4.10.11 Convert VAX Floating to VAX Floating

Format:

CVTxy

Fb.rx,Fc.wx

!Floating-point Operate format

Operation:

Fc {conversion of Fbv}

Exceptions:

Invalid Operation

Overflow

Underflow

Instruction mnemonics:

CVTDG

Convert D_floating to G_floating

CVTGD

Convert G_floating to D_floating

CVTGF

Convert G_floating to F_floating

Qualifiers:

Rounding:

Chopped (/C)

Trapping:

Exception Completion (/S)

 

Underflow Enable (/U)

Description:

The floating operand in register Fb is converted to the specified alternate floating format and written to register Fc. Register Fa must be F31.

An invalid operation trap is signaled if the operand has exp=0 and is not a true zero (that is, VAX reserved operands and dirty zeros trap). The contents of Fc are UNPREDICTABLE if this occurs.

See Section 4.7.7 for details of the stored result on overflow or underflow.

Notes:

The only arithmetic operations on D_floating values are conversions to and from G_floating. The conversion to G_floating rounds or chops as specified, removing three fraction bits. The conversion from G_floating to D_floating adds three low-order zeros as fraction bits, then the 8-bit exponent range is checked for overflow/underflow.

The conversion from G_floating to F_floating rounds or chops to single precision, then the 8-bit exponent range is checked for overflow/underflow.

No conversion from F_floating to G_floating is required, since F_floating values are always stored in registers as equivalent G_floating values.

4–116Alpha Architecture Handbook

Page 172
Image 172
Compaq ECQD2KCTE manual Convert VAX Floating to VAX Floating, Cvtdg