INTRODUCTION TO MEMORY BUSES
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INTRODUCTION TO MEMORY BUSES

Computers as a representation of binary digits, each digit is called a bit, the minimum storage element. The address may come from the CPU, from a cache memory, or from a memory mapped I/O device. A bus interface unit controls instructions and data transfers to and from CPU. Hence, like the buses between CPU and main memory, in similar manner system uses buses for connecting CPU and I/O devices, which are known as data bus and address bus.

When any program execution takes place in the CPU, it requires frequent interaction between the CPU and main memory. The connectivity between the CPU and memory is established through a channel known as BUS. Physically bus is a set of wires which carry group of bits. Buses are electrical data pathways within CPU and between CPU and other components of the motherboard through which bites are transmitted. The capacity of a bus is called a bus width which means the number of bits they carry at one time. Because storage of data in Digital Computers as a representation of binary digits, each digit is called a bit, the minimum storage element. The address may come from the CPU, from a cache memory, or from a memory mapped I/O device. A bus interface unit controls instructions and data transfers to and from CPU. Hence, like the busses between CPU and main memory, in similar manner system uses buses for connecting CPU and I/O devices, which are known as data bus and address bus.

Data Bus: The data bus is called bio-directional, as it can serve to write data to and read data from the memory system. New data can be read from or written to the memory system every cycle time. Physically, bus is a group of parallel wires that form an electrical path that connects the CPU, Memory, and I/O devices. Thus, a chip may be 8-bit, 16-bit or 32-bit chip. The number of wires in the bus affects the speed of the data transfer, because each wire can transfer one bit at a time, thus, 8 wire buses can move eight bits or one bit at one time. The 16 bit bus that transmits data along 8 or 16 data lines depending on the adapter card in the expansion slot is known as ISA bus (Indian Standard Architecture). PC buses are designed to match the capabilities of the devices attached to them. Further, advancement in computer improved the performance by the use of 32-bit bus that transmits data along 32 data lines, known as MCA (Micro Channel Architecture). It is faster than ISA bus, because MCA expansion slot requires only 32-bit adapter card. Since MCA bus could not work with ISA bus, therefore, EISA (Extended Indian Standard Architecture) was design to solve this problem. Although, EISA is 32-bit bus, it could accept old ISA expansion slot. EISA is faster than ISA.

Address Bus: it transmits signals for locating a given address in the primary memory (RAM). The numbers of wires in the address bus determines the maximum number of memory address, i.e. if the address bus could carry 8 bit at one time, then CPU could address 256 (= 28) bytes of RAM.

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