ww+++- M7A - Mode 7 Matrix A (also used with $2134/6)
ww+++- M7B - Mode 7 Matrix B (also used with $2134/6)
ww+++- M7C - Mode 7 Matrix C
ww+++- M7D - Mode 7 Matrix D
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Note that these are "write twice" registers, first the low byte is
written then the high. Current theory is that writes to the register
work like this:
Reg = (Current<<8) | Prev;
Prev = Current;
Note that there is only one Prev shared by all these registers. This
Prev is NOT shared with the BGnxOFS registers, but it IS shared with
the M7xOFS registers.
These set the matrix parameters for Mode 7. The values are an 8-bit
fixed point, i.e. the value should be divided by 256.0 when used in
calculations. See below for more explanation.
The product A*(B>>8) may be read from registers $2134/6. There is
supposedly no important delay. It may not be operative during Mode 7
rendering.
See the section "BACKGROUNDS" below for details.