Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 831

Instruction Emulation and Other Fault
Handlers
This chapter introduces several common emulation handlers that an Itanium
architecture-based operating system must support. A general overview is given for:
• Unaligned Reference Handler – emulation of misaligned memory references that
the processor hardware cannot handle, or has been configured to fault on.
• Unsupported Data Reference Handler – emulation of memory operations that the
processor hardware does not support. Examples are semaphore, ldfe or stfe
operations to uncacheable memory.
• Illegal Dependency Fault Handler – this is a fatal condition that operating system
needs to provide error logging functionality for.
• Long Branch Handler – the Itanium processor does not implement the long branch
instruction. When encountered on the Itanium processor, long branches must be
emulated by the operating system.
Floating-point software assist emulation handlers are not discussed here, but are
presented in
"Efficient Interruption Handling" on page 2:102
code in the Itanium architecture.
7.1 Unaligned Reference Handler
Misaligned memory references that are not supported by the processor cause Unaligned
Reference Faults. This behavior is implementation specific but typically occurs in cases
where the access crosses a cache line or page boundary. In cases where the operating
system chooses to emulate misaligned operations, some special cases need to be
considered:
• Emulation of control and data speculative loads as well as advanced check and
"regular" loads requires special attention. For details consult
"Unaligned Handler" on page
• Emulation of unaligned semaphores, especially when interacting with IA-32 code
require special attention. For details consult
Uncacheable and Misaligned Semaphores" on page
IA-32 programs do not use the Itanium architecture-based handler to support
unaligned references. The hardware that supports IA-32 execution provides the
appropriate behavior if alignment checking is disabled through EFLAGS.ac. If an
unaligned reference occurs in IA-32 code when EFLAGS.ac is set to enable alignment
checking, alignment faults are delivered to a different vector from the unaligned
reference handler. Specifically they are delivered to the
IA_32_Exception(AlignmentCheck) vector; see
Descriptions"
Volume 2, Part 2: Instruction Emulation and Other Fault Handlers
Chapter 8, "Floating-point System Software."
2:581.
for details.
Additionally,
discusses more details about emulation
Section 6.3.1,
Section 2.1.3.2, "Behavior of
2:509.
Chapter 9, "IA-32 Interruption Vector
7
Section 5.5.1,
2:583