ARM Community: Software Enablement - ARM Community

ARM is a 32-bit CPU architecture where every instruction is 32 bits long. Any constants which are part of an instruction must be encoded within the 32 bits of the given instruction and this naturally limits the range of constants that can be represented in one instruction. This post will show you how we can deal with these limitations and how the latest revision of the ARM architecture (ARMv7) provides a simple and efficient solution.

Most arithmetic and logical ARM instructions accept 3 parameters:
The destination: always a register. Operand 1: always a register. Operand 2: a register, an immediate constant value or a shifted register. We'll cover shifted registers in a future post. For now, we're only interested in the constants. Examples of such instructions are:

An Operand 2 immediate must obey the following rule to fit in the instruction: an 8-bit value rotated right by an even number of bits between 0 and 30 (inclusive). This allows for constants such as 0xFF (0xFF rotated right by 0), 0xFF00 (0xFF rotated right by 24) or 0xF000000F (0xFF rotated right by 4).

Operand 2 immediates are also valid immed...

We have seen how to load and store data with NEON, and how to handle the leftovers resulting from vector processing. Let us move on to doing some useful data processing – multiplying matrices.

Matrices

In this post, we will look at how to efficiently multiply four-by-four matrices together, an operation frequently used in the world of 3D graphics. We will assume that the matrices are stored in memory in column-major order – this is the format used by OpenGL-ES.

Algorithm

We start by examining the matrix mutiply operation in detail, by expanding the calculation, and identifying sub-operations that can be implemented using NEON instructions.

Notice that in the diagram, we multiply each column of the first matrix (in red) by a corresponding single value in the second matrix (blue) then add together the results for each element to give a column of results. This operation is repeated for each of the four columns in the result matrix.

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