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prog8/compiler/res/prog8lib/math.asm
Irmen de Jong 0ab98033b5 add rom-compatible random number routines
2025-04-14 22:04:11 +02:00

1028 lines
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NASM
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; Internal Math library routines - always included by the compiler
; Generic machine independent 6502 code.
;
; some more interesting routines can be found here:
; http://6502org.wikidot.com/software-math
; http://codebase64.org/doku.php?id=base:6502_6510_maths
; https://github.com/TobyLobster/multiply_test
; https://github.com/TobyLobster/sqrt_test
multiply_bytes .proc
; -- multiply 2 bytes A and Y, result as byte in A (signed or unsigned)
; https://github.com/TobyLobster/multiply_test/blob/main/tests/mult29.a
_multiplicand = P8ZP_SCRATCH_B1
_multiplier = P8ZP_SCRATCH_REG
sty _multiplicand
lsr a
sta _multiplier
lda #0
ldx #2
-
bcc +
clc
adc _multiplicand
+
ror a
ror _multiplier
bcc +
clc
adc _multiplicand
+
ror a
ror _multiplier
bcc +
clc
adc _multiplicand
+
ror a
ror _multiplier
bcc +
clc
adc _multiplicand
+
ror a
ror _multiplier
dex
bne -
; tay ; if you want 16 bits result in AY, enable this again
lda _multiplier
rts
.pend
multiply_words .proc
; -- multiply two 16-bit words into a 32-bit result (UNSIGNED)
; input: A/Y = first 16-bit number, multiply_words.multiplier = second 16-bit number
; output: multiply_words.result, 4-bytes/32-bits product, LSB order (low-to-high) low 16 bits also in AY.
; you can retrieve the upper 16 bits via math.mul16_last_upper()
; NOTE FOR NEGATIVE VALUES:
; The routine also works for NEGATIVE (signed) word values, but ONLY the lower 16 bits of the result are correct then!
; Prog8 only uses those so that's not an issue, but math.mul16_last_upper() no longer gives the correct result here.
; mult62.a
; from: https://github.com/TobyLobster/multiply_test/blob/main/tests/mult62.a
; based on Dr Jefyll, http://forum.6502.org/viewtopic.php?f=9&t=689&start=0#p19958
; - adjusted to use fixed zero page addresses
; - removed 'decrement to avoid clc' as this is slower on average
; - rearranged memory use to remove final memory copy and give LSB first order to result
; - removed temp zp storage bytes
; - unrolled the outer loop
; - unrolled the two inner loops once
;
; 16 bit x 16 bit unsigned multiply, 32 bit result
; Average cycles: ~442 ?
; 93 bytes
_multiplicand = P8ZP_SCRATCH_W2 ; 2 bytes
multiplier = result
; 16 bit x 16 bit unsigned multiply, 32 bit result
;
; On Entry:
; (multiplier, multiplier+1): two byte multiplier, four bytes needed for result
; (multiplicand, multiplicand+1): two byte multiplicand
; On Exit:
; (result, result+1, result+2, result+3): product
sta _multiplicand
sty _multiplicand+1
lda #0 ;
sta result+2 ; 16 bits of zero in A, result+2
; Note: First 8 shifts are A -> result+2 -> result
; Final 8 shifts are A -> result+2 -> result+1
; --- 1st byte ---
ldy #4 ; count for inner loop
lsr result
; inner loop (8 times)
_inner_loop
; first time
bcc +
tax ; retain A
lda result+2
clc
adc _multiplicand
sta result+2
txa ; recall A
adc _multiplicand+1
+
ror a ; shift
ror result+2
ror result
; second time
bcc +
tax ; retain A
lda result+2
clc
adc _multiplicand
sta result+2
txa ; recall A
adc _multiplicand+1
+
ror a ; shift
ror result+2
ror result
dey
bne _inner_loop ; go back for 1 more shift?
