Errata
Specification Update
15
BH3
Writing the Local Vector Table (LVT) when an Interrupt is Pending
May Cause an Unexpected Interrupt
Problem:
If a local interrupt is pending when the LVT entry is written, an interrupt may be taken
on the new interrupt vector even if the mask bit is set.
Implication:
An interrupt may immediately be generated with the new vector when a LVT entry is
written, even if the new LVT entry has the mask bit set. If there is no Interrupt
Service Routine (ISR) set up for that vector the system will GP fault. If the ISR does
not do an End of Interrupt (EOI) the bit for the vector is left set in the in-service
register and mask all interrupts at the same or lower priority.
Workaround:
Any vector programmed into an LVT entry must have an ISR associated with it, even
if that vector was programmed as masked. This ISR routine must do an EOI to clear
any unexpected interrupts that may occur. The ISR associated with the spurious
vector does not generate an EOI; therefore the spurious vector should not be used
when writing the LVT.
Status:
For the steppings affected, see the Summary Tables of Changes.
BH4
MOV To/From Debug Registers Causes Debug Exception
Problem:
When in V86 mode, if a MOV instruction is executed to/from a debug registers, a
general-protection exception (#GP) should be generated. However, in the case when
the general detect enable flag (GD) bit is set, the observed behavior is that a debug
exception (#DB) is generated instead.
Implication:
With debug-register protection enabled (i.e., the GD bit set), when attempting to
execute a MOV on debug registers in V86 mode, a debug exception is generated
instead of the expected general-protection fault.
Workaround:
In general, operating systems do not set the GD bit when they are in V86 mode. The
GD bit is generally set and used by debuggers. The debug exception handler should
check that the exception did not occur in V86 mode before continuing. If the exception
did occur in V86 mode, the exception may be directed to the general-protection
exception handler.
Status:
For the steppings affected, see the Summary Tables of Changes.
