fflags

Floating-Point Accrued Exceptions

The accrued exception flags indicate the exception conditions that have arisen on any floating-point arithmetic instruction since the field was last reset by software.

The base RISC-V ISA does not support generating a trap on the setting of a floating-point exception flag.

As allowed by the standard, we do not support traps on floating-point exceptions in the F extension, but instead require explicit checks of the flags in software. We considered adding branches controlled directly by the contents of the floating-point accrued exception flags, but ultimately chose to omit these instructions to keep the ISA simple.

Attributes

CSR Address	0x1
Defining extension	F, version >= F@2.2.0
Length	32-bit
Privilege Mode	U

CSR Address

0x1

Defining extension

F, version >= F@2.2.0

Length

32-bit

Privilege Mode

Format

Figure 1. fflags format

Field Summary

Name Location Type Reset Value

Name	Location	Type	Reset Value
`NV`	4	RW-H	UNDEFINED_LEGAL
`DZ`	3	RW-H	UNDEFINED_LEGAL
`OF`	2	RW-H	UNDEFINED_LEGAL
`UF`	1	RW-H	UNDEFINED_LEGAL
`NX`	0	RW-H	UNDEFINED_LEGAL

NV

RW-H

UNDEFINED_LEGAL

DZ

RW-H

UNDEFINED_LEGAL

OF

RW-H

UNDEFINED_LEGAL

UF

RW-H

UNDEFINED_LEGAL

NX

RW-H

UNDEFINED_LEGAL

Fields

`NV`

Location: fflags[4]
Description: Set by hardware when a floating point operation is invalid and stays set until explicitly cleared by software.
Type

RW-H	Read-Write with Hardware update Field is writable by software. Any value that fits in the field is acceptable. Hardware also updates the field without an explicit software write.

Reset value

UNDEFINED_LEGAL

Software write

This field has special behavior when written by software (e.g., through csrrw).

When software tries to write csr_value, the field will be written with the return value of the function below.

fcsr.NV = csr_value.NV;
return csr_value.NV;

`DZ`

Location: fflags[3]
Description: Set by hardware when a floating point divide attempts to divide by zero and stays set until explicitly cleared by software.
Type

RW-H	Read-Write with Hardware update Field is writable by software. Any value that fits in the field is acceptable. Hardware also updates the field without an explicit software write.

Reset value

UNDEFINED_LEGAL

Software write

This field has special behavior when written by software (e.g., through csrrw).

When software tries to write csr_value, the field will be written with the return value of the function below.

fcsr.DZ = csr_value.DZ;
return csr_value.DZ;

`OF`

Location: fflags[2]
Description: Set by hardware when a floating point operation overflows and stays set until explicitly cleared by software.
Type

RW-H	Read-Write with Hardware update Field is writable by software. Any value that fits in the field is acceptable. Hardware also updates the field without an explicit software write.

Reset value

UNDEFINED_LEGAL

Software write

This field has special behavior when written by software (e.g., through csrrw).

When software tries to write csr_value, the field will be written with the return value of the function below.

fcsr.OF = csr_value.OF;
return csr_value.OF;

`UF`

Location: fflags[1]
Description: Set by hardware when a floating point operation underflows and stays set until explicitly cleared by software.
Type

RW-H	Read-Write with Hardware update Field is writable by software. Any value that fits in the field is acceptable. Hardware also updates the field without an explicit software write.

Reset value

UNDEFINED_LEGAL

Software write

This field has special behavior when written by software (e.g., through csrrw).

When software tries to write csr_value, the field will be written with the return value of the function below.

fcsr.UF = csr_value.UF;
return csr_value.UF;

`NX`

Location: fflags[0]
Description: Set by hardware when a floating point operation is inexact and stays set until explicitly cleared by software.
Type

RW-H	Read-Write with Hardware update Field is writable by software. Any value that fits in the field is acceptable. Hardware also updates the field without an explicit software write.

Reset value

UNDEFINED_LEGAL

Software write

This field has special behavior when written by software (e.g., through csrrw).

When software tries to write csr_value, the field will be written with the return value of the function below.

fcsr.NX = csr_value.NX;
return csr_value.NX;

Software read

This CSR may return a value that is different from what is stored in hardware.

Pruned
Original

return (fcsr.NV `<< 4) | (fcsr.DZ `<< 3) | (fcsr.OF `<< 2) | (fcsr.UF `<< 1) | fcsr.NX;

return (fcsr.NV `<< 4) | (fcsr.DZ `<< 3) | (fcsr.OF `<< 2) | (fcsr.UF `<< 1) | fcsr.NX;