Virtual Control Program Interface
Encyclopedia
In computing, the Virtual Control Program Interface (VCPI) is a specification published in 1989 by Phar Lap Software
that allows a DOS
program to run in protected mode
, granting access to many features of the processor not available in real mode
. It was supplanted by DOS Protected Mode Interface
(DPMI) shortly after being introduced, due in large part to VCPI's inability to work in Windows 3.0
's protected mode.
, VCPI is provided by an expanded memory
manager in DOS (e.g. CEMM
, QEMM
, later EMM386
). It was eclipsed by DPMI, most notably because it was not supported for DOS programs run in Windows 3.0's native protected mode
(called 386 enhanced mode) and because VCPI runs programs in Ring
0, which defeated the purpose of x86 protection. It also did not work with OS/2
2.0 and later. VCPI was only supported in Windows 3.0 real mode, some programs could run in Windows 3.x standard mode. Standard mode Windows 3.1 (but not 3.0) itself was VCPI compliant (it was a VCPI client). Earlier Windows/386 2.1 was not compatible with DOS extenders at all. Windows NT dos box did not support VCPI either.
VCPI also had a comparatively limited scope in that it allowed a protected mode DOS program to run only when the program was started from DOS already running inside a virtual 8086 mode
task. (This was typically accomplished through a memory manager
operating as a virtual [mode] control program for the processor.) Because the virtual 8086 mode isolates programs from the hardware, it is not possible for a program to switch to protected mode without some support from the control program.
In the late 1980s an extended version of the specification XVCPI addressed some of these problems and was implemented or used by a small number of products including INTERACTIVE UNIX
and Digital Research
operating systems.
Phar Lap (company)
Phar Lap was a software company specializing in software development tools for DOS operating systems. They were most noted for their software allowing developers to access memory beyond the 640 KiB limit of DOS and were an author of the VCPI standard.Phar Lap Software, Inc. was founded in April...
that allows a DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
program to run in protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
, granting access to many features of the processor not available in real mode
Real mode
Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...
. It was supplanted by DOS Protected Mode Interface
DOS Protected Mode Interface
In computing, the DOS Protected Mode Interface is a specification introduced in 1989 which allows a DOS program to run in protected mode, giving access to many features of the processor not available in real mode...
(DPMI) shortly after being introduced, due in large part to VCPI's inability to work in Windows 3.0
Windows 3.0
Windows 3.0, a graphical environment, is the third major release of Microsoft Windows, and was released on 22 May 1990. It became the first widely successful version of Windows and a rival to Apple Macintosh and the Commodore Amiga on the GUI front...
's protected mode.
Overview
Developed in cooperation with Quarterdeck Office SystemsQuarterdeck Office Systems
Quarterdeck Office Systems, later Quarterdeck Corporation , was an American computer software company. It was founded by Therese Myers and Gary Pope in 1981and incorporated in 1982...
, VCPI is provided by an expanded memory
Expanded memory
In DOS memory management, expanded memory is a system of bank switching introduced April 24, 1985 that provided additional memory to DOS programs beyond the limit of conventional memory. Expanded memory uses parts of the address space normally dedicated to communication with peripherals for program...
manager in DOS (e.g. CEMM
CEMM
CEMM, for Compaq Expanded Memory Manager was the first so-called PC "memory manager" for Intel 80386 CPUs, able to transform "XMS" extended memory into "EMS" expanded memory by using the virtual memory features and the virtual 8086 mode of the CPU....
, QEMM
QEMM
Quarterdeck Expanded Memory Manager , was a memory manager produced by Quarterdeck Office Systems in the late 1980s through late 1990s. It was the most popular memory manager for the MS-DOS and other DOS operating systems.-QEMM product ranges:QRAM: A memory manager for 286 or higher CPU. It...
, later EMM386
EMM386
The name EMM386 was used for the expanded memory managers of both Microsoft's MS-DOS and Digital Research's DR-DOS, which created expanded memory using extended memory on Intel 80386 CPUs. There also is an EMM386.EXE available in FreeDOS....
). It was eclipsed by DPMI, most notably because it was not supported for DOS programs run in Windows 3.0's native protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
(called 386 enhanced mode) and because VCPI runs programs in Ring
Ring (computer security)
In computer science, hierarchical protection domains, often called protection rings, are a mechanism to protect data and functionality from faults and malicious behaviour . This approach is diametrically opposite to that of capability-based security.Computer operating systems provide different...
0, which defeated the purpose of x86 protection. It also did not work with OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
2.0 and later. VCPI was only supported in Windows 3.0 real mode, some programs could run in Windows 3.x standard mode. Standard mode Windows 3.1 (but not 3.0) itself was VCPI compliant (it was a VCPI client). Earlier Windows/386 2.1 was not compatible with DOS extenders at all. Windows NT dos box did not support VCPI either.
VCPI also had a comparatively limited scope in that it allowed a protected mode DOS program to run only when the program was started from DOS already running inside a virtual 8086 mode
Virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running a protected mode operating system.VM86 mode uses a segmentation scheme identical to that of real mode In...
task. (This was typically accomplished through a memory manager
Memory manager
In IBM PC compatible computing, DOS memory management refers to software and techniques employed to give applications access to more than 640K of "conventional memory". The 640kB limit was specific to the IBM PC and close compatibles; other machines running MS-DOS had different limits, for example...
operating as a virtual [mode] control program for the processor.) Because the virtual 8086 mode isolates programs from the hardware, it is not possible for a program to switch to protected mode without some support from the control program.
In the late 1980s an extended version of the specification XVCPI addressed some of these problems and was implemented or used by a small number of products including INTERACTIVE UNIX
INTERACTIVE UNIX
INTERACTIVE UNIX System V/386 is a port of the UNIX System V operating system for Intel x86 processors.The system was first released by INTERACTIVE Systems Corporation as 386/ix in 1985. At that time it was based on System V.3.0. Later versions were based on System V.3.2...
and Digital Research
Digital Research
Digital Research, Inc. was the company created by Dr. Gary Kildall to market and develop his CP/M operating system and related products. It was the first large software company in the microcomputer world...
operating systems.