Creating a NetPC/FlexNet bootable disk image from a FLEXCMI.DSDD80-5.IMA
image. The advantage of using a CPUXXXCMI Flex bootable image is that the
console driver can be reused. Only the disk driver have to be replaced.

To be able to boot from the monitor, I used the SBUG 2.6 code and replaced 
the IDE code with a read routine to load the boot sector.

First the preliminary work, creating disk drivers that use the NetPC/FlexNet 
protocol instead of a Floppy Controller and physical disk or images (Gotek) 
and adapt the Monitor to load the boot sector. Of the routines described in 
the Alteration Guide, only the Read, Write and Verify matter for basic 
operation. The Verify is currently trival as it always returns success. 
The Read and Write routines are developed and tested with the TestProg6.asm 
program. This program is cross-assembled and downloaded with the monitor L 
command. 
For the boot sector, the Alteration Guide Loader program is adapted to use 
a simplified (no checking) version of the Read Routine.
Another required utility is a program to write the loader to the boot sector, 
replacing the missing PUTLDR. 

Finally the Monitor code is modified to contain code to load the boot sector 
from the NetPC/FlexNet server. On the CPUXXXCMI board, there is room for four
2K versions, selectable with the R_A11 and R_A12 jumpers.

For the testing phase, both the Monitor code and the Loader do not jump to the 
loaded code on completion, but return to the Monitor. When all works as it
should, replacing the jump location will result in a self booting system 
with just one key stroke in the monitor.

Details

Common to both are the monitor code to load the Loader from the boot sector, and 
the actual Loader. Before running the Loader the location the .SYS file location 
can be modified to load another .SYS file.

The procedure: 
- replace the standard SBUG 2k monitor V 1.6 with the modified V1.6f.
- start the monitor
- select 'F'
  This loads the boot sector (drive 0, track 0, sector 1) into $C100-$C1FF.
- optionally modify $C105 and $C106 locations to select any .SYS file on Drive 0.
- select '^P' and enter the Loader entry address, $C100.
- select 'G'.
  This loads all sectors of the .SYS file.
- select 'L'.
  This allows downloading the disk driver dump into the Flex O.S. memory, 
  overwriting the FDC code.
- select '^P' and enter the Flex cold entry point, $CD00.
- select 'G'.
  This starts Flex, to the Date entry point and the prompt '+++'.
- execute "save FLEXFN.SYS,C700,DFFF,CD00"
  This creates a working Flex for my CPUXXXCMI based system.

Background information:
- 6809 Flex Adaptation Guide. Contains a skeleton of the Loader program and 
  information on the disk driver API.
- 6809 Flex Advanced Programmer's Guide. Detailed memory map of the configuration 
  parts of Flex.
- https://github-wiki-see.page/m/nealcrook/multicomp6809/wiki/FLEX-internals
  Lots of information on Flex, including a detailed memory map of the Flex core. 

fjkraan@electrickery.nl, 2026-09-26