This chapter describes the standard instruction-set extension for single-precision floating-point, which is named "F" and adds single-precision floating-point computational instructions compliant with the IEEE 754-2008 arithmetic standard cite:[ieee754-2008]. The F extension depends on the "Zicsr" extension for control and status register access.

The floating-point control and status register, fcsr, is a RISC-V control and status register (CSR). It is a 32-bit read/write register that selects the dynamic rounding mode for floating-point arithmetic operations and holds the accrued exception flags, as shown in Floating-Point Control and Status Register.

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The fcsr register can be read and written with the FRCSR and FSCSR instructions, which are assembler pseudoinstructions built on the underlying CSR access instructions. FRCSR reads fcsr by copying it into integer register rd. FSCSR swaps the value in fcsr by copying the original value into integer register rd, and then writing a new value obtained from integer register rs1 into fcsr.

The fields within the fcsr can also be accessed individually through different CSR addresses, and separate assembler pseudoinstructions are defined for these accesses. The FRRM instruction reads the Rounding Mode field frm (fcsr bits 7—​5) and copies it into the least-significant three bits of integer register rd, with zero in all other bits. FSRM swaps the value in frm by copying the original value into integer register rd, and then writing a new value obtained from the three least-significant bits of integer register rs1 into frm. FRFLAGS and FSFLAGS are defined analogously for the Accrued Exception Flags field fflags (fcsr bits 4—​0).

Bits 31—​8 of the fcsr are reserved for other standard extensions. If these extensions are not present, implementations shall ignore writes to these bits and supply a zero value when read. Standard software should preserve the contents of these bits.

Floating-point operations use either a static rounding mode encoded in the instruction, or a dynamic rounding mode held in frm. Rounding modes are encoded as shown in Rounding mode encoding.. A value of 111 in the instruction’s rm field selects the dynamic rounding mode held in frm. The behavior of floating-point instructions that depend on rounding mode when executed with a reserved rounding mode is reserved, including both static reserved rounding modes (101-110) and dynamic reserved rounding modes (101-111). Some instructions, including widening conversions, have the rm field but are nevertheless mathematically unaffected by the rounding mode; software should set their rm field to RNE (000) but implementations must treat the rm field as usual (in particular, with regard to decoding legal vs. reserved encodings).

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, as shown in Accrued exception flag encoding.. The base RISC-V ISA does not support generating a trap on the setting of a floating-point exception flag.

The following instructions are added by this extension in the generic_rv64 configuration:

This extension has the following implementation options: