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include vmm.inc VMMcall _ModifyPageBits, <VM, VMLinPgNum, nPages, bitAND, bitOR, pType, flags> or eax, eax ; nonzero if modified, zero if error jz not_modified |
The _ModifyPageBits service modifies the page attribute bits associated with PG_HOOKED pages in the V86 address space of a virtual machine. Virtual devices use this service to modify the P_PRES, P_WRITE, and P_USER bits and the PG_TYPE, if appropriate.
VM
Specifies a handle identifying the virtual machine owning the pages to modify.
VMLinPgNum
Specifies the linear page number of the first page to modify. All pages must be in the 1 megabyte V86 address space. Page numbers below the first page of the specified virtual machine or above 10Fh cause an error.
nPages
Specifies the number of pages to modify.
bitAND
Specifies the AND mask for the page attribute bits. All AND mask bits, except the P_PRES, P_WRITE, and P_USER bits, must be set to 1. The P_PRES, P_WRITE, and P_USER bits can be 0 or 1 to clear or preserve the corresponding page attributes.
bitOR
Specifies the OR mask for the page attribute bits. All bits, except the P_PRES, P_WRITE, and P_USER bits, must be set to 0. The P_PRES, P_WRITE, and P_USER bits can be 0 or 1 to preserve or set the corresponding page attributes.
pType
Specifies the page type. It can be one of the following values:
| Value | Meaning |
| PG_HOOKED | Changes the page type to hooked. This value must be specified if the service clears any one of the P_PRES, P_WRITE and P_USER bits. |
| PG_IGNORE | Leaves the current page type unchanged. |
All other values are reserved.
flags
Specifies the operation flags. This parameter must be set to 0.
The EAX register contains a nonzero value if the service is successful. Otherwise, EAX contains zero to indicate an error such as an invalid virtual machine handle, invalid bits in AND and OR masks, an invalid type, or a bad page range.
This service always clears the P_DIRTY and P_ACC bits regardless of the AND and OR mask values.
If a virtual device clears the P_PRES, P_WRITE, or P_USER bits, the virtual device must set the page type for all such pages to PG_HOOKED. Also the virtual device must have previously installed a hook page fault handler for these pages.
This service cannot be used to set the P_PRES bit. Virtual devices can use the _MapIntoV86 or _PhysIntoV86 service to make pages present. Since the _MapIntoV86 service resets the page type to the same page type as the memory specified by the memory handle, the virtual devices should set the page type to PG_HOOKED when it creates the memory handle using the _PageAllocate service. Since the _PhysIntoV86 service sets the page type to PG_SYS, the virtual devices must use the _ModifyPageBits service to change the page type to PG_HOOKED if it uses _PhysIntoV86 to map physical pages into a hooked page region.
If using the P_WRITE bit to simulate ROM in a virtual machine, a virtual device should map the pages using the _PhysIntoV86 service and immediately call the _ModifyPageBits service to clear the P_WRITE bit and change the page type to PG_HOOKED.
EAX