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57 lines
1.7 KiB
ArmAsm
57 lines
1.7 KiB
ArmAsm
; Fast mutiply
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; Note for our purposes we only care about 8.8 x 8.8 fixed point
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; with 8.8 result, which means we only care about the middle two bytes
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; of the 32 bit result. So we disable generation of the high and low byte
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; to save some cycles.
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;
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; The old routine took around 700 cycles for a 16bitx16bit=32bit mutiply
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; This routine, at an expense of 2kB of looku tables, takes around 250
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; If you reuse a term the next time this drops closer to 200
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; This routine was described by Stephen Judd and found
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; in The Fridge and in the C=Hacking magazine
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; http://codebase64.org/doku.php?id=base:seriously_fast_multiplication
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; The key thing to note is that
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; (a+b)^2 (a-b)^2
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; a*b = ------- - --------
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; 4 4
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; So if you have tables of the squares of 0..511 you can lookup and subtract
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; instead of multiplying.
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; Table generation: I:0..511
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; square1_lo = <((I*I)/4)
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; square1_hi = >((I*I)/4)
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; square2_lo = <(((I-255)*(I-255))/4)
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; square2_hi = >(((I-255)*(I-255))/4)
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; Fast 8x8 bit unsigned multiplication, 16-bit result
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; Input: M1xM2
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; Result: M2:M1
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;
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multiply_u8x8:
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lda M1
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sta sm1a+1 ; 3
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sta sm3a+1 ; 3
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eor #$ff ; invert the bits for subtracting ; 2
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sta sm2a+1 ; 3
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sta sm4a+1 ; 3
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ldx M2
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sec
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sm1a:
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lda square1_lo,X
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sm2a:
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sbc square2_lo,X
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sta M1
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sm3a:
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lda square1_hi,X
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sm4a:
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sbc square2_hi,X
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sta M2
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rts
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