ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Application Level Programmers’ Model
A2.7.3 VFP double-precision format
The double-precision floating-point format used by the VFP extension is as defined by the IEEE 754
standard.
This description includes VFP-specific details that are left open by the standard. It is only intended as an
introduction to the formats and to the values they can contain. For full details, especially of the handling of
infinities, NaNs and signed zeros, see the IEEE 754 standard.
A double-precision value consists of two 32-bit words, with the formats:
Most significant word:
31 30 20 19 0
S exponent fraction[51:32]
Least significant word:
31 0
fraction[31:0]
When held in memory, the two words must appear consecutively and must both be word-aligned. The order
of the two words depends on the endianness of the memory system:
In a little-endian memory system, the least significant word appears at the lower memory address and
the most significant word at the higher memory address.
In a big-endian memory system, the most significant word appears at the lower memory address and
the least significant word at the higher memory address.
Double-precision values represent numbers, infinities and NaNs in a similar way to single-precision values,
with the interpretation of the format depending on the value of the exponent:
0 < exponent < 0x7FF
The value is a normalized number and is equal to:
–1S × 2exponent–1023 × (1.fraction)
The minimum positive normalized number is 2 –1022, or approximately 2.225 × 10 –308.
The maximum positive normalized number is (2 – 2 –52 ) × 21023 , or approximately
1.798 × 10308 .
exponent == 0
The value is either a zero or a denormalized number, depending on the fraction bits:
fraction == 0
The value is a zero. There are two distinct zeros that behave analogously to the
two single-precision zeros:
+0 when S==0
–0 when S==1.
A2-36 Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. ARM DDI 0406B