80286
The most substantial difference between the 80286 and the 8086-8088
is the addition of a protected mode. In protected mode,
segment registers became pointers into a table of memory descriptors
rather than being a direct part of the address. Among other things,
protected mode allows up to 16 MB of memory (instead of 64KB) to be
addressed and allows safe execution of multiple programs at once
by protecting each program in memory. DOS normally operates
in real mode, in which segment registers act just as they do in
the 8086-8088. Protected mode is beyond the scope of this class
(CMSC211) and is covered in CMSC421, Principles of Operating Systems.
It is used by Microsoft Windows, IBM's OS/2, and UNIX.
The 80286 also has a few new instructions:
| push | constant |
| shl | reg-or-memory, constant |
| imul | reg, reg-or-mem, constant |
There is also a enter and leave instruction that assist in maintaining
stack frames.
To make use of this new instructions, you must use the directive
.286.
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211 F'99 |
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Monday, 07-Feb-2000 21:24:47 EST