doMultiply:
;returns HL = HL times A. No overflow protection.
push bc
push de
    rrca                     ;test if A is odd or even by dividing A by 2.
    jr c, isOdd
        ;is even

        ld b,a
loop_multiplyByEvenNumber:
        add hl,hl           ;double A until B runs out.
        djnz loop_multiplyByEvenNumber
pop de
pop bc
ret

isOdd:
    push hl
    pop de                  ;de contains original HL. We'll need it later.
    ld b,a
loop_multiplyByOddNumber:
    add hl,hl
    djnz loop_multiplyByOddNumber
    add hl,de             ;now add in original HL for the leftover add.
pop de
pop bc
ret