| High Memory Area |
Article Index for High |
Website Links For High |
Information AboutHigh Memory Area |
| CATEGORIES ABOUT HIGH MEMORY AREA | |
| computer memory | |
| ibm pc and compatibles memory architecture | |
| x86 memory management | |
|
By a strange design glitch the numbers) in Real Mode . In the Intel 8086 and Intel 8088 processors, unable to handle more than 1 megabyte of memory, addressing wrapped around, that is, address FFFF:0010 was equivalent to 0000:0000.In order to allow running existing MS-DOS programs which relied on this feature on their newer IBM PC AT computers, IBM added special Circuitry on the Motherboard to simulate the wrapping around. This circuit was a simple Logic Gate which could disconnect the microprocessor's addressing line A20 from the rest of the motherboard. This gate could be controlled, initially through the Keyboard controller, to allow running programs which wanted to access the entire RAM. So-called A20 Handler s could control the addressing mode dynamically, thereby allowing programs to load themselves into the 1024–1088 KB region and run in real mode. The first user of the HMA among Microsoft products was Windows/286 2.1 in 1988, which introduced the HIMEM.SYS device driver. Starting with versions 5.0 of DR-DOS (1990) and of MS-DOS (1991), parts of the operating system could be loaded into HMA as well, freeing up to 46 KB of Conventional Memory . Other components, such as device drivers and TSR s, could be loaded into the Upper Memory Area . SEE ALSO
|
|
|