notes

Any C00000 access switches to mode 0.
On reset mode=0.

00.. .... .... ..bb = set upper 2 bits of addr

mode 0:
00aa aaaa aaaa aaaa = mode->1, set low  14 bits of addr, xmode=0xxx
01aa aaaa aaaa aaaa = mode->1, set low  14 bits of addr, xmode=10xx
10.a aaaa bbbb bbbb = mode->0, set register a to value b
11.. .... .ccc ccc. = mode->1, set low  14 bits of addr, xmode=11xx

mode 1:
.... .... 0.00 ..bb = mode->0, set upper 2 bits of addr, xmode=xx00
.... .... 0.01 ..bb = mode->0, set upper 2 bits of addr, xmode=xxx1
.... .... 0.10 ..bb = mode->0, set upper 2 bits of addr, xmode=xx1x

xmode:
mode 0 read c00000:
0000 = read gfx mem
0001 = read vscroll mem
0010 = read cmap mem
0011 = read gfx mem, high 8 bits unpredictable
1... = lockup
mode 1 read c00000:
.... = lockup

mode 0 write c00000:
0... = xmode->1111
1000 = write gfx mem
1001 = write vscroll mem
1010 = NOP
1011 = NOP
1100 = write cmap mem
1101 = NOP
1110 = NOP
1111 = NOP
mode 1 write c00000:
.... = mode switch to 0
0... = mode->1111
1000 = write gfx mem
1001 = write vscroll mem
1010 = NOP
1011 = NOP
1100 = write cmap mem
1101 = NOP
1110 = NOP
1111 = NOP

Writing 80-ff to register $97 and writing 0000-3fff,00c0 to c00004
causes a block move to take place, source is gfx mem, dest is gfx mem.