NorthStar S-100 documentation and history

This document (c) copyright Herbert R. Johnson updated Dec 30 2019.
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On this Web page:

  • Northstar documents I have available - ask for details
  • Northstar systems, floppy controllers, other information
  • a bit of Northstar history
  • Northstar DOS and ROM code

    Northstar documents

         disassembly, NorthStar DOS2 and BOOT2, disassembled by Barry
              Watzman. 20 pgs. Also available here on my Web site by
              permission for download, see above.
         disassembly, "NOrthStar DOS2 and BOOT2. Assembled without errors to
            original." Well commented. Also PROM boot code. 20 pgs.
            No guarantees on this!
         disassembly, "DOS 2, BOOT 2" (simple disassembly, no comments). 22 pgs
           No guarantees on this!
         Disassembly, NorthStar DOS release 4 Aug 1978. 25 pages
           No guarantees on this!
         disassembly, PROM from N* DD disk controller, 2 pgs. No guarantees!
         disassembly, NorthStar Double density DOS file. DOS 5.1 1979.
           26 pgs. No guarantees on this!
         Horizon HD-5 HD5x Hard disk Installation Guide, 1981. 20 pgs
         Z80 Macro Assembler manual - copy, ask
         HRD-Z manual - copy, ask
         Hard Disk Operating System (HDOS 2.1.X, 1982)
            I/O entry points, routines included. 210 pgs
         Northstar User Guide (unpacking, use) 16 pgs
         Northstar advertizing and flyers, 8 pgs
         Lifeboat Assoc. CP/M 2 on NorthStar DD/QD User's notes, 25 pgs
         HRAM User/Technical manual 66 pgs
         Micro-Disk System MDS-A manual (rev 5 or no rev) 32 pgs each
         Micro-Disk System MDS-A-D double density manual (rev 2);
             AD3 schematics, 1979 errata: 40 pgs
         Micro-DIsk System MDC schematics ONLY, 4 pgs.
         ZPB-A Manual, 33 pgs
         ZPB-A Manual, rev 3, 34 pgs
         North Star Disk Operating System Ver 2, 20 pgs
         North Star Disk Operating System Ver 2 release 2 rev 5, 20 pgs
         North Star System Software Manual Soft Doc, rev 2.1A, 260 pgs
         North Star System Software Manual Soft Doc, rev 2.1, 260 pgs
         North Star System Software Manual Soft Doc, rev 1, with errata, 260 pgs
         North Star BASIC Version 6 Ver 6 FPB rev 5, 30 pgs
         DDSC #1, The DOS mover (relocating the DOS), 18 pgs
         North Star Monitor, Ver 1, 14 pgs
         Release 4 System Software changes, June 1978, 11 pgs
         Diskette Controller schematics, 4 pgs
         Floating Point Board FPB-A, 22 pgs
         Advantage PIO PCB schematic, 2 pgs
         Advantage AVD PIO Parallel schematic, 2 pgs
         Northstar HRZ-MB-4 addenum, schematic (motherboard), June 1980, 10 pgs
         Northstar 32K RAM, 2 versions, 40 pgs
         Northstar Horizon Computer System manual, Double Density: includes
            MDS-AD2, HRZ-MB2, power supply schematics, assembly and descriptions,
