HDOS

HDOS or HeathDOS is an early operating system from Heathkit.

Gordon Letwin, who at the company Microsoft, the leading developer and chief architect of the operating system OS / 2 was later, the operating system developed from 1978. Originally the software for the Heathkit H8 has been implemented, but without changes to the published later H89 run. At first only a few system programs such as an assembler were available for HDOS. Over time, additional software was developed by other manufacturers. Even games such as an implementation of Space Invaders and Pac -Man clone Munchkin were created. Also, the programming languages ​​BASIC, COBOL and FORTRAN were implemented for HDOS.

1983 the cost of a license from HDOS about U.S. $ 150. For a surcharge of around 200 U.S. $ the printed on paper source could be acquired.

Technical Description

HDOS is an 8 -bit operating system for the single-user mode.

Device driver

HDOS was - in contrast to the widely used operating system CP / M - from version 2.0 One of the first operating systems that used rechargeable device driver to achieve a certain degree of device independence and extensibility. A device driver consists of a file with the name XX.dvd, where XX represents the specific character abbreviation of each device. Using the SET command, a device drivers are loaded into memory and configured using various parameters.

Memory management

With its 8-bit wide data bus of the Z80 processor of H89 can address a total of 64 kB of memory. In the lower 8 kB is the ROM that contains the routines for boats and parts of the administrative routines for floppy disk drives. The actual RAM area beginning at the address 2000hex. Between this address and the address 27FFhex an area for various system parameters and system tables as well as for the program stack is allocated by HDOS. The address 2280hex, the usable range for application of the memory, which extends to the address D732hex begins. The area from the address EB56hex is reserved by the operating system, between this address and the address EDAFhex an area for additional device driver is held.

The highest memory block between EDB0hex and FFFFhex contains the memory-resident portion of the system with all system routines such as for interrupt and the device drivers for the attached disk drives.

The area between D733hex and EB55hex is a so-called overlay area. Here, the operating system stores routines that are not constantly needed, and loaded into memory only when needed. If this memory area occupied by an application that accesses these routines, it is cached on a floppy disk. Then, the system routines are stored in the area and run the system call. When the system call is completed, the outgoing parts of the program can be reloaded from the disk back into the memory.

Files

The operating system is divided into four areas:

• The command interpreter. It is included in the SYSCMD.SYS file.
• The memory-resident portion of the system.
• The two parts of the overlay area.
• The device drivers.

Resident: part and the overlay items are included in the HDOS.SYS file.

Included Software

Together with HDOS a line-oriented text editor, an assembler and BASIC variant Extended Benton Harbor BASIC were delivered. The assembler gave the - unusual for this time - opportunity to outsource parts of code into separate files and to include in the source code as needed, by statement.

Version History

As the official publication versions exist 1.0 ( released in 1980 ), 1.5, 1.6, 2.0, 3.0 and 3.02. The first four versions were written by Gordon Letwin. William G. Parrott, David T. Carroll, Dale L. Wilson and Richard Musgrave developed version 3.0 and after the departure of Parrott, Carroll and Wilson extended Richard Musgrave the system even to the latest version 3.02. On 19 July 1989, the version 2.0 of Jim Buszkiewicz, managing editor of the Heathkit Users Group, was issued as Public Domain.