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include vmm.inc
 
VMMcall _SetFreePhysRegCalBk, <Callback, flags>
 
or      eax, eax            ; nonzero if installed, zero if error
jz      not_installed
 
The _SetFreePhysRegCalBk service installs a callback procedure for managing
the free list for a free physical region. The system calls the callback
procedure whenever the memory manager puts a page on the free list or wants
to obtain a physical page.
 
Any number of callback procedures can be installed.
 
This service is only available during initialization.
 
Parameter  Description
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Callback   Points to the callback procedure to install. See the Comments
           section for more information about this procedure.
 
flags      Specifies the operation flags. This parameter must be set to 0.
 
Return Value
 
The EAX register contains a nonzero value if the service is successful.
Otherwise, EAX contains zero to indicate an error such as insufficient
memory to define the callback procedure.
 
Comments
 
When the system places a page on the free list, the system gives a callback
procedure the option to take the page and map it in a free physical region
using the _MapFreePhysReg service. When the system needs a page, the system
requests pages from the callback procedures. They provide pages by using the
_UnmapFreePhysReg service to unmap the pages from the free physical region
and put it back on the free list.
 
The system calls the callback procedure as follows:
 
mov     eax, Request    ; 0 for pages available, 1 for pages needed
mov     ecx, Pages      ; count of pages available or needed
call    [Callback]
 
jc      dont_chain      ; carry set if request satisfied
 
The Request parameter specifies whether the call is a notification of free
pages or a request for free pages. This parameter is 0 if the pages are
available and 1 if the pages are needed. All other values are reserved.
 
The Pages parameter specifies the count of page either currently available
in the free list, or requested by the memory manager to be placed on the
free list.
 
The callback procedure sets the carry flag to indicate that it carried out
the request, and to prevent the system from calling the next callback
procedure in the chain. If the callback procedure cannot complete the entire
request, it must clear the carry flag.
 
The callback procedure can modify all registers except the segment registers
and EBP.
 
If more than one callback procedure is installed, the system calls each
procedure in the chain unless a procedure returns with the carry flag set.
The system varies the order in which it calls the procedures, ensuring that
every procedure is periodically the first to be called.
 
When Request is 1, the callback procedure must release pages for regions
which were mapped without the PageFixed value.
 
The callback procedure must release all free physical region pages which
were mapped without the PageFixed value.
 
The callback procedure can release more pages than requested. The extra
pages remain on the free list until the next operation places pages on the
free list. At this point, the system sends a notification (Request is 0) to
the callback procedure for all of the pages currently on the free list.
 
In general, a callback procedure should not call any services other than the
_MapFreePhysReg and _UnmapFreePhysReg services. Calling other services,
particularly nonasynchronous services, may cause the system to be
re-entered.
 
The system does not begin calling the free-physical-region callback
procedures until after the Init_Complete message has been processed by all
virtual devices.
 
Uses
 
EAX
 
See Also
 
_MapFreePhysReg, _PageAllocate
 
 
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