To access expanded memory for use with Windows 3.x programs, Windows 3.x used DPMI. Even though these applications were still 16-bit, it allowed them to use additional memory. DOS extenders are particularly common in DOS games because they allow them to access far more of the system's memory than most real-mode applications can. The CPU is switched in and out of real mode by these DOS extenders. Those that operate on Windows make advantage of the built-in DPMI interface, which allows them to share a portion of the system's expanded memory. 

Another exception in real mode is that, despite the fact that it's not intended to be possible, the first 64KB of extended memory is really available to the PC in real mode. This is due to a bug in the original IBM AT that affects the 21st memory address line, sometimes known as A20 (A0 is the first address line). Real-mode software can gain access to the first 64KB of extended memory—the first 64KB of memory beyond the first megabyte—by manipulating the A20 line. The high memory area is the name given to this part of the brain (HMA). 

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