            HRZ-D Version 1. (yellow paper cover) 92 pgs
         Northstar Horizon Computer System manual, Double Density, 25011B: includes
            MDS-AD, HRZ-MB, power supply schematics for 1977-8, 
            technical descriptions. Blue/white cover. 64 pgs
         RAM 16A manual - copy, ask
         BASIC cross reference - copy, ask
         Engineering drawings available: schematics, circuit board layouts,
            parts lists, mechanical layouts. Contact me for details, unusual
            "inside" information from NorthStar products as follows:
            HRZ chassis, motherboard (120V or 230V)
            HRAM (32K, 48K, 64K)
            HRZ-HD5 (including power supply, cables)
            HRZ-UP8 (including PROM source listings, schematics; 26 pages
            HRZ-TIO (paddle board)
            Horizon 8/16: fabrication, power transformers, chassis
            RAM 32-A (schematic only)
    Additional manuals as of April 2004: all by Northstar unless noted otherwise
    	Disk operating system Ver 2 rel 3 1977 revision 5, 18 pgs
    	Hard Disk System users manual #25031B, w/schematic of HDS-18, about 150 pages
    	ADVANTAGE user manual #0451, 1981, hard drive system, some source code, 66 pgs
    	HSIO-4 User/Technical manual 00187b, 1981, 106 pgs
    	System Software Manual, rev 2.1, over 100 pgs
    	Addendum to System Software Manual, rev 2.1 1980, 18pgs + 6 pgs additional
    	BASIC Ver 6 FPB 1977 rev 5, 26 pgs
    	G-CPM preface #00482RD1, 1981 (graphic CP/M), 80 pgs
    	Monitor Version 1 Rev 1 1977 14 pgs
     	64K RAM board Rev 1 1977, 40 pgs
    	64K RAM board rev 2 1978, 40 pgs
    	Microdisk System manual, MDS-A D, DD, rev 2 1978, 40 pgs
    	G-DOS Disk Operating System manual (graphics), #00485 RD1 1981 108 pgs
    	Soft Doc System Software Manual rev 2.1 1979 about 100 pgs
    	Soft Doc Errata Rev 2.1A 1979 10 pgs + 18 pgs notes
    	Z80A Processor Board ZPB-A rev 1 1977 34 pgs
    	User Guide to NorthStar Basic by R. R. Rogers, Interactive Computers publisher, 240 pgs
    	ZILOG Z80 CPU tech Manual (distributed by N*) 1977, 75 pgs
    	CP/M BIOS Manual ref 2.27, 3/82 by Centra Systems 54 pgs
    	CP/M 2.2 preface by N*, #25045A, 46 pgs
    Additional Northstar related manuals, 2006
          InfoSoft NS BASIC double density Program Interface manual, 25 pgs
          HDS Hybrid Development System by Allen Ashley, 1979, 32 pgs
          Northstar "addendum" for various manuals:
            System Software Manual Rev 2.1 July 1980, source, 14 pgs 
            System Software Manual Rev 1 Dec 1978, incomplete. 4 pgs.
    Northstar User Group NSUG newsletter/magazine "The Compass"
    	Vol I, #1, 24 pgs
    	Vol I, #2, 24 pgs
    	Vol I, #3 38 pgs
    	Vol I, #4 42 pgs
    	Vol II, #1, 46 pgs (copy)
    	Vol II, #2, 44 pgs
    	Vol II #3, 40 pgs
    	Vol II, #4, 40 pgs
    	Vol III #1 34 pgs
    	Vol III #3, 60 pgs
    	Vol IV #1, 36 pgs
    	Vol IV special 10 pgs

