PART III COMPATIBILITY
PART III COMPATIBILITY
Chapter 13 Executing 80286 Protected-Mode Code
Chapter 13 Executing 80286 Protected-Mode Code
13.1 80286 Code Executes as a Subset of the 80386
Chapter 13 Executing 80286 Protected-Mode Code
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13.1 80286 Code Executes as a Subset of the 80386
In general, programs designed for execution in protected mode on an 80286
execute without modification on the 80386, because the features of the 80286
are a subset of those of the 80386.
All the descriptors used by the 80286 are supported by the 80386 as long as
the Intel-reserved word (last word) of the 80286 descriptor is zero.
The descriptors for data segments, executable segments, local descriptor
tables, and task gates are common to both the 80286 and the 80386. Other
80286 descriptors--TSS segment, call gate, interrupt gate, and trap
gate--are supported by the 80386. The 80386 also has new versions of
descriptors for TSS segment, call gate, interrupt gate, and trap gate that
support the 32-bit nature of the 80386. Both sets of descriptors can be
used simultaneously in the same system.
For those descriptors that are common to both the 80286 and the 80386, the
presence of zeros in the final word causes the 80386 to interpret these
descriptors exactly as 80286 does; for example:
Base Address The high-order eight bits of the 32-bit base address are
zero, limiting base addresses to 24 bits.
Limit The high-order four bits of the limit field are zero,
restricting the value of the limit field to 64K.
Granularity bit The granularity bit is zero, which implies that the value
of the 16-bit limit is interpreted in units of one byte.
B-bit In a data-segment descriptor, the B-bit is zero, implying
that the segment is no larger than 64 Kbytes.
D-bit In an executable-segment descriptor, the D-bit is zero,
implying that 16-bit addressing and operands are the
default.
For formats of these descriptors and documentation of their use refer to
the iAPX 286 Programmer's Reference Manual.
13.2 Two Ways to Execute 80286 Tasks
13.2 Two ways to Execute 80286 Tasks
When porting 80286 programs to the 80386, there are two cases to consider:
1. Porting an entire 80286 system to the 80386, complete with 80286
operating system, loader, and system builder.
In this case, all tasks will have 80286 TSSs. The 80386 is being used
as a faster 286.
2. Porting selected 80286 applications to run in an 80386 environment
with an 80386 operating system, loader, and system builder.
In this case, the TSSs used to represent 80286 tasks should be
changed to 80386 TSSs. It is theoretically possible to mix 80286 and
80386 TSSs, but the benefits are slight and the problems are great. It
is recommended that all tasks in a 80386 software system have 80386
TSSs. It is not necessary to change the 80286 object modules
themselves; TSSs are usually constructed by the operating system, by
the loader, or by the system builder. Refer to Chapter 16 for further
discussion of the interface between 16-bit and 32-bit code.
13.3 Differences from 80286
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
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
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
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
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.
* Bit test and change: BTS, BTR, BTC.
* Exchange: XCHG.
* One-operand arithmetic and logical: INC, DEC, NOT, and NEG.
* Two-operand arithmetic and logical: ADD, ADC, SUB, SBB, AND, OR, XOR.
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
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.