As well as providing support for DPMI service requests issued by applications, the operating system must also provide support for the internal control functions of the DPMI host. This support is provided by various components of the operating system kernel.
8086 Emulation
The 8086 Emulation component of MVDM emulates the 8086 hardware environment, and therefore provides a number of services which are used by DPMI.
When the application calls the protect mode entry to switch to protected mode, 8086 Emulation sets up tables for reflection of interrupts and exceptions. If the DPMI API Layer fails to complete the creation call, 8086 Emulation cleans up and returns the error to the application.
All support for VDH services to the DPMI API Layer is provided through 8086 Emulation.
Protected mode applications get and set vectors as in DOS. 8086 Emulation maintains tables of protected mode interrupt handlers and exception handlers.
8086 Emulation virtualizes interrupts for VDMs. The reflection of "real mode" interrupts to protected mode for DPMI applications is therefore performed with the aid of 8086 Emulation.
8086 Emulation virtualizes the IF flag while in protected mode. In V86 mode, IOPL is usually 3 and applications directly change IF without trapping. IF flag virtualization is not done while in V86 mode because IOPL must be 3 to cut down on overhead. In protected mode, IOPL cannot be 3; otherwise no port protection is possible. Therefore, the IF flag is virtualized. This prevents VDMs from blocking real interrupts when running in protected mode.
To determine if interrupts are allowed in a VDM that has a DPMI application running, the real IF bit in the CRF is checked. If interrupts are disabled here, then they are disabled. Otherwise, the virtual IF flag indicates whether interrupts are disabled.
8086 Emulation exports a VDH service to accept notification from the VPIC when it starts and stops hardware interrupt reflection. 8086 Emulation also tracks which hardware interrupts are hooked. The VPIC allocates and initializes the buffer at creation time in each VDM.
Any VDD can use this structure to determine if a particular IRQ is hooked. The timer VDD, for example, can use this to avoid delivering timer ticks when the timer tick interrupts are not hooked in either protected mode or in V86 mode.
When software interrupts are hooked, 8086 Emulation refers to this structure to determine if the interrupt is a hardware interrupt.
Services called when the client is in protected mode, and which manipulate the protected mode client address space, must be written to handle protected mode user space access exceptions. Services that cannot be called when the client is in protected mode must specify this in their headers.
When a service can fault in protected mode, it must return a failure indication
to the DPMI client. The client then cleans up and exits protected mode
so that the exception can be reflected to the VDM (in V86 mode). This error
also indicates whether a protected mode exception handler will be called.
Debug Watchpoint Management
Coordinates watchpoint use with OS/2 protected mode and kernel debugger.
Memory Management
Among the kernel services provided by the Virtual Memory Manager are:
Once descriptors are allocated they are changed directly by the DPMI API layer. Applications set descriptors only through requests to the DPMI API layer, which prevents settings that compromise protection.