; --- 2nd byte ---
ldy #4 ; count for inner loop
lsr result+1
; inner loop (8 times)
_inner_loop2
; first time
bcc +
tax ; retain A
lda result+2
clc
adc _multiplicand
sta result+2
txa ; recall A
adc _multiplicand+1
+
ror a ; shift
ror result+2
ror result+1
; second time
bcc +
tax ; retain A
lda result+2
clc
adc _multiplicand
sta result+2
txa ; recall A
adc _multiplicand+1
+
ror a ; shift
ror result+2
ror result+1
dey
bne _inner_loop2 ; go back for 1 more shift?
sta result+3 ; ms byte of hi-word of result
lda result
ldy result+1
rts
.section BSS
result .byte ?,?,?,? ; routine could be faster if this were in Zeropage...
.send BSS
.pend
divmod_b_asm .proc
; signed byte division: make everything positive and fix sign afterwards
sta P8ZP_SCRATCH_B1
tya
eor P8ZP_SCRATCH_B1
php ; save sign
lda P8ZP_SCRATCH_B1
bpl +
eor #$ff
sec
adc #0 ; make it positive
+ pha
tya
bpl +
eor #$ff
sec
adc #0 ; make it positive
tay
+ pla
jsr divmod_ub_asm
sta _remainder
plp
bpl +
tya
eor #$ff
sec
adc #0 ; negate result
tay
+ rts
.section BSS
_remainder .byte ?
.send BSS
.pend
divmod_ub_asm .proc
; -- divide A by Y, result quotient in Y, remainder in A (unsigned)
; division by zero will result in quotient = 255 and remainder = original number
sty P8ZP_SCRATCH_REG
sta P8ZP_SCRATCH_B1
lda #0
ldx #8
asl P8ZP_SCRATCH_B1
- rol a
cmp P8ZP_SCRATCH_REG
bcc +
sbc P8ZP_SCRATCH_REG
+ rol P8ZP_SCRATCH_B1
dex
bne -
ldy P8ZP_SCRATCH_B1
rts
.pend
divmod_w_asm .proc
; signed word division: make everything positive and fix sign afterwards
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
lda P8ZP_SCRATCH_W1+1
eor P8ZP_SCRATCH_W2+1
php ; save sign
lda P8ZP_SCRATCH_W1+1
bpl +
lda #0
sec
sbc P8ZP_SCRATCH_W1
sta P8ZP_SCRATCH_W1
lda #0
sbc P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W1+1
+ lda P8ZP_SCRATCH_W2+1
bpl +
lda #0
sec
sbc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W2
lda #0
sbc P8ZP_SCRATCH_W2+1
sta P8ZP_SCRATCH_W2+1
+ tay
lda P8ZP_SCRATCH_W2
jsr divmod_uw_asm
plp ; restore sign
bpl +
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
lda #0
sec
sbc P8ZP_SCRATCH_W2
pha
lda #0
sbc P8ZP_SCRATCH_W2+1
tay
pla
+ rts
.pend
divmod_uw_asm .proc
; -- divide two unsigned words (16 bit each) into 16 bit results
; input: P8ZP_SCRATCH_W1 in ZP: 16 bit number, A/Y: 16 bit divisor
; output: P8ZP_SCRATCH_W2 in ZP: 16 bit remainder, A/Y: 16 bit division result
; division by zero will result in quotient = 65535 and remainder = divident
dividend = P8ZP_SCRATCH_W1
remainder = P8ZP_SCRATCH_W2
result = dividend ;save memory by reusing divident to store the result
sta _divisor
sty _divisor+1
lda #0 ;preset remainder to 0
sta remainder
sta remainder+1
ldx #16 ;repeat for each bit: ...
- asl dividend ;dividend lb & hb*2, msb -> Carry
rol dividend+1
rol remainder ;remainder lb & hb * 2 + msb from carry
rol remainder+1
lda remainder
sec
sbc _divisor ;substract divisor to see if it fits in
tay ;lb result -> Y, for we may need it later
lda remainder+1
sbc _divisor+1
bcc + ;if carry=0 then divisor didn't fit in yet
sta remainder+1 ;else save substraction result as new remainder,
sty remainder
inc result ;and INCrement result cause divisor fit in 1 times
+ dex
bne -
lda result
ldy result+1
rts
.section BSS
_divisor .word ?
