| Bit Set | Meaning | Comments |
|---|
| 0 |
Reserved |
This is mainly for debugging purposes. Internally the memory subsystem
handles page requests and maintains the invariant that every page that is
not free has this bit set. |
| 1 |
User Page |
Any page in to-Space or from-Space is a User page. Pages for queues
and such are not user pages. |
| 2 |
System Page |
Pages marked as System pages are used for internal data structures.
Mostly these are queues. Pages in the free list are not considered system
pages. |
| 3 |
Pinned |
Used to denote a page that cannot be moved. |
| 4 |
Old |
This bit designates the page as old(i.e. belonging to an older
generation). |
| 5 |
Modified |
Used for card marking in the generational scheme. |
| 6 |
Blacklisted |
If blacklisting is in effect this marks a page as blacklisted. |
| 7 |
Bitmask Updated |
Earlier we had associated a bitmask with each page to denote which
words were header words and which weren't. This has been abandoned since
but a lot of code is still lying around which may be reused at some future
date. |
| ?-31 |
Offset |
The offset records where the first header word occurs on this page. We
are currently not using this at all. But it may be useful for squeezing
multiple large objects on the same page.
|
If no bits are set the page is "free" or unused.