GCDHL_BC

GCDHL_BC:
;Inputs:
;     HL is a number
;     BC is a number
;Outputs:
;     A is 0
;     BC is the GCD
;     DE is 0
;Destroys:
;     HL
;Size:  25 bytes
;Speed: 30 to 49708 cycles
;       -As slow as about 126 times per second at 6MHz
;       -As fast as about 209715 times per second at 6MHz
;Speed break down:
;     If HL=BC, 30 cycles
;     24+1552x
;     If BC>HL, add 20 cycles
;     *x is from 1 to at most 32 (because we use 2 16-bit numbers)
;
     or a \ sbc hl,bc     ;B7ED42    19
     ret z                ;C8        5|11
     add hl,bc            ;09        11
     jr nc,$+8            ;3006      11|31
       ld a,h             ;7C        --
       ld h,b             ;60        --
       ld b,a             ;47        --
       ld a,l             ;7D        --
       ld l,c             ;69        --
       ld c,a             ;4F        --
Loop:
     call HL_Div_BC       ;CD****    1511
     ld a,d \ or e        ;7AB2      8
     ret z                ;C8        5|11
     ld h,b \ ld l,c      ;6069      8
     ld b,d \ ld c,e      ;424B      8
     jr $-10              ;18F8      12