When the lock is "locked", the secondary ROM mapping register does not exist (see Section 2.6). It is therefore impossible to select (or to deselect) the memory mapped register page.
Note: As one may see, the nybbles in the sequence are based on two 4bit shift registers. For one reason or another, Amstrad has patented the verification mechanism ([[Media:Patent GB2243701A.pdf|GB2243701A]]). The patent seems to focus on ''verifying'' (rather than on ''sending'') the sequence, so its legal use is a bit unclear.
===Eight bit printer support===