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== Technical ==
The following has been derived from the schematic and the ROM code.
The onboard 8KB Ram is read/written via port FCxx and FDxx with the following address decoding:
1111110aaaaaaaaa
'a' form part of the address for the RAM. The data read/written to the port is from the RAM. NOTE: The order of the data bits are scrambled/remapped. See the schematic.
The other bits are controlled by port FExx with this address decoding:
1111111xxxxxxxxx
Here D3-D0 define the other parts of the RAM address.
NOTE: The addresses to the RAM are also scrambled/remapped. See the schematic.
The ROM is made visible by pressing the red button which causes an NMI. It maps the ROM into location 0000-1fff with a mirror at 2000-ffff.
The ROM replaces the internal CPC ROM and also disables the RAM where it is mapped.
The ROM state is also toggled by reading an opcode from an address >0x03000 and less than <0x03fff. The ROM itself uses 3FFB which contains a RET
instruction. It uses this to transition in and out of the ROM to read the font from the BASIC ROM.
== Manual ==