Next, we'll look at support for a built-in game.
ColecoVision with 64K + 64K?
Well, not really, at least you can't address 64K RAM plus 64K ROM. But, you can have 64K RAM plus 8K BIOS plus 32K ROM, so a total of 104K memory. Plus support for carts, of course. Not bad, huh?
The hardware supports 32K of "extended ROM" to be mapped in place of the cartridge, with e.g. a built-in game of up to 32K that is automatically started if no cartridge is plugged in. Obviously this requires a 64K EPROM instead of an 8K/32K EPROM. So, to enable this feature, set the solder jumpers SJ1 and SJ2 to 64K to support a 27C512 EPROM.
Since most of what we need - especially the memory configuration register bits - is already in place, we just need to add one component:
U50 74LS139D (SO16)
which is responsible for enabling either the top 32 K of RAM or the top 32K of the BIOS EPROM. Once it's in place, just
Set SJ6
to enable mapping the top 32K of BIOS EPROM to cart space. Don't worry, as long as you use the standard BIOS, it will leave the memory map register alone and nothing will ever be mapped there.
Custom BIOS #1
To prepare a custom BIOS that checks if a cart is inserted and enables the extended ROM if that is the case, we just need a few steps.
Using a hex-editor, load a ColecoVision BIOS ROM and go to location $13FF and change these seven bytes:
$21 -> $3E ;ld a,07h ;set D3=0, D2=1 (enable Exp. ROM), D1=1 (enable BIOS)
$79 -> $07
$14 -> $D3 ;out ($7F),a ;write memory mapping register
$11 -> $7F
$AA -> $C3 ;jp $13AB ;retest for cartridge
$01 -> $AB
$FD -> $13
Then program the modified 8K BIOS to addresses 0-$1FFF of a 27C512 EPROM. After that, program the game of your choice starting at location $8000 up to $FFFF (depending on the size of the game, up to 32K, less is also ok).
Insert BIOS and test with and without cartridge. Verified to work with 16K Donkey Kong.