I'm writing a simple program to plot colored squares on the bitmap screen, and I had so much trouble figuring out how to save and load the files afterwards, and using a variable for a filename, that I wanted to post here how I did it. Mostly so that in a year when I want to know again, I can just Google it!
It seems to work fine, but please let me know if my procedures are wrong.
To save the bitmap screen, with A$ = filename:
BSAVE ""+A$+".BT",B15,P8192 TO P16181:REM BITMAP DATA
BSAVE ""+A$+".CO",B15,P7168 TO P8167:REM COLOR DATA
To load the bitmap screen, with A$ = filename:
BLOAD ""+A$+".BT"
BLOAD ""+A$+".CO"
Maybe this'll be useful to others too!