The ROM Select port (&DFXX) is decoded and has 2 roles.
First is to select which ROM page appears in the range &C000-&FFFF of the Z80's address space. (It seems bit 7,6,5,4 must be 0).Bits 3..0 are decoded as the page number. Page 1 and 2 only seem to be visible at this time.
The second role (when bit 5,4 are used), makes a page visible in the range &4000-&FFFF of the Z80's address space.Bit 5 seems byte sent to be rom enable/disable, bit 4 this port is set for the second role (0 for other), and bits 3..0 are the page number.decoded as:x0rrrrrr
The exact decoding of Bit 6 must be 0. Bit 7 is ignored. Bits 5..0 (rrrrrr) define the ROM page to show. Brunword pages appear in ROM select port is unknownpositions 1,2,3 only. They are repeated every 64. No other pages of the Brunword ROM can be selected in this way.The exact function upper ROM enable/disable mechanism works with them, and so does the write through to RAM. They operate just like standard CPC Roms. The second role makes a page visible in the range &4000-&7FFF of all the Z80's address space. The following applies:x11bbb0pp bbb define the 64K Bank. pp define the 16K page within that bank.bit 7 is ignored.Bit 6 and 5 must be 1. Bit 2 must be 0. All pages of the Brunword ROM can be selected this way including 1,2,3. The upper ROM enable/disable mechanism doesn't work, so the ROM data bits appears in the range &4000-&7fff until another ROM selection is made. I (Arnoldemu) can't yet determine if the RAM write through works either. The page is always visible in the range &4000-&7fff. The values repeat every 128. If bit 6,5 and 2 are not defined as above then no page is selected, the ROM page is not knownvisible and it works just like normal RAM.
'''MK4'''
