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13.3 Differences From 80286

The few differences that do exist primarily affect operating system code.

13.3.1 Wraparound of 80286 24-Bit Physical Address Space

With the 80286, any base and offset combination that addresses beyond 16M bytes wraps around to the first megabyte of the 80286 address space. With the 80386, since it has a greater physical address space, any such address falls into the 17th megabyte. In the unlikely event that any software depends on this anomaly, the same effect can be simulated on the 80386 by using paging to map the first 64K bytes of the 17th megabyte of logical addresses to physical addresses in the first megabyte.

13.3.2 Reserved Word of Descriptor

Because the 80386 uses the contents of the reserved word (last word) of every descriptor, 80286 programs that place values in this word may not execute correctly on the 80386.

13.3.3 New Descriptor Type Codes

Operating-system code that manages space in descriptor tables often uses an invalid value in the access-rights field of descriptor-table entries to identify unused entries. Access rights values of 80H and 00H remain invalid for both the 80286 and 80386. Other values that were invalid on for the 80286 may be valid for the 80386 because of the additional descriptor types defined by the 80386.

13.3.4 Restricted Semantics of LOCK

The 80286 processor implements the bus lock function differently than the 80386. Programs that use forms of memory locking specific to the 80286 may not execute properly when transported to a specific application of the 80386.

The LOCK prefix and its corresponding output signal should only be used to prevent other bus masters from interrupting a data movement operation. LOCK may only be used with the following 80386 instructions when they modify memory. An undefined-opcode exception results from using LOCK before any other instruction.

A locked instruction is guaranteed to lock only the area of memory defined by the destination operand, but may lock a larger memory area. For example, typical 8086 and 80286 configurations lock the entire physical memory space. With the 80386, the defined area of memory is guaranteed to be locked against access by a processor executing a locked instruction on exactly the same memory area, i.e., an operand with identical starting address and identical length.

13.3.5 Additional Exceptions

The 80386 defines new exceptions that can occur even in systems designed for the 80286.
Exception #6 -- invalid opcode
This exception can result from improper use of the LOCK instruction.
Exception #14 -- page fault
This exception may occur in an 80286 program if the operating system enables paging. Paging can be used in a system with 80286 tasks as long as all tasks use the same page directory. Because there is no place in an 80286 TSS to store the PDBR, switching to an 80286 task does not change the value of PDBR. Tasks ported from the 80286 should be given 80386 TSSs so they can take full advantage of paging.

up: Chapter 13 -- Executing 80286 Protected-Mode Code
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next: Chapter 14 -- 80386 Real-Address Mode