The A20 handler is
IBM PC memory manager software controlling access to the
High Memory Area.
Extended memory managers usually provide this functionality. A20 handlers are named after the 21st address line of the microprocessor, the
A20 line.The
Intel 8088 CPU used in the original IBM PC was capable of addressing memory using 20
binary digits ("bits"). But the registers in the 8088 were only 16 bits wide. A 20 bit
memory address was created by left shifting the 16 bit Segment Address (in effect multiplying it by 16) and adding the 16 bit offset address to it yielding a 20 bit result.
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