fres

Floating Reciprocal Estimate Single

fres FRT, FRB

Estimates the reciprocal of a single-precision floating-point number.

Encoding

Binary Layout

59

0:5

FRT

6:10

0

11:15

0

16:20

FRB

21:25

24

26:30

/

31

 

Format A-form

Opcode 0xEC000030

Extension Floating-Point

Operands

• FRT
  Target
• FRB
  Source
• FT
  Target Floating-Point Register
• FB
  Source Floating-Point Register

Example

fres f1, f3

Registers Altered

FPSCR, CR0

Related

More in Floating-Point

Reference

View in PowerISA v3.1C Specification ↗ OPF Power ISA v3.1C

Description

Computes an estimate of the reciprocal (1/x) of the single-precision floating-point value in FRB and stores the result in FRT. The estimate is accurate to approximately 1 part in 256 for normalized values. If Rc=1, CR1 is set based on the result; FPSCR exception bits are set according to the floating-point exception enable controls.

Operation

FRT ← estimate(1.0 / FRB)
if Rc = 1 then
  CR1 ← (FRT_exception_summary)

Programming Note

The fres instruction provides a fast estimate of the reciprocal for single-precision floating-point numbers. It's useful in performance-critical applications where exact precision is not required, but speed is essential. Be cautious with inputs that have an unbiased exponent greater than +127, as they are treated as Infinity, which might lead to unexpected results if not handled properly.