Merge (software)

Merge is a software system which allows a user to run DOS/Windows 3.1 on SCO UNIX, in an 8086 virtual machine.

Merge was originally developed to run DOS under UNIX System V Release 2 on an AT&T 6300 Plus personal computer. Development of the virtual machine began in late 1984, and AT&T announced the availability of the machine on October 9, 1985, referring to the bundled Merge software as SimulTask. (The PC6300 Plus shipped with MS-DOS in 1985 though, because its Unix System V distribution was not ready until the end March 1986.) Merge was developed by engineers at Locus Computing Corporation, with collaboration from AT&T hardware and software engineers, particularly on aspects of the system that were specific to the 6300 Plus (in contrast to a standard PC/AT).

The AT&T 6300 Plus contained an Intel 80286 processor, which did not include the support for 8086 virtual machines (virtual 8086 mode) found in the Intel 80386 and later processors in the x86 family. On the 80286, the DOS program had to run in realmode. The 6300 Plus was designed with special hardware on the bus that would suppress and capture bus cycles from the DOS program if they were directed toward addresses not assigned for direct access by the DOS virtual machine. Various system registers, such as the programmable interrupt controller, and the video controller, had to be emulated in software for the DOS process, and a watchdog timer was implemented to recover from DOS programs that would clear the interrupt flag and then hang for too long. The hardware used the Non Maskable Interrupt (NMI) to take control back to the emulation code. More detail may be seen in the patent referenced in the External Links below.

Later Merge was enhanced to make use of the virtual 8086 mode provided by the Intel 80386 processor; that version was offered with Microport SVR3 starting in 1987, and subsequently with SCO Unix. There was also a Merge/286 version that ran on an unmodified PC/AT (without any special I/O trapping hardware); it ran as long as the PC program was reasonably well-behaved, though a malicious or crashing program could take the unprotected UNIX kernel down on those machines. Even so, the notoriously ill-behaved Microsoft Flight Simulator would run on the PC/AT simultaneously with Unix. These later versions were marketed directly by Locus as well as through some OEM and ISV channels. A product-evaluation version with user manual appeared in January 1987, with retail Version 1.0 of Merge/386 shipping in October of that year.

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