While adding banking support to my own C64 emulator back in the good 'ole days (April 14, 2020, maybe only good for hunkering down on retro stuff), I also wrote a test program to help verify the results. This may help you visualize how you can access more RAM and ROM on the C64.
The program works by switching to the RAM only bank writing a signature high byte to four addresses, then cycles through the standard list of banks reading the value at those locations. Of course the first line for bank zero shows the bytes as expected. But the other banks tell a different story.
A000-BFFF is BASIC ROM. Appears ROM is active in banks 3 & 7 only, otherwise is RAM.
C000-CFFF is always RAM.
D000-DFFF is usually I/O (banks 5-7) including color RAM at D8000 (notice the high nibble changes in this run). May also appear as RAM (banks 0 & 4), CHAR ROM (banks 1-3).
E000-FFFF is BASIC ROM (banks 2, 3, 6, 7), otherwise RAM (banks 0, 1, 4, 5).
It is interesting to see the similarities and differences in this chart.
The key code to switching banks includes
sei
lda $01
and #$f8
ora #bank_selection
sta $01
rts
Only clear the interrupt flag (cli instruction) once you have returned to the normal bank (7) so IRQ vectors and the KERNAL/BASIC ROMs will work as expected.
Links
- Source: bank_test.asm
- Program: bank_test.prg
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