; Integer multiplication - <int32> * <int32>
; ==========================================
; Multiply two 32-bit integers without using MUL instruction
;
; On entry, r4=RHS b0-b15, r3=RHS b16-b31
;           r2=LHS b0-b15, r1=LHS b16-b31
;
; On exit,  r4=SUM b0-b15, r3=SUM b16-b31
;
.Mul32int
mov  r4,r0		; r0=b0-b15 of RHS
mov  r3,r5		; r5=b16-b31 of RHS
clr  r4			; Initial total=0
clr  r3
mov  #32,-(sp)		; 32 bits to shift and add
;
.fnMultiplyLp
ror  r1			; Rotate LHS through carry
ror  r2
bcc  fnMultNoAdd	; No carry, no add needed
add  r0,r4		; total=total+RHS
adc  r3
add  r5,r3
bmi  fnMultiplyOverflow	; Too big for 31-bit integer
.fnMultNoAdd
rol  r0			; Rotate LHS
rol  r5
dec  (sp)		; Decrement counter
bne  fnMultiplyLp	; Loop for all bits
tst  (sp)+		; Drop counter
;
; On exit,  r4=SUM b0-b15, r3=SUM b16-b31
;           r5,r2,r1,r0=corrupted
....


.fnMultiplyOverflow
tst  (sp)+		; Drop counter
;
; On exit,  all registers corrupted




; Integer multiplication - <int16> * <int16>
; ==========================================
; Multiply two 16-bit integers using MUL instruction giving 32-bit result
;
; On entry,            r4=RHS b0-b15, r3=RHS b16-b31
;       sp=>retaddr, exp, LHS b0-b15, LHS b16-b31
;
; On exit,             r1=SUM b0-b15, r0=SUM b16-b31
;
; Using schoolbook long multiplication:
;
;           r3    r4
;         6(sp) 4(sp) x
;         -----------
;
;          aaaa bbbb
;          cccc dddd x
;          ---------
;          ( b * d )
;      ( a * d ) 0
;      ( b * c ) 0
;  ( a * c ) 0   0   +
;  -----------------
;
;
.Mul16
mov  r3,r2		; r2=aaaa
bis  6(sp),r2		; aaaa>0 or cccc>0, not 16-bit inputs
bne  fnMultiplyBigInts	; Will be too big for 32-bit result
mov  r4,r0		; r0=bbbb
bmi  fnMultiplyBigInts	; Will overflow into bit 31
			; We now have a maximum of &FFFF*&7FFF = &7FFF8001
mul  4(sp),r0		; r0:r1=bbbb * dddd
			; a=0 and c=0, so a*d=0 and b*c=0
			; So r0:r1 is the result.
bmi  fnMultiplyBigInts	; Has become negative, should be positive
;
; On exit, r1=SUM b0-b15, r0=SUM b16-b31
;          r2 corrupted
;          r3,r4,r5 preserved
;
....


.fnMultiplyBigInts
;
; On exit, r0,r1,r2 corrupted
;          r3,r4,r5 preserved




; Integer multiplication - <int32> * <int32>
; ==========================================
; Multiply two 32-bit integers using MUL instruction giving 32-bit result
;
; On entry,            r4=RHS b0-b15, r3=RHS b16-b31
;       sp=>retaddr, exp, LHS b0-b15, LHS b16-b31
;
; On exit,             r1=SUM b0-b15, r0=SUM b16-b31
;
; Using schoolbook long multiplication:
;
;           r3    r4
;         6(sp) 4(sp) x
;         -----------
;
;          aaaa bbbb
;          cccc dddd x
;          ---------
;          ( b * d )
;      ( a * d ) 0
;      ( b * c ) 0
;  ( a * c ) 0   0   +
;  -----------------
;
;
;
.Mul32
mov  r3,r0
bic  6(sp),r0		; r0=a AND a
bne  fnMultiplyBigInts	; a*c>0, more than 32 bits
mov  r3,r0		; r0=a
mul  4(sp),r0		; r0:r1=a*d
bcs  fnMultiplyBigInts	; Overflow
mov  r1,r2		; r2=a*d
mov  r4,r0		; r0=b
mul  6(sp),r0		; r0:r1=b*c
bcs  fnMultiplyBigInts	; Overflow
add  r1,r2		; r2=(a*d)+(b*c)
bcs  fnMultiplyBigInts	; Overflow
mov  r4,r0		; r0=b
mul  4(sp),r0		; r0:r1=b*d
add  r2,r0
bcs  fnMultiplyBigInts	; Overflow
bmi  fnMultiplyBigInts	; Overflow
;
; On exit, r1=SUM b0-b15, r0=SUM b16-b31
;          r2 corrupted
;          r3,r4,r5 preserved
;
....


.fnMultiplyBigInts
;
; On exit, r0,r1,r2 corrupted
;          r3,r4,r5 preserved