Technical Reference: Base Operating System and Extensions, Volume 1

fp_trap Subroutine

Purpose

Queries or changes the mode of the user process to allow floating-point exceptions to generate traps.

Library

Standard C Library (libc.a)

Syntax

#include <fptrap.h> int fp_trap( flag) int flag;

Description

The fp_trap subroutine queries and changes the mode of the user process to allow or disallow floating-point exception trapping. Floating-point traps can only be generated when a process is executing in a traps-enabled mode.

The default state is to execute in pipelined mode and not to generate floating-point traps.

Note

The fp_trap routines only change the execution state of the process. To generate floating-point traps, you must also enable traps. Use the fp_enable (fp_any_enable, fp_is_enabled, fp_enable_all, fp_enable, fp_disable_all, or fp_disable Subroutine) and fp_enable_all subroutines to enable traps.

Before calling the fp_trap(FP_TRAP_SYNC) routine, previous floating-point operations can set to True certain exception bits in the Floating-Point Status and Control register (FPSCR). Enabling these Cexceptions and calling the fp_trap(FP_TRAP_SYNC) routine does not cause an immediate trap to occur. That is, the operation of these traps is edge-sensitive, not level-sensitive.

The fp_trap subroutine does not clear the exception history. You can query this history by using any of the following subroutines:

Parameters

flag

Specifies a query of or change in the mode of the user process:

FP_TRAP_OFF: Puts the user process into trapping-off mode and returns the previous mode of the process, either FP_TRAP_SYNC, FP_TRAP_IMP, FP_TRAP_IMP_REC, or FP_TRAP_OFF.
FP_TRAP_QUERY: Returns the current mode of the user process.
FP_TRAP_SYNC: Puts the user process into precise trapping mode and returns the previous mode of the process.
FP_TRAP_IMP: Puts the user process into non-recoverable imprecise trapping mode and returns the previous mode.
FP_TRAP_IMP_REC: Puts the user process into recoverable imprecise trapping mode and returns the previous mode.
FP_TRAP_FASTMODE: Puts the user process into the fastest trapping mode available on the hardware platform.

Note

Some hardware models do not support all modes. If an unsupported mode is requested, the fp_trap subroutine returns FP_TRAP_UNIMPL.

Return Values

If called with the FP_TRAP_OFF, FP_TRAP_IMP, FP_TRAP_IMP_REC, or FP_TRAP_SYNC flag, the fp_trap subroutine returns a value indicating which flag was in the previous mode of the process if the hardware supports the requested mode. If the hardware does not support the requested mode, the fp_trap subroutine returns FP_TRAP_UNIMPL.

If called with the FP_TRAP_QUERY flag, the fp_trap subroutine returns a value indicating the current mode of the process, either the FP_TRAP_OFF, FP_TRAP_IMP, FP_TRAP_IMP_REC, or FP_TRAP_SYNC flag.

If called with FP_TRAP_FASTMODE, the fp_trap subroutine sets the fastest mode available and returns the mode selected.

Error Codes

If the fp_trap subroutine is called with an invalid parameter, the subroutine returns FP_TRAP_ERROR.

If the requested mode is not supported on the hardware platform, the subroutine returns FP_TRAP_UNIMPL.