An ASIC is an especially manufactured custom Chip designed to fullfill special fonctions.
The main reason is to perform special tasks or combine different electronnic componnents in a single Chip.
The Amstrad CPC used one custom chip : the CRTC (or VGA ? don't remember well)
Latter CPC series included a "pre-ASIC".
The Amstrad Plus include a "second heart" simply referred as the ASIC.
The Amstrad Plus ASIC perform many additionnal features that the old CPC series couldn't...The Plus Features.
- Hardware Sprites.
- CRTC emulation.
- Hardware Scrollings.
- Maybe some interactions with the ACID chip in Cartridges.
- DMA sound channels.
- MAny many more.