NitrOS-9 boot from ROM

One of the shortcomings of this CMS 9619 system is the fairly complicated OS9 bootstrap procedure I had to use in the absence of any sort of disk drives. To summarize:

1. DEBUG19 (in ROM) starts
2. Host computer transfers a small loader program using DEBUG19's memory edit routine
3. DEBUG19 jumps to the loader program, loading the kernel track from the drivewire server and runs it
4. NitrOS-9 starts and continues loading the rest of the bootfile from drivewire

This works pretty well, but has several drawbacks:

• 8K of address space is taken up by DEBUG09 ROM
• The host computer needs a separate script to efficiently transfer the loader program using DEBUG19
• 2 x 256 byte blocks of RAM need to be reserved to hold the RAM interrrupt vectors and data for DEBUG19

I had hoped that the kernel file could run from ROM and directly load disk images from drivewire. Unfortunately, the NitrOS-9 code is tied closely to the CoCo's hardware, and always relocates the kernel from ROM to RAM. To combat this problem, I replaced NitrOS‑9's relocation routines and added a module to initialize the CMS 9619 hardware for booting directly into the NitrOS-9 kernel. From there, the NitrOS-9 booter (on ROM) will load a disk image over drivewire. This is very similar to the set-up for the CMS 9639 (and I assume other computers sold with OS9). NITROS9 ROM BOOT NitrOS-9/6809 Level 1 V3.3.0 CMS 9619A (C) 2014 The NitrOS-9 Project ** DEVELOPMENT BUILD ** ** NOT FOR DISTRIBUTION! ** Wed Dec 16 16:07:07 2020 http://www.nitros9.org * Welcome to NitrOS-9 Level 1 * * on the CMS 9619A ASBM * yyyy/mm/dd hh:mm:ss Time ? 2020/12/12 05:55:55 >> Clock Initialization Errors << December 12, 2020 05:55:55 Shell OS9:

(The Clock init Error is because the code currently expects the DEBUG19 clock setting routines to be installed in ROM, which they are not. I still need to add these.)

It took some careful configuration to ensure that the CMS 9619 can boot from the DEBUG19 loading process OR directly from ROM, with no wasted memory in either case. Since the final NitrOS-9 boot ROM is less than 4K in size, an abbreviated DEBUG19 can also be programmed into the EEPROM. The code is available on Github.

On the note of programming EEPROMs, I discovered back when I was disassembling DEBUG19 that I can use DEBUG19's memory edit routine to program an EEPROM on the board. Of course, the timing is all wrong and the editor returns an error because the EEPROM takes much longer to store the value than RAM would. With some help from an expect script, it is fairly easy (though slow) to continuously run the memory edit routine, once for each byte, and ignore the errors. So, I can program EEPROMs on board by simply moving a jumper and running the script. If anybody has one of these boards without a ROM, I would be happy to send you a fully programmed EEPROM with NitrOS-9 and DEBUG19. In fact, I think I will add that to my tindie store soon!

CMS 9619 Drivewire Boot to NitrOS-9

I finally got drivewire running on the CMS 9619 SBC. As a result, I managed to complete the bootup and have a virtual disk! Source code available on github.

OS9 shell on a real CMS 9619!

I finally got it working thanks to the advice of a kind member of the Tandy Color Computer (CoCo) discord channel. Interrupts! Once I connected the 6551 ACIA's IRQ line to the 6809's IRQ (using the CMS 9619's TS8 jumper block), the OS9 shell came alive:

You can see the system start up with the ROM's "DEBUG19" prompt. Then it proceeds to:

1. clear some memory
2. use the edit memory command to enter in the ZMODEM program and a mini boot-loader
3. jump to the boot-loader to get the first part of the kernel into memory sector by sector from a disk image
4. jump to the NitrOS9 Kernel.
5. The kernel starts loading in more sectors using its ZMODEM boot module, until it finally runs the terminal shell and gives me an "OS9:" prompt.

And now, with interrupts enabled, the shell responds! Now I need a proper ZMODEM file manager to load in and run other programs.

Fortunately... Unfortunately...

While I was waiting for my RAM to arrived, I tried burning an EEPROM for NitrOS-9 so that I could make the load time shorter and avoid the relocation problems I thought I was having. Well, it did not work.

Filled up Sockets! This would have cost
many hundreds of dollars back in the day.

Fortunately, my SRAM chips arrived early, so I got to work on installing them. Since the board expects EPROMS where I am putting the SRAM, I had to remove some jumpers that disabled writing to the sockets. I felt bad removing the beautifully wire-wrapped jumpers and replacing them with plastic jumpers, but it is for a good cause. With writing to the SRAM enabled, I tried booting. I thought it worked perfectly at first, but I realized I had run my expect script for the emulator instead of the real board. When I ran the correct script, I was sorely disappointed to find that everything was getting stuck just before the kernel was supposed to get loaded. After a lot of debugging, I figured out the problem. I had padded out the boot modules to align with the beginning of memory pages so that everything was easy to debug and compare to the source listings. Unfortunately, the emulated MC6809 was much more tolerant of scanning past the fill characters than the real MC6809. Once I got rid of the padding, everything started working... to a point. Where I was supposed to be greeted with the NitrOS-9 startup screen, I was getting gibberish:

Fortunately, this particular type of gibberish is recognizable as a baud rate mismatch, which makes sense. NitrOS-9 uses a completely different system for getting the baud rate than the CMS 9619 uses. After an adjustment to the terminal description files, I had an improvement:

YES! I was so excited until I hit the return key and... nothing. It does not respond with another prompt or an error or anything. Just stuck. More work to do...

NitrOS9 on (an emulated) CMS 9619

I have been working on some ideas to expand my CMS 9619 SBC. I thought an SD card interface would be a nice option, but I didn't want to use any modern microcontrollers or FPGAs. I had a circuit pretty much worked out, but then thought a Compact Flash card would work better for 8 bit transfers.

While working on a circuit to interface with the MC6821, I realized that I already have a nice RS232 connection to the computer, why not use it? So, I wrote a 6809 assembly tool for the CMS 9619 debugger (debug19) to download blocks of data using the zmodem protocol.

This was working so well, that I figured I could use it to boot load NitrOS-9 on the board. Since I am moving soon, my computers are all packed away, but I couldn't wait to try it. I made some changes to sbc09, a simple 6809 emulator, to emulate the CMS 9619. Here is a video of the boot process to NitrOS9 all the way to the shell:

And it works! I need to finish a real zmodem driver to get it running other commands on the disk image though. Once I move, I can test it out on a real computer.