2.0 - DPMI functions - Overview

DOS/32 Advanced DOS Extender has a built-in Advanced DOS Protected Mode Interface (ADPMI) which provides your protected mode program with nearly full subset of DPMI v0.9 functions. The "DPMI" is a standard, which allows protected mode applications to get an access to the extended memory, switch into or out from protected mode and invoke real mode software. DPMI API is accessed through software interrupt INT 31h in protected mode with function number is placed in register AX. On successful return, registers will contain values/information returned by the DPMI host and the carry flag will be cleared. If the function is not supported by the DPMI host, or DPMI host could not perform the action desired by the function, it will return with carry flag set and an error code placed in register AX. Note that some implementations of DPMI do not return error codes, but only set the carry flag. DOS/32 Advanced built-in DPMI will always return error codes in AX.

On startup, DOS/32 Advanced DOS Extender will detect what system is in place and if there is an external DPMI host present. Depending on configuration, the DOS Extender will look for VCPI and DPMI in different orders. If configured to look for VCPI first and DPMI second, the DOS Extender will detect and initialize the VCPI server even though a DPMI host might be present. However, a DPMI host may always deny the presence of VCPI. In that case the DOS Extender will detect and initialize DPMI instead. If neither VCPI nor DPMI are installed, DOS/32 Advanced will look for XMS and if even it is not present, initialize and use Clean (INT 15h) system.

When running under Clean, XMS or VCPI system softwares, DOS/32 Advanced will always install its built-in DPMI server, ADPMI. Otherwise, the present DPMI host ("external DPMI") will be initialized and used. Note that some of DPMI implementations can be quite buggy and unpredictable; some of them will run your protected mode code at CPL 0, some at CPL 3, some will execute real mode calls in Real mode and some in Virtual 8086 mode. Due to all this complications DOS/32 Advanced will try to be very careful when accessing external DPMI hosts and interfere with them as little as possible.

The following information about DOS/32 Advanced built-in DPMI host will show its major capabilities:

• descriptors reside in the GDT
• protected mode code runs at CPL 0
• user configurable order of detection of VCPI/DPMI servers
• real mode INTs/IRQs/callbacks executed in actual Real mode under Clean (INT 15h) and XMS systems, and in Virtual 8086 mode under VCPI
• modifiable number of selectors/callbacks/real&protected mode stacks that will be allocated at startup
• support for exception handling which can be turned off
• support for debugging through INT 1 and INT 3 software interrupts
• support for trapping software INTs in range INT 08-0Fh issued to emulate hardware IRQs 0-7 when not remapped (can be disabled)
• support for auto-callbacking of IRQs 0-7 and 8-15 from real mode to protected mode when installing a protected mode handler for those
• support for auto-checking of integrity of extended memory blocks at run-time when allocating using DPMI functions 05xxh (can be disabled)
• support for vendor-specific API extensions (entry point can be obtained from DPMI function 0A00h)