Difference between revisions of "NSC800 Conversion"

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(Notes on use with M100 CP/M)
(How do I get M100 CP/M loaded up?)
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Since the NSC800 and REXMGR are incompatible, use CPM.CO to start CP/M.
Since the NSC800 and RXCMGR are incompatible, use CPM.CO to start CP/M.
For added convenience, I have made a version of CPM.CO as a .DO file, for easy file transfer to the laptop.
For added convenience, I have made a version of CPM.CO as a .DO file, for easy file transfer to the laptop.

Latest revision as of 06:00, 12 October 2021



Current version! (courtesy John Wiggins)

NSC800 1.jpg

What is this all about?

Motivated by 2 things

  • discovery (to me) of the NSC800 Z80 processor that is 80C85 like
  • continuing to work in the direction of CP/M

I have invested some time and effort in developing a conversion that supports NSC800 operation in the M100, both for standard BASIC use, as well as CP/M (in conjunction with REXCPM). Partly this has been motivated simply by curiosity. However, in CP/M it actually broadens the software applicability since quite a bit of CP/M software is Z80.

If you have an interested in doing this conversion please get in touch with me at Twospruces at --the google mail service.

What are the tradeoffs?

When installed, you get an M100 that is ALMOST exactly the same. Differences:

  1.  Z80 not 80C85...ok
  2.  Still runs the same BASIC applications 
  3.  Cassette port no longer works as the 80C85 uniquely has the SID/SOD pins but NSC800 does not.
  4.  NSC800 is NOT COMPATIBLE with REXMGR software, so you can't benefit from the standard REX features when using NSC800.
  5.  almost 100% compatible with M100 software, even machine code.  Except for:
      *  anything that makes use of the hardware interrupts - Timer, RS-232, BCR port - may require a patch to work.
  6.  When combined with REXCPM, you get to run Z80 software in M100 CP/M!  See below section on M100 CP/M use.

The NSC800 processor uses the interrupts in a slightly different way, so machine code that uses the interrupts needs to change a bit. Patches need to be applied to the M100 Main ROM (see below) to tweak things a bit to work.

Required Changes

There are 2 areas of change that are needed. Firstly you have to adapt the NSC800 to the 80C85 socket. There are a couple of tradeoffs to make. Secondly, the NSC800 needs to use a slightly different Main ROM.

To get started on this project, one can leverage the work done in the past. In fact an NSC800 conversion for 80C85 was posted back in the early 80s for S100 computers equipped with an 8080/8085 processor board. The information is posted below, from Microsystems September 1984.

NSC800 conversion1.JPG

NSC800 conversion2.JPG

This article lists considerations; from my work in M100 the things that any adapter has to deal with are listed below.

  1.  Inversion of the interrupt signals
  2.  Conversion of the 50/50 duty cycle RST7.5 signal to a 100 usec low pulse (NSC800 is level triggered not edge).
  3.  4.9152 MHZ clock generation circuitry.
  4.  And in the case of a dual processor conversion, certain NSC800 signals are not tri-state in RESET.

Note: it seems that the M100 RAM/ROM and REX are all tolerant of the short opcode fetch read/write cycle. So, no wait state generator appears to be needed.

While I have implemented both a DUAL 80C85/NSC800 CPU adapter, and a SINGLE NSC800 CPU conversion, I prefer the single conversion for it's simplicity.

1CPU NSC800 conversion PCB

The current schematic is here. So far so good. May change. UPDATE: I have redesigned this board to make it fit better. I have not uploaded that schematic or layout here. Initial PoC board is below.


Layout is being optimized. Current layout is here as an example, but this board is not done yet. Gives you a sense for what's involved.

The BOM is pretty short

  - PCB
  - NSC800 CPU  3MHz variety  (UTsource is good)
  - socket for M100 main board
  - 74HC04 buffer/inverter
  - 4.9152 MHz Oscillator + 2x 27pf caps
  - low profile pin arrangement for connecting PCB to M100 80C85 socket
  Note:  due to height restrictions, you have to solder the NSC800 processor to the adapter (trying to resolve this)



Modified M100 Main ROM

As mentioned, since the interrupt control mechanisms in NSC800 are slightly different, the M100 main ROM needs to be patched to support NSC800.

The NSC800 patch needs extra code space to be created in the ROM. To do that, an original patch has been verified that creates a block of unused space in the main ROM as published by Microsoft.


Once this base patch is applied to the ROM, you have space now for the next set of patches, here.


All these patches rolled up into a binary: (here I used the T102 base ROM as the starting point)


These are what I use today. If I find bugs I will post updates.

Notes on use with M100 CP/M

Probably, you have installed an NSC800 processor and supporting M100/T102 main ROM, so that you can use M100 CP/M and gain the benefit of Z80. I suppose to be fair it is possible to run the NSC800 in the laptop, and yes it would be a Z80 - so you could write new programs that are based on Z80 opcodes. Not sure that would be worth it.

