If P is a page key then:

P(0;==>;K) returns a key like P except the read only bit is 1.

P(39;==>c;) stores zeroes in all of the page. This operation may not require having the page in core. c=0 if P is read/write, kt+2 if read-only.

P(4096+A, string; ==> c;) where 0 <= A < 4096, stores string in the page starting at address A in the page. c is 0 if ok, 1 if length of string + A > 4096, kt+2 if P has the read-only bit on.

{ni}P(41==>c) Returns c=0 if page holds 4096 zero bytes and c=1 otherwise.

P(kt;==>c;) returns binary R001000000010 in c where R is the read only bit {(p1,robit)}.

Proposed addition: It seems that in analogy with the order code NODE__COMPARE we need an order code PAGE__COMPARE.
But the same function can be obtained from a read/write page key by creating a read-only key to the same page and then comparing the two keys with the key in question using DISCRIM. This is feasible because there are only two forms of a page key, unlike the case with node keys.

A page key order to test for a zero page would be cheap to implement (without reading the page from disk) and aid such schemes as described at (nonzscan).