; Calculates the 16 bit unsigned integer square of the signed 16 bit integer in
; Numberl/Numberh. The result is always in the range 0 to 65025 and is held in
; Squarel/Squareh
;
; The maximum input range is only +/-255 and no checking is done to ensure that
; this is so.
;
; This routine is useful if you are trying to draw circles as for any circle
;
; x^2+y^2=r^2 where x and y are the co-ordinates of any point on the circle and
; r is the circle radius
;
; Destroys all registers
.ORG 8000 ; these must be in RAM
Numberl ; number to square low byte
Numberh = Numberl+ ; number to square high byte
.word $FFFF
Squarel ; square low byte
Squareh = Squarel+1 ; square high byte
.word $FFFF
Tempsq ; temp byte for intermediate result
.byte $00
.ORG 8192 ; any address will do
Square
LDA #$00 ; clear A
STA Squarel ; clear square low byte
; (the high byte gets shifted out)
LDA Numberl ; get number low byte
LDX Numberh ; get number high byte
BPL NoNneg ; if +ve don't negate it
; else do a two's complement
EOR #$FF ; invert
SEC ; +1
ADC #$00 ; and add it
NoNneg:
STA Tempsq ; save ABS(number)
LDX #$08 ; set bit count
Nextr2bit:
ASL Squarel ; low byte *2
ROL Squareh ; high byte *2+carry from low
ASL A ; shift number byte
BCC NoSqadd ; don't do add if C = 0
TAY ; save A
CLC ; clear carry for add
LDA Tempsq ; get number
ADC Squarel ; add number^2 low byte
STA Squarel ; save number^2 low byte
LDA #$00 ; clear A
ADC Squareh ; add number^2 high byte
STA Squareh ; save number^2 high byte
TYA ; get A back
NoSqadd:
DEX ; decrement bit count
BNE Nextr2bit ; go do next bit
RTS
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