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 internal registers in the 8088 were only 16 bits wide. A 20 bit address was created by tacking four zero bits onto the end of a 16 bit Segment Address and adding the 16 bit address to it yielding a 20 bit result. In reality, the result can be 21 bits wide since the total of the two registers can be more than FFFFF hexadecimal. It is possible to generate references to slightly more than 65000 bytes that are outside the addressing range of the 8088. When the 80286 CPU which had more than 20 address lines was developed, it became necessary to deal with code that referenced the 64K addressable by ordinary addressing but not present in the 8088. As it turned out, it was sometimes desirable to set the 21st address line, and sometimes desirable not to. A logic gate was added on the motherboard to control the 21st Address line therefore.

Controlling the A20 line was an important feature at one stage in the growth of the IBM PC architecture, as it added access to an additional 65520 bytes of memory in real mode without dramatic software changes.

See also

• Computer storage
• Software

References

External links

• A20 - a pain from the past