2 fpexc, the floating-point exception register, 3 fpsid, the floating-point system id register – ARM VERSION 1.2 User Manual

Page 250

Vector Floating-point Programming

6-12

ARM DUI 0068B

6.5.2

FPEXC, the floating-point exception register

You can only access the

FPEXC

in privileged modes. It contains the following bits:

bit[31]

is the

bit. You can read it in all VFP implementations. In some

implementations you might also be able to write to it.

If the value is 0, the only significant state in the VFP system is the
contents of the general purpose registers plus

FPSCR

and

FPEXC

If the value is 1, you need implementation-specific information to save
state (see the technical reference manual for the VFP coprocessor you are
using).

bit[30]

is the

bit. You can read and write it in all VFP implementations.

If the value is 1, the VFP coprocessor is enabled and operates normally.

If the value is 0, the VFP coprocessor is disabled. When the coprocessor
is disabled, you can read or write the

FPSID

FPEXC

registers, but other

VFP instructions are treated as undefined instructions.

bits[29:0]

might be used by particular implementations of VFP. You can use all the
VFP functions described in this chapter without accessing these bits.

You must not alter these bits except in accordance with their use in a
particular implementation (see the technical reference manual for the
VFP coprocessor you are using).

To alter some bits without affecting other bits, use a read-modify-write procedure (see
Modifying individual bits of a VFP system register).

6.5.3

FPSID, the floating-point system ID register

The

FPSID

is a read-only register. You can read it to find out which implementation of

the VFP architecture your program is running on.

6.5.4

Modifying individual bits of a VFP system register

To alter some bits of a VFP system register without affecting other bits, use a
read-modify-write procedure similar to the following example:

FMRX r10,FPSCR ; copy FPSCR into r10
BIC r10,r10,#0x00370000 ; clears STRIDE and LEN
ORR r10,r10,#0x00030000 ; sets STRIDE = 1, LEN = 4
FMXR FPSCR,r10 ; copy r10 back into FPSCR

See FMRX, FMXR, and FMSTAT on page 6-33.