9.21 Control of scalar, vector, and mixed operations

Whether a VFP arithmetic instruction operates on scalars, vectors, or a mixture of both depends on the LEN bits in the FPSCR and also on which register bank the destination and operand registers are in.

Use the LEN bits in the FPSCR to control the length of vectors. When LEN is 1 all VFP operations are scalar.

When LEN is greater than 1, the VFP operation can be scalar, vector or mixed. The behavior of VFP arithmetic operations depends on which register bank the destination and operand registers are in.

The first bank of registers, s0 to s7 or d0 to d3 and the fifth bank of registers d16 to d19 are scalar banks. All other banks are vector banks. A vector operation or mixed operation is one where the destination register is in one of the vector banks.

Given instructions of the following general forms:

    Op  Fd,Fn,Fm
    Op  Fd,Fm

where:

Op

is the VFP instruction.

Fd

is the destination register.

Fn

is an operand register.

Fm

is the only or second operand register.

the behavior of the operation is as follows:

  • If Fd is in the first or fifth bank of registers then the operation is scalar.

  • If Fm is in the first or fifth bank of registers, but Fd is not, then the operation is mixed.

  • If neither Fd nor Fm are in the first or fifth bank of registers, the operation is vector.

In scalar operations, Op acts on the value in Fm, and the value in Fn if present. The result is placed in Fd.

In vector operations, Op acts on the values in the vector starting at Fm, together with the values in the vector starting at Fn if present. The results are placed in the vector starting at Fd.

In mixed operations, with a single operand, Op acts on the single value in Fm and LEN copies of the result are placed in the vector starting at Fd.

In mixed operations, with two operands, Op acts on the single value in Fm, together with the values in the vector starting at Fn. The results are placed in the vector starting at Fd.

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