Compaq ECQD2KCTE Laptop User Manual


 
4–138 Alpha Architecture Handbook
4.11.4 Exception Barrier
Format:
Operation:
{EXCB does not appear to issue until completion of all
exceptions and dependencies on the Floating-point Control
Register (FPCR) from prior instructions.}
Exceptions:
Instruction mnemonics:
Qualifiers:
Description:
The EXCB instruction allows software to guarantee that in a pipelined implementation, all pre-
vious instructions have completed any behavior related to exceptions or rounding modes before
any instructions after the EXCB are issued.
In particular, all changes to the Floating-point Control Register (FPCR) are guaranteed to have
been made, whether or not there is an associated exception. Also, all potential floating-point
exceptions and integer overflow exceptions are guaranteed to have been taken. EXCB is thus a
superset of TRAPB.
If a floating-point exception occurs for which trapping is enabled, the EXCB instruction acts
like a fault. In this case, the value of the Program Counter reported to the program may be the
address of the EXCB instruction (or earlier) but is never the address of an instruction follow-
ing the EXCB.
The relationship between EXCB and the FPCR is described in Section 4.7.8.1.
EXCB
! Memory format
None
EXCB Exception Barrier
None