BCD_mul

;This code is as yet UNTESTED.
;Feel free to experiment.
;This is the algorithm I personally use when doing arithmetic by hand of numbers with more than ~6 digits.
;It's a modified 'schoolbook' method.
;First create a table of values of one of the multiplicands multiplied by 1,2,3,...,9
;Now use this table to quickly look up partial products.

BCD_mul:
;Input:
;  HL points to the first multiplicand (big endian, 14-digit integer)
;  DE points to the second multiplicand (big endian, 14-digit integer)
;  'scrap' starts at an address ending in 00, and at least 94 bytes long.
;Output:
;
    push de
    ld de,scrap
    xor a
    ld (de),a
    inc de
    call mov7
    pop hl
    ld de,scrap+72
    ld (de),a
    inc de
    call mov7
    ld h,a
    ld l,a
    ld (scrap+80),hl
    ld (scrap+82),hl
    ld (scrap+84),hl
    ld (scrap+86),hl
    ld (scrap+88),hl
    ld (scrap+90),hl
    ld (scrap+92),hl
    call mullutgen

    ld bc,scrap+79
    ld de,srap+93
    call bcdmul_sub_0
    xor a
    ld hl,scrap+71
    ld b,72
    rrd
    dec hl
    djnz $-3
    ld bc,scrap+79
    ld de,srap+93
    call +_
_:
    call +_
_:
    call +_
_:
    ld a,(bc)
    inc bc
    and 15
    ret z
    rra
    and %01111000
    jp bcdmul_sub_1
bcdmul_sub_0:
    call +_
_:
    call +_
_:
    call +_
_:
    ld a,(bc)
    inc bc
    and 15
    ret z
    add a,a
    add a,a
    add a,a
bcdmul_sub_1:
sub 73
    cpl
    ld h,b
    ld l,a
    ld a,(de) \ add a,(hl) \ ld (de),a \ dec de \ dec hl
    ld a,(de) \ adc a,(hl) \ ld (de),a \ dec de \ dec hl
    ld a,(de) \ adc a,(hl) \ ld (de),a \ dec de \ dec hl
    ld a,(de) \ adc a,(hl) \ ld (de),a \ dec de \ dec hl
    ld a,(de) \ adc a,(hl) \ ld (de),a \ dec de \ dec hl
    ld a,(de) \ adc a,(hl) \ ld (de),a \ dec de \ dec hl
    ld a,(de) \ adc a,(hl) \ ld (de),a \ dec de \ dec hl
    ld a,(de) \ adc a,(hl) \ ld (de),a
    ld hl,-6
    add hl,de
    ex de,hl
    ret
mov7:
    ldi
    ldi
    ldi
    ldi
    ldi
    ldi
    ldi
    ret
mullutgen:
    ld hl,scrap+71
    ld bc,scrap+63
    call mullutgen
    call +_
_:
    call +_
_:
    call mullutgen
mullutgen_sub:
    ld de,scrap+71
    or a
    call +_
_:
    call +_
_:
    call +_
_:
    ld a,(de)
    adc a,(hl)
    daa
    ld (bc),a
    dec hl
    dec de
    dec bc
    ret