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include vmm.inc mov eax, PriorityBoost ; priority boost (can be 0) mov ebx, VM ; VM handle mov ecx, Flags ; option flags mov edx, OFFSET32 RefData ; points to reference data mov esi, OFFSET32 EventCallback ; points to event callback procedure mov edi, TimeOut ; number of milliseconds for time out VMMcall Call_Priority_VM_Event mov [Event], esi ; event handle or zero if callback called |
The Call_Priority_VM_Event service either calls the callback procedure immediately, or schedules a priority event for the specified virtual machine. This service schedules the event if the virtual device is processing a hardware interrupt that interrupted the VMM, or the current virtual machine is not the specified virtual machine, or the Flags parameter specifies the PEF_Always_Sched value. In all other cases, the service calls the callback procedure and returns without scheduling an event.
This is an asynchronous service.
PriorityBoost
Specifies a positive or negative priority boost for the virtual machine. This parameter must be a value such that when added to the current execution priority, the result is within the range Reserved_Low_Boost to Reserved_High_Boost. This parameter can be 0 if no boost is necessary.
The following lists some common priority boost values:
| Value | Meaning |
| Critical_Section_Boost | The system uses this value for virtual machines specified in a call to the Begin_Critical_Section service. |
| Cur_Run_VM_Boost | The time-slice scheduler uses this value to force a virtual machine to run for its allotted time slice. |
| High_Pri_Device_Boost | Virtual devices use this value for events that need timely processing, but should not circumvent operations that have a critical section boost. |
| Low_Pri_Device_Boost | Virtual devices use this value for events that need timely processing, but are not time critical. |
| Reserved_High_Boost | Reserved for use by system. |
| Reserved_Low_Boost | Reserved for use by system. |
| Time_Critical_Boost | Virtual devices use this value for events that must be processed even when another virtual machine is in a critical section. For example, VPICD uses this when simulating hardware interrupts. |
VM
Specifies a handle identifying the virtual machine to process the event.
Flags
Specifies how to carry out the event. This parameter can be a combination of the following values:
| Value | Meaning |
| PEF_Always_Sched | Event is always scheduled, meaning the callback procedure is never called immediately. |
| PEF_Dont_Unboost | Priority of the virtual machine is not reduced after return from callback procedure. |
| PEF_Time_Out | Specifies that time-out value in the EDI register should be used. Available in Windows version 3.1 or later. |
| PEF_Wait_For_STI | Callback procedure is not called until the virtual machine enables interrupts. |
| PEF_Wait_Not_Crit | Callback procedure is not called until the virtual machine is not in a critical section or time-critical operation. |
All other values are reserved.
RefData
Points to reference data to be passed to the callback procedure.
EventCallback
Points to the callback procedure to install. For more information about the callback procedure, see the following “Comments” section.
TimeOut
Specifies the number of milliseconds until the event times out. The service uses this parameter only if the PEF_Time_Out value is specified by the Flags parameter.
The ESI register is zero if the callback procedure was called immediately. Otherwise, the ESI register contains the event handle. The handle can be used in a subsequent call to the Cancel_Priority_VM_Event service to cancel the event.
The system carries out a task switch to the specified virtual machine if it is not the current virtual machine. If the PriorityBoost parameter is not zero, the service boosts the priority of the virtual machine before calling the callback procedure.
If the amount of time specified by the TimeOut parameter elapses before the system can switch to the virtual machine, the system sets the carry flag and calls the callback procedure immediately regardless of which virtual machine is currently running. In this case, any requested priority boost is canceled even if the Flags parameter specified the PEF_Dont_Unboost value. The callback procedure should always check the carry flag to determine whether a time out occurred.
The callback procedure can carry out any actions, and use any VMM services. The system calls the event callback procedure as follows:
mov ebx, VM ; current VM handle
mov edx, OFFSET32 RefData ; points to reference data
mov ebp, OFFSET32 crs ; points to a Client_Reg_Struc
call [EventCallback]
The VM parameter is a handle identifying the current virtual machine. If a time out occurred, this handle may not be valid. In such cases, the callback procedure should use the Get_Cur_VM_Handle service to get the handle of the current virtual machine.
The RefData parameter points to reference data supplied by the virtual machine that scheduled the event and the crs parameter points to a Client_Reg_Struc structure containing the contents of the current virtual machine's registers.
The callback procedure can modify the EAX, EBX, ECX, EDX, ESI, EDI, and Flags registers.
Flags
Adjust_Exec_Priority, Call_VM_Event, Call_When_Not_Critical, Call_When_VM_Ints_Enabled, Cancel_Priority_VM_Event