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FPA page Structure of pages
204 U929-J-Z125-9-76
Structure of an FPA extent page with a length of 2048 bytes or of an FPA page with a
length of 4000 or 8096 bytes
Figure 49: Structure of an FPA extent page with a length of 2048 bytes or of an FPA page with a length of 4000 or
8096 bytes
Meaning of the bytes
Byte Meaning
1-20 Page control information; type=FPA_PAGE;
contains, among other things, the act-key that indicates the page address.
25 Starting with this byte, there is a 2-byte entry for each page of the realm
indicating the number of free bytes in the respective page.
This is equal to 0 for
– the act-key-0 page, the act-key-N page,
– FPA pages,
– DBTT anchor pages
– DBTT pages,
– CALC pages,
since these pages are exclusively reserved for specific purposes and cannot
therefore be assigned additional data via the Free Place Administration. The
effectively usable storage space for empty pages is indicated as 2048 bytes,
3980 bytes or 8076 bytes, respectively, due to the 20 bytes taken up by the
the page header.
Table 17: Meanings of the bytes of an FPA extent page with a length of 2048 bytes or of an FPA page with a length
of 4000 or 8096 bytes
page control information
page 0=0
page 1
page n
free
number
of free
bytes
in
. . .
20
2
2
2
5th. byte: X‘03‘
1
21
23
25
4000/8096
(identifier for FPA page)