Extended memory switch

Ø  The 286 and the 386 can work in both real and protected modes. Real mode limits memory access to 1MByte but provides compatibility with the 8088/86 processor and MS-DOS applications.

Ø  Extended memory is available beyond 1MByte but only in protected mode, i.e. real mode applications cannot use extended memory (with the exception of the HMA).

Ø  The XMS standard allows protected mode software to allocate extended memory without conflict. It also controls access to the HMA- a 64KByte area of extended memory that real mode applications can make use of. It has replaced the older INT 15 or top down allocation method.

Ø  A DOS extender is a way of allowing protected mode applications to use MS-DOS in real mode.

Ø  Expanded memory is a way of adding memory beyond 1MByte that is accessible by real mode applications.

Ø  Extended memory can be converted into expanded memory using nothing but software.

Ø  VCPI was the first standard controlling how protected mode applications should share extended memory. It has been superseded by the DPMI standard introduced with Windows 3.0.

Ø  The 386’s memory management can be used to create not only expanded memory from extended but also Upper Memory Blocks (UMBs) to increase conventional memory, shadow RAM to increase speed and virtual memory to increase the total memory.

Ø  Advanced SCSI drives that make use of bus mastering need special treatment by protected mode operating systems.