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

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

Defining Extension

F, version >= F@2.2.0

CSR Address

0x1

Length

32-bit

Privilege Mode

Format

Figure 1. fflags format

Field Summary

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

Name

Location

Type

Reset Value

fflags.NV

RW-H

UNDEFINED_LEGAL

fflags.DZ

RW-H

UNDEFINED_LEGAL

fflags.OF

RW-H

UNDEFINED_LEGAL

fflags.UF

RW-H

UNDEFINED_LEGAL

fflags.NX

RW-H

UNDEFINED_LEGAL

Fields

`NV`

Location: 4
Description: Set by hardware when a floating point operation is invalid and stays set until explicitly cleared by software.
Type: RW-H
Reset value: UNDEFINED_LEGAL

`DZ`

Location: 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
Reset value: UNDEFINED_LEGAL

`OF`

Location: 2
Description: Set by hardware when a floating point operation overflows and stays set until explicitly cleared by software.
Type: RW-H
Reset value: UNDEFINED_LEGAL

`UF`

Location: 1
Description: Set by hardware when a floating point operation underflows and stays set until explicitly cleared by software.
Type: RW-H
Reset value: UNDEFINED_LEGAL

`NX`

Location: 0
Description: Set by hardware when a floating point operation is inexact and stays set until explicitly cleared by software.
Type: RW-H
Reset value: UNDEFINED_LEGAL

Software write

This CSR may store a value that is different from what software attempts to write.

When a software write occurs (e.g., through csrrw), the following determines the written value:

NV = CSR[fcsr].NV = csr_value.NV;
return csr_value.NV;

DZ = CSR[fcsr].DZ = csr_value.DZ;
return csr_value.DZ;

OF = CSR[fcsr].OF = csr_value.OF;
return csr_value.OF;

UF = CSR[fcsr].UF = csr_value.UF;
return csr_value.UF;

NX = CSR[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.

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