Neha Patil (Editor)

86 DOS

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Written in  8086-Assembly language
Working state  Historic, Unsupported
OS family  DOS
Source model  Closed source
Developer  Seattle Computer Products / Tim Paterson
Initial release  1980; 37 years ago (1980)

86-DOS is a discontinued operating system developed and marketed by Seattle Computer Products (SCP) for its Intel 8086-based computer kit. Initially known as QDOS (Quick and Dirty Operating System) the name was changed to 86-DOS once SCP started licensing the operating system in 1980.


86-DOS had a command structure and application programming interface that imitated that of Digital Research's CP/M operating system, which made it easy to port programs from the latter. The system was purchased by Microsoft and developed further as MS-DOS and PC DOS.


86-DOS was created because sales of the Seattle Computer Products 8086 computer kit, demonstrated in June 1979 and shipped in November, were languishing due to the absence of an operating system. The only software which SCP could sell with the board was Microsoft's Standalone Disk BASIC-86, which Microsoft had developed on a prototype of SCP's hardware. SCP wanted to offer the 8086-version of CP/M that Digital Research had announced, but its release date was uncertain. This was not the first time Digital Research had lagged behind hardware developments; two years earlier it had been slow to adapt CP/M for new floppy disk formats and hard disks. In April 1980 SCP assigned 24-year-old Tim Paterson to develop a substitute for CP/M-86.

Using a CP/M-80 manual as reference Paterson modeled 86-DOS after its architecture and interfaces, but adapted to meet the requirements of Intel's 8086 16-bit processor, for easy (and partially automated) source-level translatability of the many existing 8-bit CP/M programs; porting them to either DOS or CP/M-86 was about equally difficult. At the same time he made a number of changes and enhancements to address what he saw as CP/M's shortcomings. CP/M cached file system information in memory for speed, but this required a user to force an update to a disk before removing it; if the user forgot, the disk would be corrupt. Paterson took the safer but slower approach of updating the disk with each operation. CP/M's PIP command, which copied files, supported several special file names that referred to hardware devices such as printers and communication ports. Paterson built these names into the operating system as device files so that any program could use them. He gave his copying program the more intuitive name COPY. Rather than implementing CP/M's file system, he drew on Microsoft Standalone Disk BASIC-86's FAT filesystem.

By mid-1980 SCP advertised 86-DOS, priced at $95 for owners of its $1290 8086 board and $195 for others. It touted the software's ability to read Z80 source code from a CP/M disk and translate it to 8086 source code, and promised that only "minor hand correction and optimization" was needed to produce 8086 binaries.

IBM interest

In October 1980, IBM was developing what would become the original IBM Personal Computer. CP/M was by far the most popular operating system in use at the time, and IBM felt it needed CP/M in order to compete. IBM's representatives visited Digital Research and discussed licensing with Digital Research's licensing representative, Dorothy Kildall (née McEwen), who hesitated to sign IBM's non-disclosure agreement. Although the NDA was later accepted, Digital Research would not accept IBM's proposal of $250,000 in exchange for as many copies as IBM could sell, insisting on the usual royalty-based plan. In later discussions between IBM and Bill Gates, Gates mentioned the existence of 86-DOS and IBM representative Jack Sams told him to get a license for it.

Creation of PC DOS

Microsoft purchased a non-exclusive license for 86-DOS from Seattle Computer Products in December 1980 for $25,000. In May 1981, it hired Tim Paterson to port the system to the IBM PC, which used the slower and less expensive Intel 8088 processor and had its own specific family of peripherals. IBM watched the developments daily, submitted over 300 change requests before it accepted the product and wrote the user manual for it.

In July 1981, a month before the PC's release, Microsoft purchased all rights to 86-DOS from SCP for $50,000. It met IBM's main criteria: it looked like CP/M, and it was easy to adapt existing 8-bit CP/M programs to run under it, notably thanks to the TRANS command which would translate source files from 8080 to 8086 machine instructions. Microsoft licensed 86-DOS to IBM, and it became PC DOS 1.0. This license also permitted Microsoft to sell DOS to other companies, which it did. The deal was spectacularly successful, and SCP later claimed in court that Microsoft had concealed its relationship with IBM in order to purchase the operating system cheaply. SCP ultimately received a 1 million dollar settlement payment.

Intellectual property dispute

When Digital Research founder Gary Kildall examined PC DOS and found that it duplicated CP/M's programming interface, he wanted to sue IBM, which at the time claimed that PC DOS was its own product. However, Digital Research's attorney did not believe that the relevant law was clear enough to sue. Nonetheless, Kildall confronted IBM and persuaded them to offer CP/M-86 with the PC in exchange for a release of liability.