.send BSS
.pend
randword .proc
; -- 16 bit pseudo random number generator into AY
; default seed = $00c2 $1137. NOTE: uses self-modifying code so won't work in ROM (use randword_rom instead)
; routine from https://codebase64.org/doku.php?id=base:x_abc_random_number_generator_8_16_bit
inc x1
clc
x1=*+1
lda #$00 ;x1
c1=*+1
eor #$c2 ;c1
a1=*+1
eor #$11 ;a1
sta a1
b1=*+1
adc #$37 ;b1
sta b1
lsr a
eor a1
adc c1
sta c1
ldy b1
rts
.pend
randword_rom .proc
; -- 16 bit pseudo random number generator into AY. Can run from ROM.
; NOTE: you have to set the initial seed using randseed_rom! (a good default seed = $00c2 $1137)
; routine from https://codebase64.org/doku.php?id=base:x_abc_random_number_generator_8_16_bit
inc _x1
clc
lda _x1
eor _c1
eor _a1
sta _a1
adc _b1
sta _b1
lsr a
eor _a1
adc _c1
sta _c1
ldy _b1
rts
.section BSS
_x1 .byte ?
_c1 .byte ?
_a1 .byte ?
_b1 .byte ?
.send BSS
.pend
randbyte = randword ; -- 8 bit pseudo random number generator into A (by just reusing randword) NOTE: can not run from ROM
randbyte_rom = randword_rom ; -- 8 bit pseudo random number generator into A (by just reusing randword). Can run from ROM
; ----------- optimized multiplications (in-place A (byte) and ?? (word)) : ---------
mul_byte_3 .proc
; A = A + A*2
sta P8ZP_SCRATCH_REG
asl a
clc
adc P8ZP_SCRATCH_REG
rts
.pend
mul_word_3 .proc
; AY = AY*2 + AY
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
tay
lda P8ZP_SCRATCH_W1
rts
.pend
mul_byte_5 .proc
; A = A*4 + A
sta P8ZP_SCRATCH_REG
asl a
asl a
clc
adc P8ZP_SCRATCH_REG
rts
.pend
mul_word_5 .proc
; AY = AY*4 + AY
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
tay
lda P8ZP_SCRATCH_W1
rts
.pend
mul_byte_6 .proc
; A = (A*2 + A)*2
sta P8ZP_SCRATCH_REG
asl a
clc
adc P8ZP_SCRATCH_REG
asl a
rts
.pend
mul_word_6 .proc
; AY = (AY*2 + AY)*2
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
tay
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
sta P8ZP_SCRATCH_W1+1
tya
asl a
rol P8ZP_SCRATCH_W1+1
ldy P8ZP_SCRATCH_W1+1
rts
.pend
mul_byte_7 .proc
; A = A*8 - A
sta P8ZP_SCRATCH_REG
asl a
asl a
asl a
sec
sbc P8ZP_SCRATCH_REG
rts
.pend
mul_word_7 .proc
; AY = AY*8 - AY
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
sec
sbc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
sbc P8ZP_SCRATCH_W2+1
tay
lda P8ZP_SCRATCH_W1
rts
.pend
mul_byte_9 .proc
; A = A*8 + A
sta P8ZP_SCRATCH_REG
asl a
asl a
asl a
clc
adc P8ZP_SCRATCH_REG
rts
.pend
mul_word_9 .proc
; AY = AY*8 + AY
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
tay
lda P8ZP_SCRATCH_W1
rts
.pend
mul_byte_10 .proc
; A=(A*4 + A)*2
sta P8ZP_SCRATCH_REG
asl a
asl a
clc
adc P8ZP_SCRATCH_REG
asl a
rts
.pend
mul_word_10 .proc
; AY=(AY*4 + AY)*2
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
