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91 lines
2.2 KiB
ArmAsm
91 lines
2.2 KiB
ArmAsm
; Fast mutiply -- setup tables
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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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.ifndef square1_lo
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square1_lo = $2000
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square1_hi = $2200
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square2_lo = $2400
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square2_hi = $2600
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.endif
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; for(i=0;i<512;i++) {
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; square1_lo[i]=((i*i)/4)&0xff;
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; square1_hi[i]=(((i*i)/4)>>8)&0xff;
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; square2_lo[i]=( ((i-255)*(i-255))/4)&0xff;
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; square2_hi[i]=(( ((i-255)*(i-255))/4)>>8)&0xff;
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; }
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init_multiply_tables:
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; Build the add tables
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ldx #$00
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txa
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.byte $c9 ; CMP #immediate - skip TYA and clear carry flag
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lb1: tya
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adc #$00 ; 0
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ml1: sta square1_hi,x ; square1_hi[0]=0
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tay ; y=0
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cmp #$40 ; subtract 64 and update flags (c=0)
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txa ; a=0
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ror ; rotate
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ml9: adc #$00 ; add 0
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sta ml9+1 ; update add value
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inx ; x=1
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ml0: sta square1_lo,x ; square1_lo[0]=1
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bne lb1 ; if not zero, loop
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inc ml0+2 ; increment values
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inc ml1+2 ; increment values
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clc ; c=0
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iny ; y=1
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bne lb1 ; loop
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; Build the subtract tables based on the existing one
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ldx #$00
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ldy #$ff
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second_table:
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lda square1_hi+1,x
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sta square2_hi+$100,x
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lda square1_hi,x
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sta square2_hi,y
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lda square1_lo+1,x
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sta square2_lo+$100,x
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lda square1_lo,x
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sta square2_lo,y
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dey
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inx
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bne second_table
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rts
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