Problems with floating-point support on ARM
sebastian.huber at embedded-brains.de
Thu Sep 10 09:06:01 UTC 2009
RTEMS currently supports only soft-float ARM so this will be assumed. The
default floating-point model for GCC on ARM is FPA.
That means (excerpt from the corresponding ARM document):
Little-endian double-precision values are neither pure little-endian nor pure
This differs from that more recent VFP (Vectored Floatin-Point architecture).
In IEEE 754 we have the following double format:
least significant word (lsw): fraction [31:0]
moast significant word (msw): sign | exponent | fraction [51:32]
This will be stored in different ways:
VFP LE VFP BE FPA
lower address lsw msw msw
higher address msw lsw lsw
The important thing here is that on little-endian the msw and lsw are switched.
Newlib does not support the FPA floating-point model on little-endian. In this
case it produces wrong results and writes to arbitrary memory locations.
A possible fix is to add the following options to the GCC configuration command
The VFP floating-point hardware model is not implemented in GCC, thus the
hard-float multilibs have to be disabled in gcc/config/arm/t-rtems.
Have a nice day!
Sebastian Huber, embedded brains GmbH
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E-Mail : sebastian.huber at embedded-brains.de
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