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Old 7th May 2019, 3:31 pm   #19
Join Date: Aug 2009
Location: Worthing, Sussex, UK.
Posts: 519
Default Re: Gemini 80-Bus System

Just been looking at RPM V2.0 - the boot code does the following

1. Check disk is rotating by looking at the index signal
2. Read the first 128 bytes of disk A:, track 0, sector 0 to address 80h
3. Check that the bytes at 80h (first two of track 0, sector 0) are both 47h
4. If the above check passes then copy the 128 bytes at 80h (First 128 bytes
of sector 0) down to location 0.
5. jump to location 2.

The code at sector 0, starting at offset 2 is responsible for loading CP/M

I hope that helps.

PS - If you are thinking of downloading the first sector code from linux, don't use 80h as
the location as RPM uses that is its line input buffer. Use 100h instead.

to copy the buffer at 100h down to zero and execute - try the following

21 00 01 ld hl,100h
11 00 00 ld de,0h
01 80 00 ld bc,80h
ed b0 ldir
c3 02 00 jp 2

This code can be run at any address above 180h and is relocatable. it does the
copy to 0 and then jumps to zero.

Remember the boot sector starts at offset 8 in the .DMP file I sent you.

(At location fc42 in my version of the RPM is the xor a which sets for sector 0). I don't know if Getek can handle sector 0 access).

The first two bytes are GG which might mean Gemini disk as in the above post.

Last edited by JohnBHanson; 7th May 2019 at 3:41 pm. Reason: More comments
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