Q. Explain Segment Registers in bus interface unit?
These are very significant registers of CPU. Why? We will answer this later. In 8086 microprocessor memory is a byte organized which is a memory address is byte address. But the number of bits fetched is 16 at an instance. The segment registers are used to compute the address of memory location accompanied by other registers. The Segment-register is 16 bits long. The BIU comprise four 16-bit registers which follows the CS: Code Segment, the DS: Data Segment, the SS: Stack Segment and ES: Extra Segment. However what is the requirement of the segments: Segments reasonably divide a program in logical entities of Data, Code and Stack each having a particular size of 64 K. A Segment- register holds the upper 16 bits of starting address of a logical group of memory known as the segment. However what are the benefits of using segments? The main benefits of employing segments are:
- Logical division of program so increasing the overall possible memory use and minimise wastage.
- Addresses which required to be used in programs are relocatable as they are offsets. So the segmentation supports relocatability.
- Though size of address is 20 bits however only maximum segment size which is 16 bits requires to be kept in instruction so decreasing instruction length.