FPMODE(1)                                                            FPMODE(1)


NAME
     fpmode - run a command in specified floating point and/or memory mode

SYNOPSIS
     fpmode precise|performance|smm|nsmm|spec|nonspec command [ arguments ]

DESCRIPTION
     fpmode executes command, forcing it to use the specified floating point
     exception and memory access mode. This is only meaningful on processors
     that support multiple modes. On these processors, the default mode is
     usually performance mode (also called imprecise exception mode) and non-
     sequential memory (nsmm) mode.  On all other processors, the default is
     precise exception and sequential (smm) mode.

     The R8000 is the only processor which supports both a precise exception
     mode and a performance mode.  All other processors support only precise
     exceptions, though automatic flushing of denormalized results to zero may
     be controlled through the setting of the FS bit in the FP control and
     status register by calling set_fpc_csr().

     In precise exception mode, all floating point exceptions/interrupts are
     reported on the instruction that caused the exception, so floating point
     signal handlers can clean up the FP state and continue execution. In
     performance mode, floating point interrupts are asynchronous and the
     reported program counter is meaningless. Precise mode is provided as a
     means of debugging processes and/or backward compatibility for programs
     that have knowledge of the FP state. It causes significant performance
     degradation.

     When a program is run in performance mode on an R8000, the floating point
     processor automatically flushes all denormalized results to zero
     regardless of the setting of the FS (flush denormalized results to zero)
     bit in the FP control and status register. This bit is turned on at
     exec() time.  Turning it off (by calling set_fpc_csr()) will cause
     performance degradation. When the same program is run in precise mode,
     the FS bit is turned off at exec() time (for backward compatibility with
     older processors). Note that this means a given program may generate
     different results when run in performance and precise modes.

     Non-sequential memory access mode allows the processor to do floating
     point and integer memory operations out of order, thus possibly providing
     better performance.  This is only supported on R8000 processors.

     Turning on speculative execution (spec) mode tells the kernel to ignore
     all memory access faults generated by the program. This mode is used by
     the compiler to provide better performance in some cases by allowing
     eager instruction scheduling. Note that this may also cause significantly
     worse performance if used indiscriminately and should be avoided when
     debugging since it masks potential problems.


     The floating point mode is inherited across fork() and exec() system
     calls. To modify both exception and memory modes, use fpmode twice; e.g.,

          fpmode precise fpmode smm command


SEE ALSO
     syssgi(2), get_fpc_csr(3), set_fpc_csr(3).

WARNINGS
     In the case of the following command

          fpmode precise command1; command2

     fpmode applies only to command1.  The command

          fpmode precise (command1; command2)

     is syntactically incorrect.


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