With CGH programs you have perfect tools to build programs for your PC-1500.
Writing a program in ML (Machine Language) is hard to do and the best is to use some existing code.
The best library is the system ROM (Read Only Memory).
There is a lot of useful routines that will help you.

Some of my documents are from the early 80’s and written by Reiner PUCH.
Here is a list of really interesting ROM routines :
Download

Thank you Herr Reiner PUCH !
If you have same interesting papers please contact me.