sta P8ZP_SCRATCH_W1+1
lda P8ZP_SCRATCH_W1
asl a
rol P8ZP_SCRATCH_W1+1
ldy P8ZP_SCRATCH_W1+1
rts
.pend
mul_byte_11 .proc
; A=(A*2 + A)*4 - A
sta P8ZP_SCRATCH_REG
asl a
clc
adc P8ZP_SCRATCH_REG
asl a
asl a
sec
sbc P8ZP_SCRATCH_REG
rts
.pend
; mul_word_11 is skipped (too much code)
mul_byte_12 .proc
; A=(A*2 + A)*4
sta P8ZP_SCRATCH_REG
asl a
clc
adc P8ZP_SCRATCH_REG
asl a
asl a
rts
.pend
mul_word_12 .proc
; AY=(AY*2 + AY)*4
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
sta P8ZP_SCRATCH_W1+1
lda P8ZP_SCRATCH_W1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
ldy P8ZP_SCRATCH_W1+1
rts
.pend
mul_byte_13 .proc
; A=(A*2 + A)*4 + A
sta P8ZP_SCRATCH_REG
asl a
clc
adc P8ZP_SCRATCH_REG
asl a
asl a
clc
adc P8ZP_SCRATCH_REG
rts
.pend
; mul_word_13 is skipped (too much code)
mul_byte_14 .proc
; A=(A*8 - A)*2
sta P8ZP_SCRATCH_REG
asl a
asl a
asl a
sec
sbc P8ZP_SCRATCH_REG
asl a
rts
.pend
; mul_word_14 is skipped (too much code)
mul_byte_15 .proc
; A=A*16 - A
sta P8ZP_SCRATCH_REG
asl a
asl a
asl a
asl a
sec
sbc P8ZP_SCRATCH_REG
rts
.pend
mul_word_15 .proc
; AY = AY * 16 - AY
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
sec
sbc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
sbc P8ZP_SCRATCH_W2+1
tay
lda P8ZP_SCRATCH_W1
rts
.pend
mul_byte_20 .proc
; A=(A*4 + A)*4
sta P8ZP_SCRATCH_REG
asl a
asl a
clc
adc P8ZP_SCRATCH_REG
asl a
asl a
rts
.pend
mul_word_20 .proc
; AY = AY * 10 * 2
jsr mul_word_10
sty P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
ldy P8ZP_SCRATCH_REG
rts
.pend
mul_byte_25 .proc
; A=(A*2 + A)*8 + A
sta P8ZP_SCRATCH_REG
asl a
clc
adc P8ZP_SCRATCH_REG
asl a
asl a
asl a
clc
adc P8ZP_SCRATCH_REG
rts
.pend
mul_word_25 .proc
; AY = (AY*2 + AY) *8 + AY
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
sta P8ZP_SCRATCH_W1+1
lda P8ZP_SCRATCH_W1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
tay
lda P8ZP_SCRATCH_W1
rts
.pend
mul_byte_40 .proc
and #7
tay
lda _forties,y
rts
_forties .byte 0*40, 1*40, 2*40, 3*40, 4*40, 5*40, 6*40, 7*40 & 255
.pend
mul_word_40 .proc
; AY = (AY*4 + AY)*8
sta P8ZP_SCRATCH_W1
sty P8ZP_SCRATCH_W1+1
sta P8ZP_SCRATCH_W2
sty P8ZP_SCRATCH_W2+1
asl a
rol P8ZP_SCRATCH_W1+1
asl a
rol P8ZP_SCRATCH_W1+1
clc
adc P8ZP_SCRATCH_W2
sta P8ZP_SCRATCH_W1
lda P8ZP_SCRATCH_W1+1
adc P8ZP_SCRATCH_W2+1
asl P8ZP_SCRATCH_W1
rol a
asl P8ZP_SCRATCH_W1
rol a
asl P8ZP_SCRATCH_W1
rol a
tay
lda P8ZP_SCRATCH_W1
rts
.pend
mul_byte_50 .proc
and #7
tay
lda _fifties, y
rts
_fifties .byte 0*50, 1*50, 2*50, 3*50, 4*50, 5*50, 6*50 & 255, 7*50 & 255
.pend
mul_word_50 .proc
; AY = AY * 25 * 2
jsr mul_word_25
sty P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
ldy P8ZP_SCRATCH_REG
rts
.pend
mul_byte_80 .proc
and #3
tay
lda _eighties, y
rts
_eighties .byte 0*80, 1*80, 2*80, 3*80
