Interrupt 2Fh Function 1685h

mov bx, [VM_Id] ; virtual-machine identifier

mov cx, [Flags] ; switch conditions

mov dx, word ptr [Priority+2]

mov si, word ptr [Priority] ; dx:si contains 32-bit priority boost

mov es, seg [CallBack]

mov di, offset [CallBack] ; es:di contains callback function address

mov ax, 1685h ; Switch VMs and CallBack

int 2Fh ; multiplex interrupt

Switch VMs and CallBack (Interrupt 2Fh Function 1685h) directs Windows to switch to a specific virtual machine and begin execution. After Windows switches, it calls the specified callback function allowing a device driver or TSR to access the data associated with the specified virtual machine. This function is typically used by MS-DOS device drivers and TSRs that support networks, and that need to perform functions in a specific virtual machine.

Parameters

VM_Id

Identifies the virtual machine to switch to.

Flags

Specifies when to switch. This parameter is a combination of the following bit values.

Bit

Meaning

0	Set to 1 to wait until interrupts are enabled.
1	Set to 1 to wait until critical section is released.

All other bits are reserved and must be 0.

Priority

Specifies the priority boost for the virtual machine. It can be one of the following values.

Value

Meaning

Critical_Section_Boost	VM priority is boosted by this value when Begin_Critical_Section is called.
Cur_Run_VM_Boost	Time-slice scheduler boosts each VM in turn by this value to force them to run for their allotted time slice.
High_Pri_Device_Boost	Time critical operations that should not circumvent the critical section boost should use this boost.
Low_Pri_Device_Boost	Used by virtual devices that need an event to be processed in a timely fashion but that are not extremely time critical.
Reserved_High_Boost	Reserved; do not use.
Reserved_Low_Boost	Reserved; do not use.
Time_Critical_Boost	Events that must be processed even when another VM is in a critical section should use this boost. For example, VPICD uses this when simulating hardware interrupts.

The DX register contains the high-order word, the SI register the low-order word.

CallBack

Points to the callback function.

Return Value

The return value is a cleared carry flag if successful. Otherwise, the function sets the carry flag and sets the AX register to one of the following error values.

Value

Meaning

0001h	Invalid VM ID
0002h	Invalid priority boost
0003h	Invalid flags

Comments

Windows calls the callback function as soon as the conditions specified by the Flags parameter are met. This may be before or after Switch VMs and CallBack returns.

The callback function can carry out any action, but must save and restore all registers it modifies. The function must execute an iret instruction to return to Windows. The priority for the virtual machine remains at the level specified by Priority until the callback function executes the iret instruction.