Answer to Question #151443 in Assembler for shan

Question

What is a descriptor in 80286 microprocessors? Write how the size of a memory
segment is determined during real mode memory addressing and during protected mode
memory addressing.

Answer 1

The 80286 operate in either the real or protected mode.

Real Address Mode

The contents of segment registers are used as segment base addresses. The other registers (size 16bits), depending upon the addressing mode, contain the offset addresses. As in 8086, the physical memory is organized in terms of segments of 64Kbyte maximum size.

RAM can be divided into logical blocks of 64 KB, called segments, with each segment starting at a multiple of 16 bytes. Thus, the first segment has a start address of 0, the second is at 16 (or 10 in hex), and so on

Protected Mode Memory Addressing

The descriptor contains the physical address of the beginning of the segment in memory (base address), its size, and other information. The base address is located in 24 bits (and not 20, as for real mode), so the segments do not need to be aligned to an address that is a multiple of 16. The segment size is specified in 16 bits, and can take any values less than or equal to 64 KB . This is in contrast to real mode, where each segment is 64KB by default.