# Fast signed multiply

From Nesdev wiki

The following code multiplies two 8-bit signed integers, of range (-64..+64). The output is a signed 16-bit result, in range (-4096..4096). It uses a 256 byte lookup table, but is quite fast.

;Fast table driven signed multiply routine ;It multiplies Y with A, and relies on the mathematical fact (a+b)^2-(a-b)^2 = 4*a*b ;Also both inputs and outputs are SIGNED and are never supposed to be outside of the range (-64; +64) ;Return with YA = 4*A*Y (result can be shifted as necessary) Multiply sty Temp pha clc adc Temp ;Add two multiplicands together bpl + eor #$ff clc adc #$01 ;If result is negative, force it to be positive + asl A tax ;The square of a negative number is equal to its pos counterpart anyways pla sec sbc Temp ;Compute difference of two multiplicands bpl + eor #$ff ;Again, force the result to be positive clc adc #$01 + asl A tay lda SquareTbl,X sec sbc SquareTbl,Y ;Compute (a+y)^2-(a-y)^2 pha lda SquareTbl+1,X sbc SquareTbl+1,Y tay pla rts ;This is a square table for the multiply routine ;Note that this macro is for WLA-DX, but can be adapted ;For most assemblers SquareTbl .def i = 0 .rept 128 .dw i^2 ;Create a square table with 128 word entries .redef i i+1 .endr .ends