of_bit_mirror:
        mov cx, dx
        test bx, bx
        jnz .large
        cmp dx, byte 64
        jb .normal
.large:
        xor bx, bx              ; mirror count 64 or higher:
        xor dx, dx              ;  all 32 bits mirrored with (nonexistent) zero bits
        retn
.normal:
        mov dx, word [hhvar]
        mov bx, word [hhvar+2]
        cmp cl, 1
        jbe .ret                ; mirror count one or zero, return input -->
        push si
        push di

        push cx
        mov di, -1
        mov si, di
.loopmask:
        shl di, 1
        rcl si, 1
        loop .loopmask          ; create mask of bits not involved in mirroring
        and si, bx
        and di, dx              ; get the uninvolved bits
        pop cx

        push si
        push di                 ; save them
        xor si, si
        xor di, di              ; initialize mirrored register
.loop:
        shr bx, 1
        rcr dx, 1               ; shift out of original register's current LSB
        rcl di, 1
        rcl si, 1               ;  into other register's current LSB
        loop .loop
        pop dx
        pop bx                  ; restore uninvolved bits
        or bx, si
        or dx, di               ; combine with mirrored bits

        pop di
        pop si
.ret:
        retn