; IL65 integer math library for 6502
; (floating point math is done via the C-64's BASIC ROM routines)
;
;  some more interesting routines can be found here http://6502org.wikidot.com/software-math
;
; Written by Irmen de Jong (irmen@razorvine.net)
; License: GNU GPL 3.0, see LICENSE
;
; indent format: TABS, size=8


output raw

~ math {
		; note: the following two ZP scratch registers must be the same as in c64lib
		memory  SCRATCH_ZP1	= $02		; scratch register #1 in ZP
		memory  SCRATCH_ZP2	= $03		; scratch register #2 in ZP



sub  multiply_bytes  (byte1: X, byte2: Y) -> (A, X?)  {
	; ---- multiply 2 bytes, result as byte in A  (signed or unsigned)
	asm {
		stx  SCRATCH_ZP1
		sty  SCRATCH_ZP2
		ldx  #8
-               asl  a
		asl  SCRATCH_ZP1
		bcc  +
		clc
		adc  SCRATCH_ZP2
+               dex
		bne  -
		rts
	}
}


sub  multiply_bytes_16  (byte1: X, byte2: Y) -> (A?, XY)  {
	; ---- multiply 2 bytes, result as word in X/Y (unsigned)
	asm {
		lda  #0
_m_with_add	stx  SCRATCH_ZP1
		sty  SCRATCH_ZP2
		ldx  #8
		lsr  SCRATCH_ZP1
-               bcc  +
		clc
		adc  SCRATCH_ZP2
+               ror  a
		ror  SCRATCH_ZP1
		dex
		bne  -
		tay
		ldx  SCRATCH_ZP1
		rts
	}
}


sub  multiply_bytes_addA_16  (byte1: X, byte2: Y, add: A) -> (A?, XY)  {
	; ---- multiply 2 bytes and add A, result as word in X/Y (unsigned)
	asm {
		jmp  multiply_bytes_16._m_with_add
	}
}


sub  divide_bytes  (numerator: X, denominator: Y) -> (X, A)  {
	; ---- divide X by Y, result quotient in X, remainder in A   (unsigned)
	;      division by zero will result in quotient = 255 and remainder = original number
	asm {
		stx  SCRATCH_ZP1
		sty  SCRATCH_ZP2
		lda  #0
		ldx  #8
		asl  SCRATCH_ZP1
-		rol  a
		cmp  SCRATCH_ZP2
		bcc  +
		sbc  SCRATCH_ZP2
+		rol  SCRATCH_ZP1
		dex
		bne  -
		ldx  SCRATCH_ZP1
		rts
	}
}

}