366
March 2002 Release
1-800-255-7778
Virtex-II Pro™ Platform FPGA Documentation
Chapter 3:
User Programming Model
R
Exception Summary
An exception is an event that can be caused by a number of sources, including:
•
Error conditions arising from instruction execution.
•
Internal timer resources.
•
Internal debug resources.
•
External peripherals.
When an exception occurs, the processor can interrupt the currently executing program so
that system software can deal with the exception condition. The action taken by an
interrupt includes saving the processor context and transferring control to a
predetermined exception-handler address operating under a new context. When the
interrupt handler completes execution, it can return to the interrupted program by
executing a
return-from-interrupt
instruction.
Exceptions are handled by privileged software. The exception mechanism is described in
. Following is a list of exceptions that can be caused
by the execution of an instruction in user mode.
•
Data-Storage Exception.
An attempt to access data in memory that results in a memory-protection violation
causes the data-storage interrupt handler to be invoked.
•
Instruction-Storage Exception.
An attempt to access instructions in memory that result in a memory-protection
violation causes the instruction-storage interrupt handler to be invoked.
•
Alignment Exception.
An attempt to access memory with an invalid effective-address alignment (for the
specific instruction) causes the alignment-interrupt handler to be invoked.
•
Program Exception.
Three different types of interrupt handlers can be invoked when a program exception
occurs: illegal instruction, privileged instruction, and system trap. The conditions
causing a program interrupt include:
-
An attempt to execute an illegal instruction causes the illegal-instruction interrupt
handler to be invoked.
-
An attempt to execute an optional instruction not implemented by the PPC405
causes the illegal-instruction interrupt handler to be invoked.
-
An attempt by a user-level program to execute a supervisor-level instruction
causes the privileged-instruction interrupt handler to be invoked.
-
An attempt to execute a defined instruction with an invalid form causes either the
illegal-instruction interrupt handler or the privileged-instruction interrupt
handler to be invoked.
-
Executing a trap instruction can cause the system-trap interrupt handler to be
invoked.
•
Floating-Point Unavailable Exception.
On processors that support floating-point instructions, executing such instructions
when the floating-point unit is disabled (MSR[FP]
=
0) invokes the floating-point-
unavailable interrupt handler.
•
System-Call Exception.
The execution of an
sc
instruction causes the system-call interrupt handler to be
invoked. The interrupt handler can be used to call a system-service routine.
•
Data TLB-Miss Exception.
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