A Special [[Cartridge]] with a socket that a allows easy modification of the build in ROM software.
Such thing can be made by "cannibalizing" an existing Cartridge (Let's say... a GX4000 Burnin'Rubber Cartridge, as it is the most common one).
Remove properly the ROM (the biggest chip) but beware to harm nothing. Then but a hole on your Cartridge, add a ROM/EPROM slot...there you are.
Of course the most important chip remains the [[Amstrad Cartridge Identification Device| ACID]].
The Multi EPROM cartridge shown here seems of a proffessionnal quality. The Chip slot is quite good as it allows to remove the EPROM/ROM without harming it.
[[Image:CPC GX4000-Multi EPROM Cartridge-1.jpg|thumb|300px|The Multi Eprom Cartridge]]