Mods to ROMBOOT.ASM

[Peter Sterrantino]

Hi All,  

Based on a Mike Douglas YouTube video titled "CP/M on the Altair 8800 without a Floppy Controller or Disk Drive" and a conversation with Joe Corda, I decided my next thing would be to try and get my IMSAI up and running with CP/M using this method before i invested in drives and figuring out my Compupro Disk 1 card. Basically, it would be a good introduction to CP/M since I know nothing about it. 

After a few viewings of the video and a review of the links, I believe I will need to modify the serial bootloader ROMBOOT.ASM for my Interfacer 1 card, but otherwise I think that would work. I would add ROMBOOT.ASM into my ROM at FC00h and see that  it would cohabitate nicely with my Interfacer 1 modified ALTMON at F800h in the last 2k of the 64k address space.

I've downloaded all the software and see I need to modify ROMBOOT.ASM to correct the port addresses and the control flag equates. Also noticed an optimization I ran into when i modified ALTMON for the Interfacer 1 card. The srByte function (specifically in srbLoop) where the data ready flag is rotated (via a RRC instruction) into the carry flag vs. a masking operation with SIORDRF. I fixed this with an extra RRC since the flags are reversed on the Interfacer. I assume this was done for speed as Mike really knows his stuff. I only see this once, but wondering if there is anything else I need to check for? 

Also wondering if there is anything I could do to check proper operation of the serial bootloader before i attempt to connect to the FDC Server, just to make sure things are working properly. And also seeking confirmation that the whole scheme is a valid thing to try if anyone else has done this....  

Thanks in advance. 

Pete

[Patrick Linstruth]

Peter, you will also need to modify the CP/M disk for the interfacer 1 board. The first track of the disk image contains the second stage boot loader which is looking for a 2SIO board, and the BIOS is also configured for 2SIO. Port 0 console and Port 1 serial drive server.

If you just want to experiment, I can loan you a 2SIO board.

It’s also possible to make a new CP/M disk that will work with Interfacer 1 using a simulator.

Does the interfacer 1 have two serial ports?

Sent from my iPhone

On Feb 23, 2026, at 10:06 AM, Peter Sterrantino <sterra...@gmail.com> wrote:

Hi All,  

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[Peter Sterrantino]

Hi Patrick, 

Thanks for that. Looking thru the bootloader code I did get the feeling that there was more to it. 

And yes, the Interfacer I has two serial ports. 

When you say: 

It’s also possible to make a new CP/M disk that will work with Interfacer 1 using a simulator

Are you implying SimH? I have been meaning to try that out. The Interfacer 1 has two ports but the I/O address is different, status and data registers are reversed, and the bit flags for TX buffer empty and RX data available are different. I can modify everything but the  bit flags by jumpers, but I would then still have to modify the Second stage bootloader and the bios anyway. 

I’d love to try your board, but before I ask, let me look into SimH and the details of misting the disk image as you suggest. 

Pete