[Info-vax] Virtual memory 101 (was Re: Android development...)

[VAXman- at SendSpamHere.ORG]

In article <54222358$0$24420$c3e8da3$f6268168 at news.astraweb.com>, JF Mezei <jfmezei.spamnot at vaxination.ca> writes:
>On 14-09-23 21:30, Stephen Hoffman wrote:
>
>> On VAX and Alpha, the change-mode operation — an instruction on VAX, a 
>> PALcode call on Alpha — triggers what is called an exception, and the 
>> exception is then received and processed by the system service 
>> dispatcher, which looks at how it was called, and (it's a dispatcher, 
>> after all) dispatches to the system service.
>
>So basically, the mode switches from user to either exec or kernel only
>when the process executes a particular opcode that triggers that
>exception, and the exception is granted or rejected based on where the
>instruction is located in memory ?
>
>Is this grant or reject decision  properties given to the page of memory
>containing that code ? Or is there a page range that defines system
>space, and that range is specified in hardware specific physical memory
>location ?

On VAX and Alpha, no.  On VAX, the CHMx instruction creates a trap which
causes the change of access mode to that of 'x' (User, Super, Exec, Kernel)
On Alpha, CHMx is a PAL code providing, essentially, the same thing.  It
does not matter where in the address space the CHMx or CALL PAL_CHMx are
located.

On Itanium, the EPC instruction is used to change the access mode.  This
instruction DOES depend where it is located.  EPC causes the access mode
to change to that of the page on which it resides.

http://groups.csail.mit.edu/pag/application_communities/manuals/IA64-vol3.pdf
 
Search for "EPC"  (found on page 3:48).

Hoff's URL containing Ruth's discussion of SSI also contains details of 
the system service dispatch mechanism of each of the architectures.  I'd
highly suggest you read that article and then, if you still have problems
comprehending the vehicle which gets one to inner access mode, read the
associated Internals and Data Structures manuals.

-- 
VAXman- A Bored Certified VMS Kernel Mode Hacker    VAXman(at)TMESIS(dot)ORG

I speak to machines with the voice of humanity.