.pend
mul_word_80 .proc
; AY = AY * 40 * 2
jsr mul_word_40
sty P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
ldy P8ZP_SCRATCH_REG
rts
.pend
mul_byte_100 .proc
and #3
tay
lda _hundreds, y
rts
_hundreds .byte 0*100, 1*100, 2*100, 3*100 & 255
.pend
mul_word_100 .proc
; AY = AY * 25 * 4
jsr mul_word_25
sty P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
ldy P8ZP_SCRATCH_REG
rts
.pend
mul_word_320 .proc
; AY = A * 256 + A * 64 (msb in Y doesn't matter)
sta P8ZP_SCRATCH_B1
ldy #0
sty P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
asl a
rol P8ZP_SCRATCH_REG
pha
clc
lda P8ZP_SCRATCH_B1
adc P8ZP_SCRATCH_REG
tay
pla
rts
.pend
mul_word_640 .proc
; AY = (A * 2 * 320) (msb in Y doesn't matter)
asl a
jmp mul_word_320
.pend
; ----------- end optimized multiplications -----------
; support for bit shifting that is too large to be unrolled:
lsr_byte_A .proc
; -- lsr signed byte in A times the value in Y
cpy #0
beq +
cmp #0
bpl lsr_ubyte_A
- sec
ror a
dey
bne -
+ rts
.pend
lsr_ubyte_A .proc
; -- lsr unsigned byte in A times the value in Y
cpy #0
beq +
- lsr a
dey
bne -
+ rts
.pend
asl_byte_A .proc
; -- asl any byte in A times the value in Y
cpy #0
beq +
- asl a
dey
bne -
+ rts
.pend
lsr_word_AY .proc
; -- lsr signed word in AY times the value in X
cpx #0
beq +
cpy #0
bpl lsr_uword_AY
sty P8ZP_SCRATCH_B1
- sec
ror P8ZP_SCRATCH_B1
ror a
dex
bne -
ldy P8ZP_SCRATCH_B1
+ rts
.pend
lsr_uword_AY .proc
; -- lsr unsigned word in AY times the value in X
cpx #0
beq +
sty P8ZP_SCRATCH_B1
- lsr P8ZP_SCRATCH_B1
ror a
dex
bne -
ldy P8ZP_SCRATCH_B1
+ rts
.pend
asl_word_AY .proc
; -- asl any word in AY times the value in X
cpx #0
beq +
sty P8ZP_SCRATCH_B1
- asl a
rol P8ZP_SCRATCH_B1
dex
bne -
ldy P8ZP_SCRATCH_B1
+ rts
.pend
square .proc
; -- calculate square of signed word (actually -255..255) in AY, result in AY
; routine by Lee Davison, source: http://6502.org/source/integers/square.htm
; using this routine is a lot faster as doing a regular multiplication (for words)
;
; 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
numberl = P8ZP_SCRATCH_W1 ; number to square low byte
numberh = P8ZP_SCRATCH_W1+1 ; number to square high byte
squarel = P8ZP_SCRATCH_W2 ; square low byte
squareh = P8ZP_SCRATCH_W2+1 ; square high byte
tempsq = P8ZP_SCRATCH_B1 ; temp byte for intermediate result
sta numberl
sty numberh
lda #$00 ; clear a
sta squarel ; clear square low byte
; (no need to clear the high byte, it gets shifted out)
lda numberl ; get number low byte
ldx numberh ; get number high byte
bpl _nonneg ; if +ve don't negate it
; else do a two's complement
eor #$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
lda squarel
ldy squareh
rts
.pend
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