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Udate the mode7 notes

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This commit is contained in:
Vince Weaver 2018-05-01 19:12:30 -04:00
parent 5d5519e0dd
commit fe7218b597
1 changed files with 3 additions and 3 deletions
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6
mode7_demo/docs/mode7_demo.tex
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@ -490,8 +490,8 @@ for a 8.8 x 8.8 fixed point multiply.
% Note, this is Quarter-square multiplication, apparently an ancient algorithm
% https://en.wikipedia.org/wiki/Multiplication_algorithm#Quarter_square_multiplication
We improved this by using the fast multiply algorithm
described by Stephen Judd.
We improved this by using the ancient quarter-square
multiply algorithm, first described for 6502 use by Stephen Judd.
This works by noting these factorizations:
\[(a+b)^{2} = a^{2}+2ab+b^{2}\]
@ -502,7 +502,7 @@ If you subtract these you can simplify to
For 8-bit values if you create a table of squares from 0 to 511
(all 8-bit a+b and a-b fall in this range) then you can convert a multiply
into two table lookups and a subtraction.
This does have the downside of requiring 2kB of square lookup tables
This does have the downside of requiring 2kB of lookup tables
(which can be generated at startup) but it reduces the multiply
cost to the order of 250 cycles or so.