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

F.2 Vector length and stride control
VFP Vector Operation Support
The FPSCR.LEN field, bits [18:16], controls the vector length for VFP instructions that operate on short
vectors, that is, how many registers are in a vector operand. Similarly, the FPSCR.STRIDE field,
bits [21:20], controls the vector stride, that is, how far apart the registers in a vector lie in the register bank.
For information about the FPSCR see Floating-point Status and Control Register (FPSCR) on page A2-28.
The permitted combinations of LEN and STRIDE are shown in Table F-1. All other combinations of LEN
and STRIDE produce UNPREDICTABLE results.
The combination LEN == 0b000, STRIDE == 0b00 is called scalar mode. When it is in effect, all arithmetic
instructions specify scalar operations. Otherwise, most arithmetic instructions specify a scalar operation if
their destination is in the range:
S0-S7 for a single-precision operation
D0-D3 or D16-D19 for a double-precision operation.
The full rules used to determine which operands are vectors and full details of how vector operands are
specified can be found in VFP instruction type selection on page AppxF-7.
The rules for vector operands do not permit the same register to appear twice or more in a vector. The
permitted LEN and STRIDE combinations listed in Table F-1 never cause this to happen for
single-precision instructions, so single-precision scalar and vector instructions can be used with all of these
LEN and STRIDE combinations.
For double-precision vector instructions, some of the permitted LEN and STRIDE combinations would
cause the same register to appear twice in a vector. If a double-precision vector instruction is executed with
such a LEN and STRIDE combination in effect, the instruction is UNPREDICTABLE. The last column of Table
2-2 indicates which LEN and STRIDE combinations this applies to. Double-precision scalar instructions
work normally with all of the permitted LEN and STRIDE combinations.
Table F-1 Vector length and stride combinations
LEN STRIDE Vector length Vector stride Double-precision vector instructions
0b000 0b00 1 - All instructions are scalar
0b001 0b00 2 1 Work as described in this appendix
0b001 0b11 2 2 Work as described in this appendix
0b010 0b00 3 1 Work as described in this appendix
0b010 0b11 3 2 UNPREDICTABLE
0b011 0b00 4 1 Work as described in this appendix
0b011 0b11 4 2 UNPREDICTABLE
0b100 0b00 5 1 UNPREDICTABLE
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. AppxF-3