High Memory Area

(H-M-A)

The first 64K (minus 16 bytes) of extended memory (the 64K directly above the one megabyte point on an MS-DOS/Windows computer). The HMA is under the control of the driver HIMEM.SYS.

The Intel 8086 and 8088 CPUs could only handle 1MB of memory and any address over that was wrapped around to the bottom of the memory address space (e.g., ffff:0010 was seen as 0000:0000). The 80x86 processors kept that default wrapping behavior for compatibility purposes but it could be turned off using the A20 line. Special "A20 handlers" were developed that loaded into the 1024 to 1088 KB region and ran in real mode, taking advantage of this unique area. Often, parts of MS-DOS were loaded there to free up lower memory for programs.

More Information

• PC Memory Architecture