Memory

The System Logical Memory Layout

The original PC had a total of addressable memory and the top 384K of that was reserved for use by the system.The constant pressures on system and peripheral manufacturers to maintain compatibility by never breaking from the original memory scheme of the first PC has resulted in a system memory structure that is a mess.In an AT system,the memory map extends beyond the 1M boundry and can continue to 16M on a system based on the 286 or higher processor.Any memory past 1M is called extended memory.

If the processor is running in real mode,only the first megabyte is accessible. If the processor is in protected mode,the full 16M are accessible.Each symbol is equal to 1K of memory and each line or segment is 64K.