🚀 go-pugleaf

In article <1992Jul26.015003.1@matrix.cs.wright.edu>, dwagoner@matrix.cs.wright.edu writes:
> Can anyone explain the difference between P0 and P1 space on a VAX?

P0 space starts at 0 and goes to 0x3FFFFFFF.  P1 space starts at 0x7FFFFFFF
and extends down to 0x40000000.  P0 is used for text, data and bss.  P1
is used for the stack. [I hope i've got the numbers right; anyway it's
all to do with the top two bits of the address:

   31 30               ...            0
    o  o  o  o         ...            o
    ^  ^
    |  |---------   P0/P1
    |
    |-----------   user/kernel space

kernel P1 is invalid].

> Text and initialized data segments (on BSD 4.3, at least) wind up 
> in P0 space; unitialized data (i.e. stack) is  placed in P1 space.

Uninitialised data is known as bss and belongs in P0.

> The two regions are accessed using different base registers; P1 is
> a bit tricky to work with in comparison to P0. 

Well, I always thought that it was counter intuitive for the human
but was designed to be simple for the machine.  I think the idea
was to use the same arithmetic for both regions.  From memory, the P1
length register indicated how much is _not_ mapped.


Boyd Roberts			boyd@prl.dec.com

``When the going gets wierd, the weird turn pro...''