So, how does one get M100 CP/M rolling with the NSC800? CP/M 2.2 is generally based on 8080, so the operating system itself is fine. Well you for sure need a REXCPM, either 2MB or 4MB. I will assume that you would have already been familiar with setting up M100 CP/M using the standard config with 80C85 installed.

There are a few major considerations.

  1.  RXCMGR software does NOT support NSC800!  RXCMGR makes extensive use of the 80C85 undocumented opcodes, so it won't run on NSC800.
  2.  Installing and running CP/M relies solely on the tools provided for M100 CP/M.
  3.  In order to load up the M100 CP/M utilities, you will need a DOS of some kind.  See below.
  4.  M100 CP/M, as it comes in standard form, IS generally compatible with NSC800.  
  5.  IMPORT and EXPORT are not written using 8080 opcodes, so you will need replacements.  See below.
  6.  If you want to use the BCR Hack for external video over MVT100, you need to (1) change the hack slightly and (2) patch CP/M.  See below.
  7.  Also note - the utilities for backup/restore of REXCPM also do not work on NSC800.

How do I transfer files into the laptop?

TELCOM application in M100 still works with M100. So, any .DO file is loadable using the standard methods with TELCOM.

  But why?  What about the interrupt issue?  
  ... in the modified T102 main rom, I have patched it up so that the serial port still works correctly.
  The T102 main rom provides routines for getting and sending data over RS-232 and those routines work fine.
  However, any serial port software that uses POLL MODE will not work correctly as the NSC800 does not have an opcode like SIM/RIM.
  --> so as a general rule I would say - assume that serial port and BCR port work fine with NSC800, but it is possible that 3rd party software
  might try to POLL the interrupt status and that could cause a problem.  Try first, and decide.

TELCOM alone does not solve the problem, since .CO and .BA programs are not text. What to do? Well, what about TEENY? TEENY happens to make use of SIM opcode unfortunately. So, I have built on the great work of Ron Wiesen, and modified TEENY to run on NSC800.

I have been using NTEENY below and do not observe any problems, but please do report back if you find any issues. This is for NSC800 only, please use standard TEENY on 80C85.


This is not as fancy as Ron's because it loads and runs at a fixed location 62213, but it should suffice.

How do I get M100 CP/M loaded up?

The best way to get M100 CP/M running with NSC800 is to start with a known good M100 CP/M installation, and then convert the hardware.

The currently posted version of M100 CP/M happens to use 8085 undocumented opcodes. A modified version of M100 CP/M has been prepared that is pure 8080.


  This version of M100 CP/M includes the following changes
  1. all 8085 undoc opcodes have been removed, and the code reworked.
  2. a patch has been applied to support the NSC800 version of the BCR hack.

Because stock IMPORT/EXPORT don't work with the NSC800, that problem needs to be solved BEFORE you install the NSC800 CPU.

I modified Philip's IMPORT and EXPORT routines to eliminate the 80C85 undoc opcodes and SIM/RIM. The result are 8080 based versions called IMP80 and EXP80, below.


Steps to get M100 CP/M and NSC800 running:

  0.  Modify the M100 to put a socket in place where the 80C85 is.  Install the 80C85 for now.
  1.  Start with a freshly installed REXCPM in an M100, with no CP/M installed.   So, a blank REXCPM.
  2.  Follow Philip's procedures to get an initial M100 CP/M system up and running.  Use the posted files.
  3.  Run M100 CP/M from the laptop LCD, not RS-232 or CASS (BCR).
  4.  Using IMPORT, transfer into CP/M the modified IMP80 and EXP80.  Test that they work as expected.
  5.  Update M100 CP/M to use CPM440_patched.bs, by using CPMUPD.CO.  Change the filename to CPM440.bs.
      Switch back to M100 mode.  Run the command CPMUPD.CO CPM440.bs
      This will update the CP/M operating system.

OK, now you have a system ready to transplant the NSC800. Next..

  6.  Open the M100, and install (1) the NSC800 and (2) the modified main ROM.
  7.  Close up the laptop, and power up.  You should see that the NSC800 is working.
  8.  Now, start CP/M and you should be up and running.

Since the NSC800 and RXCMGR are incompatible, use CPM.CO to start CP/M.

For added convenience, I have made a version of CPM.CO as a .DO file, for easy file transfer to the laptop. Use this if your CPM.CO file gets corrupted.


Comments welcome on this procedure!

What about using the BCR Hack for external video?

Ah, great question! As you know M100 CP/M supports 3 types of video

  1.  M100 native LCD
  2.  VT100 video over RS-232
  3.  VT100 video over (a suitably modified) BCR port

With NSC800 processor, since we don't have the SOD pin and the SIM opcode, the BCR hack needed to change. That change is documented here:


This change requires a bit of software change to work, though. To make M100 CP/M send video data over this modified BCR hack, a patch is needed.

The patched CPM440.bs file posted above includes this patch.