I have really struggled to do ROM to RAM mapping on the C64 and Atari 8-bit. The processes is very much alike, between the two systems. The problem is making the process consistent. Additionally, re-running the same program with the same memory calls causes crashes. I have in the time being just made 1.1 versions very simple. I wanted to use the same graphics that I made for the TI-99/4A. In theory that is possible. I just have not been able to consistently do so.
My compromise:
Instead of using this code:
100 MEMTOP=PEEK(106)
110 GRTOP=MEMTOP-4
120 POKE 106,GRTOP
140 GRAPHICS 0
160 CHROM=PEEK(756)*256
170 CHRAM=GRTOP*256
500 FOR N=0 TO 1023
510 POKE CHRAM+N,PEEK(CHROM+N)
520 NEXT N
540 POKE 756,GRTOP
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