BASIC-DOS

V86

V86 [program [arguments]]

Runs a real-mode program (a COM file, with .COM added if program has no extension, found in the current directory, or if program has no drive or directory, in a directory of PATH$) in a V86 machine: 640K of memory that the program sees as an 8086 would, at addresses starting from 0, with the ROMs above it, while BASIC-DOS keeps running in protected mode. Without a program, V86 runs COMMAND.COM. Any switches after program are the program’s own, but a switch before it is a Syntax error. V86 gets PATH$ the way an external program does, from its environment, but since a session that START makes has none, START gives V86 the current PATH$ ahead of its arguments, as it does for COMMAND.COM (eg, START V86 DEBUG runs V86 PATH$="C:\;C:\EXT":DEBUG), and V86 passes that PATH$ on to its program’s environment, as PATH.

V86 machines need an 80386 (or later) running BASIC-DOS in protected mode with the 80386’s features, which is what a CPU=80386 line in CONFIG.SYS (see CPU) gives you; otherwise, V86 displays V86 mode not available.

The program can do anything an 8086 program can, except I/O, and its interrupts and exceptions go to its own vectors, which it can hook (eg, with DOS function 25h). V86 does the DOS functions that involve the machine itself (its vectors, PSP, DTA, memory, and ending), and passes the rest on to BASIC-DOS, so the program’s files, console, and devices are V86’s. So far:

• Only COM programs can run, and they can’t run other programs (DOS function 4Bh) or allocate memory (48h), although they can shrink (or regrow) their own (4Ah).
• Only DOS functions (INT 20h and 21h) work; any other interrupt that the program doesn’t handle itself (eg, a BIOS interrupt, or BASIC-DOS’s own INT 2Bh) ends it, with a CPU fault message. That’s why COMMAND.COM can’t run in a V86 machine yet.
• An instruction that real mode allows but a V86 machine doesn’t (eg, I/O, or HLT) ends the program the same way.
• The program can’t use video memory directly, and its BIOS data is a copy.

ERRORLEVEL is set to the program’s exit code, or 1 if V86 couldn’t run it. When V86 is the first program of its session (eg, START V86), its console closes when it ends, so if V86 or its program fails (a non-zero exit code), V86 first displays Press any key and waits for one, so that the messages can be read.

Source: v86.asm

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