Editing Architecture of Btrieve (section)

===Page allocation tables===
A ''page allocation table'' (PAT) maps the logical pages to the physical pages. Each PAT is just a physical page located at well-defined locations. Like the FCRs, PATs always occur in pairs, with the currently valid copy indicated by having a higher usage count. The first pair of PATs immediately follow the first two FCRs and takes up physical pages 2 and 3. A variable number of other pages follow, and a new pair of PATs in turn follow this. Each PAT has a fixed number of pointers to logical pages, with each entry that is empty having a value of zero.

The amount of logical records that can be stored in the PAT is determined by its page size. Each page pointer in versions 6.x and 7.x of the MKDE takes up 4 bytes of space, and the PAT header takes up 8 bytes, so the amount of logical pages in the PAT becomes:

:''Number of logical pages'' = (''Page size'' ÷ 4) - 8

With the introduction of MKDE version 8, the size of the page header changed and so this formula no longer applies but the principle remains the same.