    Northstar systems, floppy controllers, other information

    Northstar Horizon repairs

    A Northstar system I repaired in 2006 and later is on this Web page link. There's also a 2011 restoration described on that page.

    Confusion about Northstar MDS and MDC floppy controllers

    The Northstar floppy controller board named "MDC" was a single-density controller, first sold by Northstar around 1976. It was sold as part of their "MDS" system of controller, cables, drive cabinet with power and drives, and their operating system. This was one of Northstar's first products - a floppy disk system upgrade for S-100 computers like the MITS Altair, the IMSAI, the Processor Tech SOL, and so on. That's where the MDS/MDC confusion began.

    The Northstar floppy controller board named "MDS" was a single or double-density controller, first sold by Northstar around 1978. By then, they produced the Northstar Horizon S-100 system with this product; and they may well have continued to sell their packaged disk system.

    The two boards both have program PROMs; either one or two depending. They also use a PROM for address decoding. these are all small bipolar or fuse-link PROMS, easily overlooked as "logic ICs". Refer to parts lists and schematics for specifics on those PROMs. the locations of the PROMS confirm which schematics and docs "go with" which board.

    I don't know how cross-compatible the two controllers are. I believe the single-density disks are compatible with both controllers. Of course the double-density disks can't likely be read by a single density controller. I don't use these systems enough to be confident of these statements. I'd appreciate any confirming information and corrections. - Herb Johnson, 2016

    other people's Northstar systems

    Northstar Horizon of Nick Papadonis

    NorthStar MDS damage from reversed floppy cable

    It's easy to "flip" or "reverse" the cable connector between the floppy controller and the edge connector on the drive. Andrew Lynch advised me of the risk this poses to Northstar floppy controllers, in response to some comments of mine as below:

    Most of the time, when a floppy is cabled "backward" - that is upside
    down - the drive light comes on IMMEDIATELY when the drive is powered.
    That's because the drive select lines are active low, and grounding them
    will "select" the drive. Of course the cable has to be hooked up AT BOTH
    ENDS for this result to occur. I"ve never known this mis-connection to
    damage the drive, just power down the stuff and flip the connection.
    Hooking up the drive to the WRONG connector could be a disaster,
    depending.  - Herb Johnson

    Andrew replied: I have seen this often. On PC controllers and drives it usually means nothing but on NorthStar controllers, it is instant death for the controller and the drive. The funny thing is that the red stripe on PC floppy cables is opposite to what it means on NorthStar cables. Now I never know which is which without testing to see which pins are grounded. It just depends so I like to be extra sure before taking the plunge with nearly irreplaceable items.

    As a precaution, I use a separate power supply for the drive and attach the cabling to the host computer with everything off. Then I turn on the drive's power supply, while keeping the computer off, and if it does what you describe, I know the cable is hooked up backwards. Since I am sensitive to this issue, I rarely ever see it but it might save my FDC someday.

    [As to where this is documented,] it is mentioned at the very end of the NorthStar MDS-A-D manual, revison 1, page 40, errata and additional information, paragraph #2. - Andrew Lynch

    From the manual: "Applying power to a disk drive while the ribbon cable is incorrectly plugged in to either the drive or the controller can DO DAMAGE TO THE DISK DRIVE! It is very important that the instructions given in the CABLE CONFIGURATION section of the MICRO DISK SYSTEM document be followed very carefully. DO NOT follow the instructions in the OEM manual and DO NOT use the pin 1 mark on the ribbon cable plug as an indicator of how the plug should be connected." - Herb Johnson

    NorthStar history & products


    In due course I'll have a better history of Northstar computers here. A very early S-100 manufacturer, Northstar offered a floppy disk controller and 5.25-inch floppy drive, with their own DOS, for early S-100 systems. Their early systems used hard-sectored diskettes; they continued to use hard sector for some time. They offered a floating point card built from a 74181 ALU and PROM microcode. They also built their own S-100 system (Horizon) and an integrated system with keyboard and CRT display (Advantage model). Here's a Web link to an informative NorthStar Web site by NorthStar employee Alan Bowker. as of 2007 and 2019. - HErb

    NorthStar Floating Point Board FPB-A

    A note from "Ray" in 2019: "That board uses 256 x 4 PROMs. Ten of these PROMs formed a 40-bit microinstruction word, with the eleventh as condition code decoder. The ALU was a 74181 and RAM was 256 bytes in two 256 x 4 parts. All the board did was add-subtract-multiply-divide; but it had up to fourteen decimal dights of precision (stored as packed BCD) plus one more byte for sign and exponent. And, it ran at 8 MHz."

    A version of NS BASIC, "Floating Point BASIC", operated the board. The FPB-A2 Rev 5 manual includes a schematic and a hex dump of the PROMs. (Other manuals may not include a schematic.) General operation of the microcoded processor is described. But they don't provide an "instruction set". One could reverse engineer that with patience.

