC Register.

Memory Data Register (MDR)

MDR is the register of a computer’s control unit that contains the data to be stored in the computer
storage (e.g. RAM), or the data after a fetch from the computer storage. It acts like a buffer and holds
anything that is copied from the memory ready for the processor to use it. MDR hold the information before it goes to the decoder. MDR which contains the data to be written into or read out of the addressed location. For example, to retrieve the contents of cell 123, we would load the value 123 (in binary, of course) into the MAR and perform a fetch operation. When the operation is done, a copy of the contents of cell 123 would be in the MDR. To store the value 98 into cell 4, we load a 4 into the MAR and a 98 into the MDR and perform a store. When the operation is completed the contents of cell 4 will have been set to 98, by discarding whatever was there previously.

The MDR is a two-way register. When data is fetched from memory and placed into the MDR, it is written to in one direction. When there is a write instruction, the data
to be written is placed into the MDR from another CPU register, which then puts the data into memory.
The Memory Data Register is half of a minimal interface between a micro program and computer storage, the other half is a memory address register.

Index Register

A hardware element which holds a number that can be added to (or, in some cases, subtracted from) the address portion of a computer instruction to form an effective address. Also known as base register. An index register in a computer’s CPU is a processor register used for modifying operand addresses during the run of a program.

Memory Buffer Register

MBR stands for Memory Buffer Register. This register holds the contents of data or instruction read from, or written in memory. It means that this register is used to store data/instruction coming from the memory or going to the memory.

Data Register

A register used in microcomputers to temporarily store data being transmitted to or from a peripheral device.

The execution of an instruction by the CPU, during program execution, normally involves the following steps:

1.        The control unit takes the address of the next program instruction to the executed from the program control register, and read the instruction from the corresponding memory address, into the instruction register of the control unit.
2.       The control unit then sends the operation part and the address part of the instruction, to the decoder and the memory address register, respectively.
3.         The decoder interprets the instruction, and accordingly the control unit sends signals to the appropriate unit, which needs to be involved in carrying out the task specified in an instruction. For example, if it is an arithmetic or logic operation, the signal is sent to the ALU. In this case, the control unit also ensures that the data corresponding to the address part of the instruction is loaded in a suitable register in the ALU, before the signal is sent to the ALU. The ALU performs the necessary operation on the data, and the signals the control unit as soon as it has finished.
4.        As each instruction is executed, the address of the next instruction to be executed is automatically loaded into the program control register, and steps 1 to 4 are repeated.