Shape is 32-bits.  When SHAPE is set entirely to zeros, remapping is
disabled: the register's elements are a linear (1D) vector.

| 31..30   | 29..24 | 23..21  | 20..18  | 17..12  | 11..6   | 5..0    |
| -------- | ------ | ------- | ------- | ------- | -------- | ------- |
| applydim | offset | invxyz  | permute | zdimsz  | ydimsz  | xdimsz  |

applydim will set to zero the dimensions less than this. applydim=0
applies all three. applydim=1 applies y and z. applydim=2 applys only
z. applydim=3 is reserved.

invxyz will invert the start index of each of x, y or z. If invxyz[0] is
zero then x-dimensional counting begins from 0 and increments, otherwise
it begins from xdimsz-1 and iterates down to zero. Likewise for y and z.

offset will have the effect equivalent to the sequential element loop
to appear to run for offset (additional) iterations prior to actually
generating output.

xdimsz, ydimsz and zdimsz are offset by 1, such that a value of 0 indicates
that the array dimensionality for that dimension is 1.  A value of xdimsz=2
would indicate that in the first dimension there are 3 elements in the
array.  The format of the array is therefore as follows:

    array[xdim+1][ydim+1][zdim+1]

However whilst illustrative of the dimensionality, that does not take the
"permute" setting into account.  "permute" may be any one of six values
(0-5, with values of 6 and 7 being reserved, and not legal).  The table
below shows how the permutation dimensionality order works:

| permute | order | array format             |
| ------- | ----- | ------------------------ |
| 000     | 0,1,2 | (xdim+1)(ydim+1)(zdim+1) |
| 001     | 0,2,1 | (xdim+1)(zdim+1)(ydim+1) |
| 010     | 1,0,2 | (ydim+1)(xdim+1)(zdim+1) |
| 011     | 1,2,0 | (ydim+1)(zdim+1)(xdim+1) |
| 100     | 2,0,1 | (zdim+1)(xdim+1)(ydim+1) |
| 101     | 2,1,0 | (zdim+1)(ydim+1)(xdim+1) |

In other words, the "permute" option changes the order in which
nested for-loops over the array would be done.