Kernel Interfaces

The Kernel provides the following external interfaces:

Application (R3/2) Interface.

Application access kernel services via GDT call gates. These are called either directly from the application program or via the DOSCALL1.DLL module, where additional Ring 3 processing is required before calling the kernel. Some system interfaces are able to be implemented entirely within Ring 2/3. In these cases, DOSCALL1.DLL does not make any kernel calls.

The kernel interfaces are represented by a fictitious module called DOSCALLS.DLL.

File System Driver (FSD)
The FSDs run in ring 0 as separately loaded modules. They are provided a set of interfaces to the kernel via the FSD_Hlp (File System Helper) calls.
Physical Device Driver (PDD)
The PDDs run in ring 0 as separately loaded modules. They are provided a set a interfaces to the kernel via the Dev_Hlp (Device Helper) calls.
Virtual Device Driver (VDD)
The VDDs run in ring 0 as separately loaded modules. They are provided a set a interfaces to the kernel via the VDD_Hlp (Virtual Device Driver Helper) calls.
Compatibility BIOS
The compatibility BIOS resides within the OS2LDR module. It provides a hardware implementation independent layer through which the kernel access the BIOS. The interface to the CBIOS from the kernel is provided by the Dos_Hlp (Dos Helper Services). These are not available for access by PDDs, VDDs or FSDs, however a limited set of Dos_Hlp calls are provided via the TESTCFG.SYS and OEMHLP$ device drivers.

Notes:

OEMHLP$ is not a separately loaded module - it is resident within the OS2LDR module.

OS2LDR is responsible for loading the Kernel at system initialisation time. It does not get involved with the loading of Application Programs, PDDs, VDDs for FSDs during normal running - that function is performed by the System Loader component of the Kernel.

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