fre

Floating Reciprocal Estimate

fre FRT,FRB

Estimates the reciprocal of a floating-point operand.

Encoding

Binary Layout

63

0:5

FRT

6:10

0

11:15

0

16:20

FRB

21:25

24

26:30

/

31

 

Format A-form

Opcode 0xFC000030

Extension Floating-Point

Operands

• FRT
  Target
• FRB
  Source

Example

fre f1, f3

Registers Altered

FPSCR (FPRF, FX, OX, UX, ZX, VXSNAN), CR1 (if Rc=1)

Related

More in Floating-Point

Reference

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

Description

An estimate of the reciprocal of the floating-point operand in register FRB is placed into register FRT. Unless the reciprocal would be a zero, an infinity, the result of a trap-disabled Overflow exception, or a QNaN, the estimate is correct to a precision of one part in 256 of the reciprocal of (FRB).

Operation

if 'fre' then
    FRT <- estimate(1 / (FRB))
else if 'fre.' then
    FRT <- estimate(1 / (FRB))
    CR1 <- result_class(FRT)

Programming Note

For the Floating-Point Estimate instructions, some implementations might implement a precision higher than the minimum architected precision.