Modified Creed Monitor for the 8008


The standard SCELBI-8H doesn't include any EPROM.  This makes booting the system fairly laborious, as you must toggle in a boot loader program using the front panel, before you can load any significant applications.  This issue was solved by SCELBI with the development of a rather sophisticated monitor that was released with the SCELBI-8B.   That didn't help the original owners the 8H, but at least one owner, solved the problem by hacking an EPROM onto the back of one of his SRAM cards.  Here is the back of Steve Ciarcia's original 8H with EPROM installed.  I'm not sure what's in the EPROM, but I can assume that it is some kind of simple monitor/bootloader.

Steve Ciarcia's 8H with EPROM

Though I have toyed with the idea of doing something similar with my SCELBI-8H, I just never had the motivation to do the modification.  For some reason that I have since forgotten, I was recently scanning through online copies of Micro-8 newsletter, when I ran across an 8008 monitor that could run in 256 bytes.  Micro-8 newsletter was an amateur newsletter that was put out mostly for owners or those interested in the Mark-8 Computer.  The monitor was called the Creed Monitor and was described starting on page 58 of volume 1, number 7 of the newsletter.  It was published on May 20, 1975. 

I figured that this 256 monitor would just the thing for the SCELBI-8H and I decided to see if I could get it to work in my SCELBI emulator.  After about a week of hacking I had practically rewritten the entire monitor.  I made many, many changes.  The final version of this monitor also requires the use of 3 bytes of SRAM in order to create jump instructions.  I'm calling the final result MCMON - or Modified Creed Monitor.
  1. I had to free up some memory, so I could fit the bit banged serial driver that I use - the original system apparently had a UART
  2. I removed a tape driver and added my own serial downloader
  3. The original monitor was written for BAUDOT.  I modified it to read and write ASCII
  4. The use of RST instructions was eliminated so that MCMON could be moved to anywhere in memory
The final version supports a very simple command set that accepts single character commands

Here is some example output captured on a PC being used as a terminal

>I 301-010:044
>I 301-011:007
>I 301-012:320
>I 301-013:301
>I 301-014:002
>I 301-015:002
>I 301-016:002
>010H 010-016:013
>007H 007-016:002
>D 007-015:002
>D 007-014:002
>000L 007-000:104
>D 006-377:200
>I 007-000:104
>I 007-001:026
>I 007-002:017
>D 007-001:026
>D 007-000:104

In order to use MCMON, you may need to replace the bit banged serial input and output routines with routines that suit your particular system.  This version is set up for half duplex, 2400 baud, 8 bits and no parity.  It also assumes the MSB bit of incoming and outgoing characters are zero.  Early versions of my OS/X emulator assumes ones, so this version of MCMON will only work in version 1.1 (or later) of the emulator.  That version allows for optionally setting and stripping the MSB of serial data.  This version of MCMON is set up to run from 017-000 and uses 3 bytes of SRAM located at 016-374.  Baud rate can be adjusted by tweaking timing loops.  I have provided the spreadsheet I use for these calculations, in the zip package.

Download the MCMON zip package from this link.

SCELBI-8H owners should also see my page that describes how I added a 256 byte EPROM to a SCELBI 1K SRAM card.



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