    Photos on other Web sites show a FPB-A2 board or mention -A1 and or -A2. In one photo I see MMI6301 PROMs with NorthStar labling which corresponds to the descriptions in the manual. A A1 model board has PROMs with -1 labeling. Thanks to for some of the Northstar info described here., does have some NorthStar disk-images, which may include N* FP BASIC. - Herb Johnson

    Comstar BASIC compiler for NorthStar

    In Aug 2006, Northstar software developer Dave Ashley posted the following in comp.sys.northstar, and permitted me to post it here:

    My dad's name is Allen Miller Ashley. He developed lots of software for 8080/z80, then in the 80's did a lot of business writing cross assemblers. As I recall when [my Dad and I] worked on [the Comstar compiler] I was 14 and a freshman in high school. This would have been about 1979-80. The compiler took NorthStar BASIC programs and produced Z-80 assembly language. This would then get fed to my dad's existing compiler/linker and you'd get a binary, presumably behaving the same as the original program, only it would be much faster and also obfuscated (harder to reverse engineer object code than a BASIC program).

    Also for small programs the resultant binary would be smaller than the BASIC interpreter plus the BASIC program. I remember the documentation had a graph of the likely cutoff point where your BASIC program would end up compiled larger than the original source plus interpreter. It was pretty big though.

    The whole package was $400. The pricing was set to match Microsoft's compiler, which worked for Microsoft BASIC. Maybe $395. Back in those days M$ was just another pissant.

    I wrote a bunch of the support functions. I remember I wrote the PRINT USING implementation and I had to run exhaustive experiments on what N* BASIC would do to make sure my version matched. The behaviour was documented but still you'd need to make it match exactly.

    I remember my coding style drove my dad nuts. We were programming in 8080 or z80 assembly language. I'd have some routine that would do something, and it would call some other routine. Since I was writing both routines, I'd pass arguments in registers in whatever way was convenient. Sometimes I'd use the accumulator, sometimes D, sometimes H. Sometimes DE would be a pointer, sometimes HL would be. In effect you wouldn't know how to call a function unless you went and looked at it. We weren't much on comments. What I was trying to do was avoid lots of push/pops and utilize all the registers fully. Net effect was small, efficient code but it wasn't really maintainable by other people. That didn't matter since I knew how all the functions worked together...

    My dad did the expression evaluator, which would have been pretty complicated for me to attempt at the time. I think I did the file IO stuff also. It's difficult to remember all the pieces. I think the project went on for 3 months or so. It was good money for someone my age to be making.

    I got interested in digital electronics and spent some of the money on parts + other stuff.

    I'd guess we sold between 200 and 300 copies of the compiler. Probably not more than 500. I think later my dad dropped the price to $200 and sold some more. I never paid much attention to the business details.

    Hope the above is useful. - Dave Ashley, Aug 2006

    NorthStar DOS code, ROMS

    By arrangement with Barry Watzman, I have the following disassembled Northstar software on my Web site for download:
    NSDOS.PRN, DOS version 2.? disassmbly printfile;
    NSBOOT.ASM, disassembly of disk boot code

    In 2012 I've added some disassembled ROMS or ROM listings as below. Also see the paper listings of code I have above.

    This bit of assembly code was posted by John Crane on Aug 7 2012 in comp.sys.northstar. It may be a custom ROM, says Bill Sudbrink soon after: "While I do not personally have a N* controller setup for E900, that listing doesn't look very N* to me. I suspect it may in fact be some third party implementation."

    Bill Sudbrink offers these versions, in private communication: "I disassembled the more common 0xE800 boot ROM a long time ago and published it on classiccmp [in 2002]. Allison Parent later verified that my disassembly matched her NorthStar units." I made a local copy and annotated it. Here's that copy of Bill's disassembled N* ROM.

    Copyright © 2019 Herb Johnson

    Herb Johnson
    New Jersey, USA
    to email @ me follow this link