Application-Specific Integrated Circuit (ASIC)
An ASIC is an especially manufactured custom chip designed to fulfil special functions.
The main reason is to perform special tasks or combine different electronic components into a single Integrated Circuit (chip).
Amstrad Custom chips
The Amstrad CPC used one custom chip: the video Gate Array (also called VGA – no connection with the Video PC standard).
Latter CPC cost down series included a "pre-ASIC" to merge the VGA and the CRTC.
The Amstrad Plus included a "second heart" simply referred as the ASIC.
CPC+ ASIC's part number is 40489
Emulated chips
CPC+ ASIC emulates the following chips :
- CRTC 6845
- Gate Array
- PAL (128K RAM paging)
- 8255 PPI
- Printer Port Data and Strobe
- Floppy Motor enable flip-flop, Floppy Address decoding (FDC chip Select)
New features
This Amstrad Plus ASIC performs many additional features that the old CPC series couldn't: the "Plus Features".
- Hardware Sprites.
- Soft scrolling (in complement with register 12&13 of the CRTC)
- Screen splitting
- Programmable and vectorised interrupts
- DMA sound channels
- Specific ROM switching
Known Flaws
The Amstrad Plus ASIC improved a lot of the old CPC's capability. Yet this was a bit flawed.
- Despite removing some tasks from the CPU (Z80), ASIC registers are mapped onto memory from #4000 to #7FFF range prior to other type of memory (RAM or ROM).That means this memory range is not accessible when ASIC registers are paged.
- PPI emulation is not correct as the original 8255 does not need validation.On ASIC emulation , this validation is needed so some programs written for "old CPCs" will not be able to get keyboard state.
- Z80 IM2 mode is bugged.In this mode , the Z80 I register gives the high word for vector table.ASIC gives the low word from IVR and the devices that generate interrupt (raster and DMAs channels).ASIC generates sometimes a bad values and the raster interrupt routine is called instead of DMA0 routine.The reasons of this bug are not known.
- There is a conflict between programmable interrupts and some CRTC settings (line screen split).That will cause the RAM refresh to stop and the memory content will be quickly corrupted causing machine crash.
- Reducing Horizontal BLanking could cause another internal conflict when using DMA lists.In the worst case , this conflict can cause irreversible damage to the ASIC.
- Original CPC colors emulation is not correct.