System Buses Computer Components & flow control Interrupts & Interconnection Bus design and timings Hierarchy & arbitration

What is a program?A sequence of stepsFor each step, an arithmetic or logical operation is doneFor each operation, a different set of control signals is needed

Basic TermsInstruction & Data: Info fed to comp is either inst. or data Instructions are commands that -- Govern transfer of information within comp & I/O devices.-- specify arithmetic &logic inst. To be performed. A set of inst. That perform a task is a Program.Data are nos & encoded characters that are used as operands by instruction.

2.Bits: Each no, char or inst. is encoded as a string of binary digits called bits .Bits have value 0 or1

3. Word: The MM contains a large no. of semiconductor storage cells, each capable of storing one bit of info. These cells are accessed into groups called words.

4. Word Length: No. of bits in each word is called Word-length. Wordlength ranges from 16 to 64 bits.

5. Memory access time: time require to access one word is called the memory access time.

6.Registers: Processor normally contains a no. of high speed storage elements called registers Each reg. can store one word of data.

1 byte = 8 bits Halfword = 16 bits word = 32 bits Doubleword = 64 bits

ComponentsThe Control Unit and the Arithmetic and Logic Unit constitute the Central Processing UnitData and instructions need to get into the system and results outInput/outputTemporary storage of code and results is neededMain memory

Computer components

Processor contains no. of registers used for temporary storage of data, control unit & ALU.

Instruction register: The IR holds the instruction currently being executed. Program Counter: PC keeps track of the execution of the program. It contains memory address of the next instruction to be fetched and executed. During the execution of an instruction the contents of PC are updated.MAR & MDR: Provides communication with the memory.Memory address reg. (MAR) holds address of location to or from which data are to be transferred.Memory data reg. (MDR) contains the data to be written into or read to location.

Instruction CycleInstruction processing consists of two steps.

Fetch cycle: processor fetches inst. from memory one at a time.

Execute cycle: fetched instruction is executed

The processing required for a single instruction is called an instruction cycle. Fetch cycle and execute cycle run simultaneously.

Basic instruction cycle

1. Fetch Cycle: Reading a word from memory:

PC

2. Execute cycle: Initially the fetched inst. Is loaded into reg. IR. Inst. Specifies what action the CPU is to take.The CPUs action fall into four categories

Processor-memorydata transfer between CPU and main memoryProcessor I/OData transfer between CPU and I/O moduleData processingSome arithmetic or logical operation on dataControlAlteration of sequence of operationse.g. jumpCombination of above

The various states in inst. Cycle can be described as followsInst. Address calculation.Inst. Fetch Inst. OperationOperand address calculationOperand fetchData operationOperand store

Instruction Cycle - State Diagram