The system boots via the code on the EEPROM-chip, if you wish to change the monitor, or the bootup process you'll want to update that, which is a two-part process:
- Making the appropriate change in source, and rebuilding the firmware.
- Uploading the firmware to the EEPROM, so it will actually be used.
Rebuilding the firmware should be a simple matter of running make within this repository:
$ rm *.hex
$ make
pasmo cpm.asm cpm.hex
$ ls -l *.hex
-rw-r--r-- 1 skx skx 18779 Mar 27 19:12 cpm.hex
$
As you can see the firmware is created from the source cpm.asm, and this in turn includes other files as appropriate.
To update the flash on the EEPROM you'll need a hardware programmer, I'm using a generic "TL866II Plus Universal Minipro Programmer", which I chose from AliExpress based solely the seller which claimed fastest shipping.
As I'm on a GNU/Linux host, running Debian, I installed the software from this repository:
The EEPROM is labeled "28C256 - 32K EEPROM" in the system schematic so this, which helps narrow things down nicely.
To backup the existing contents of the EEPROM:
$ minipro -p "AT28C256" -r eeprom.bin
Send the contents of cpm.hex to the device:
$ minipro -p "AT28C256" -w cpm.hex
Once written you can, and should, verify the contents, for safety:
$ minipro -p "AT28C256" -m cpm.hex