Conventional Memory
The Intel 8088 processor used in the original IBM PC had 20 address lines and so could directly address 1 mebibyte (220 bytes) of memory. Different areas of this address space were allocated to different kinds of memory used for different purposes. Starting at the lowest end of the address space, the PC had read/write random access memory (RAM) installed, which was used by PC DOS and application programs. The first part of this memory was installed on the motherboard of the system (in very early machines, 64 KiB, later revised to 256 KiB). Additional memory could be added on cards plugged into the expansion slots; each card contained straps or switches to control what part of the address space access memory and devices on that card.
On the IBM PC, all the address space up to 640 KiB was available for RAM. This part of the address space is called "conventional memory" since it is accessible to all versions of PC DOS (and MS DOS) automatically on start up. Normally expansion memory is set to be contiguous in the address space with the memory on the motherboard. If there was an unallocated gap between motherboard memory and the expansion memory, the memory would not be automatically detected as usable by PC DOS .
Read more about this topic: DOS Memory Management
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