The Processor & its components

The CPU The brain. Performs all major calculations. Controls and manages the operations of other components of the computer. Determines much of the performance of the computer. The 2 basic components of the CPU are: –The Control Unit (CU). –The Arithmetic Logic Unit (ALU).

The Control Unit (CU) It selects and interprets program instructions and then sees that they are executed. Contains: –Special purpose registers. a) instruction register (IR) and b) program control register (PCR). –A decoder to perform various activities. IR holds the current instruction to be executed. PCR holds the address of the next instruction to be executed.

The decoder has necessary circuitry to interpret the meaning of every instruction supported by the CPU. Each instruction is determined by microcode – which tell the CPU how to execute the instruction. CU does no processing by itself. It coordinates the entire computer system including I/O units. Basic task – summary: –Obtain instructions from program stored in memory. –Interpret/decode those instructions. –Issue signals. –The signals cause other units of the system to do their tasks.

The ALU Actual data processing takes place here. CU hands control over to the ALU when it encounters instructions such as add, divide etc. Contains: –Special purpose registers. –Circuitry to perform arithmetic & logic operations such as adder. Example: CU might load 2 numbers into the ALU registers and then tell the ALU to add them.

Instruction Set A ser of machine instructions supported by the processor such as add, compare etc. Each different processor has different instruction set. So machine language programs for one processor will not execute in another processor. When a manufacturer develops a CPU, they try to maintain upward compatibility.

Registers Temporary storage of data. Not a part of main memory. CPU always puts data from RAM to internal register before it processes that data Most CPU registers of today are of 32 or 64 bits. Known as word size. The bigger the word size the faster the CPU can process data.

Number of registers vary from computer to computer. Some are common to all computers: –Memory address register (MAR) Holds address of active memory location. Loaded from PCR. –Memory buffer register (MBR) Temporary storage for memory contents. Instructions are transferred to IR. Data are accessible to the ALU registers and can also be sent to I/O registers. Data needed to be written in memory is placed here first.

–Program control register (PCR): Holds memory address of next instruction to be executed. –Accumulator register (A): Holds data and result after processing. Mostly used. Result that is to be written in memory is transferred from here to MBR. –Instruction register (IR): Holds current instruction to be executed. The operation part is sent to CU. Address is part sent to MAR. –I/O register (I/O): Used to talk to I/O devices. Data & instruction from/to I/O devices are first put in this register.

Processor speed Operations of ALU and CU are synchronized by an internal clock called system clock. Generates electrical pulses. Several clock cycles are needed to execute a single instruction. (fetch, decode, execute). Measured in megahertz. MHz and now-a- days GHz.

Types of processor CISC (complex instruction set computer) –Supports a large number of instructions. –Makes it easier to translate high level languages. –Adds more circuitry and complexity to the overall design of the CPU. –Supports variable length instructions. –Supports various addressing modes. –Expensive. –Makes the job of machine level programmer easier. –Most processors of today are CISC. Intel’s Pentium.

RISC (reduced instruction set computers): –Low design complexity. –Supports few number of instructions. –Complex instructions are performed by the basic instructions. E.g. multiplication is performed by repeated addition. –Fixed length instructions. –Supports small number of addressing modes. –Puts extra burden on machine language programmers. –Less expensive. –Faster for most applications. –Motorola’s processors.