Controversy has continued to surround the similarity between the two systems. Perhaps the most sensational claim comes from Jerry Pournelle, who claims that Kildall personally demonstrated to him that DOS contained CP/M code by entering a command in DOS that displayed Kildall's name; as of 2006 Pournelle has not revealed the command and nobody has come forward to corroborate his story. A 2004 book about Kildall says that he used such an encrypted message to demonstrate that other manufacturers had copied CP/M, but does not say that he found the message in DOS; instead Kildall's memoir (a source for the book) pointed to the well-known interface similarity. Paterson insists that the 86-DOS software was his original work, and has denied referring to or otherwise using CP/M code while writing it. After the 2004 book appeared, he sued the authors and publishers for defamation. The court ruled in summary judgement that no defamation had occurred, as the book's claims were opinions based on research or were not provably false.


By 1982, when IBM asked Microsoft to release a version of DOS that was compatible with a hard disk, PC DOS 2.0 was an almost complete rewrite of DOS, so by March 1983, very little of 86-DOS remained. The most enduring element of 86-DOS was its primitive line editor, EDLIN, which remained the only editor supplied with Microsoft versions of DOS until the June 1991 release of MS-DOS 5.0, which included a TUI-based editor, MS-DOS Editor based on QBasic. EDLIN can still be used on contemporary machines, since there is an emulated DOS environment up to Windows 10 (32 bit).

Disk formats

Seattle Computer Products' 86-DOS supported the FAT12 filesystem on a range of 8.0" and 5.25" floppy disk drives on S-100 floppy disk controller hardware manufactured by Cromemco, Tarbell Electronics and North Star Computers. The Western Digital FD1771-based Cromemco and Tarbell boards supported one-sided single-density soft-sectored drives. A Tarbell double-density board utilizing the FD1791 was supported as well. At a later stage, SCP offered advanced floppy disk controllers like the Disk Master series themselves.

86-DOS itself did not take advantage of a FAT ID byte or BPB (as later DOS issues do) to distinguish between different media formats; instead different drive letters were hard-coded (at time of compilation) to be associated with different physical floppy drives, sides and densities, that is, depending on its type, a disk had to be addressed under a certain drive letter to be recognized correctly (a concept later emulated with more flexibility by DRIVER.SYS under DOS 3.x and higher).

Two logical format variants of the 86-DOS 12-bit FAT format existed, the original format with 16-byte directory entries and the later format (since 86-DOS 0.42) with 32-byte directory entries. Only the second one is logically compatible with the FAT12 format known since the release of MS-DOS and PC DOS. MS-DOS still cannot mount such volumes as in absence of a BPB it falls back to retrieve the FAT ID in the FAT entry for cluster 0 to choose among hard-coded disk geometry profiles. In all formats of a volume formatted under MS-DOS which would otherwise be supported by both systems (and typically also in all other formats), this ID is located in the first byte of logical sector 1 (that is, the volume's second sector with physical CHS address 0/0/2 or LBA address 1), since MS-DOS assumes a single reserved sector (for the boot sector). Under 86-DOS, the reserved sectors area is significantly larger (whole tracks), and therefore the prototypical FAT ID 0xFE (and 0xFF) is located elsewhere on disk making it impossible for MS-DOS to retrieve it, and even if it would, the hard-wired disk profile associated with it would not take this larger reserved sectors region under 86-DOS into account.

CP/M 2 floppy media were readable through RDCPM.

86-DOS did not offer any specific support for fixed disks by itself, but third-party solutions in form of hard disk controllers and corresponding I/O system extensions for 86-DOS were available from companies like Tallgrass, making hard disks accessible similar to superfloppies within the size limits of the FAT12 file system.

Various OEM versions of MS-DOS 1.2x and 2.x supported a number of similar 8.0" FAT12 floppy disk formats as well, although not identical to those supported by 86-DOS.

Disk formats supported by one of the last versions developed by Tim Paterson at Microsoft, MS-DOS 1.25 (March 1982) for the SCP Gazelle computer with SCP controller or Cromemco 16FDC controller (by default, this version only supported the MS-DOS-compatible variants of the 8.0" formats with a single reserved sector but it could be built to provide two extra drive letters to read and write floppies in the previous SCP 86-DOS 8.0" disk formats since 0.42 as well):

In 1984 Seattle Computer Products released an OEM version of MS-DOS 2.0 for the SCP S-100 computer with SCP-500 Disk Master Floppy controller. It added support for 5.25" DD/1S (180 KB) and DD/2S (360 KB) FAT12 formats and supported the older formats as well, although possibly with some of the parameters modified compared to MS-DOS 1.25.


86-DOS Wikipedia

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