This list shows only the internal standard I/O ports, as used in the BIOS ROM of the CPC 464/664/6128 BIOS, and in the AMSDOS ROM of the CPC 664/6128 and DDI-1. For a more complete list, including all Peripherals, see the Complete I/O Port Summary.
| I/O | Decoded as | Port | Read | Write |
| #7FXX | %01xxxxxx xxxxxxxx | Gate Array | - | Write |
| #BCXX | %x0xxxx00 xxxxxxxx | 6845 CRTC Index | - | Write |
| #BDXX | %x0xxxx01 xxxxxxxx | 6845 CRTC Data Out | - | Write |
| #BEXX | %x0xxxx10 xxxxxxxx | 6845 CRTC Status (as far as supported) | Read | - |
| #BFXX | %x0xxxx11 xxxxxxxx | 6845 CRTC Data In (as far as supported) | Read | - |
| #DFXX | %xx0xxxxx xxxxxxxx | Upper ROM Bank Number | - | Write |
| #EFXX | %xxx0xxxx xxxxxxxx | Printer Port | - | Write |
| #F4XX | %xxxx0x00 xxxxxxxx | 8255 PIO Port A (PSG Data) | Read | Write |
| #F5XX | %xxxx0x01 xxxxxxxx | 8255 PIO Port B (Vsync,PrnBusy,Tape,etc.) | Read | - |
| #F6XX | %xxxx0x10 xxxxxxxx | 8255 PIO Port C (KeybRow,Tape,PSG Control) | - | Write |
| #F7XX | %xxxx0x11 xxxxxxxx | 8255 PIO Control-Register | - | Write |
| #F8FF | N/A | Peripheral Soft Reset (MC_BOOT_PROGRAM and MC_START_PROGRAM do OUT [F8FF],FF) | - | Write |
| #FA7E | %xxxxxx0x0 0xxxxxxx | Floppy Motor Control (for 765 FDC) | - | Write |
| #FADC | ? | Amstrad Serial Interface Z80-SIO / DART port A Data Register | Read | Write |
| #FADD | ? | Amstrad Serial Interface Z80-SIO / DART port A Control Reg. | Read | Write |
| #FADE | ? | Amstrad Serial Interface Z80-SIO / DART port B Data Register | Read | Write |
| #FADF | ? | Amstrad Serial Interface Z80-SIO / DART port B Control Reg. | Read | Write |
| #FB7E | %xxxxxx0x1 0xxxxxx0 | 765 FDC (internal) Status Register | Read | - |
| #FB7F | %xxxxxx0x1 0xxxxxx1 | 765 FDC (internal) Data Register | Read | Write |
| #FBDC | ? | Amstrad Serial Interface 8253 Timer counter 0 | Read | Write |
| #FBDD | ? | Amstrad Serial Interface 8253 Timer counter 1 | Read | Write |
| #FBDE | ? | Amstrad Serial Interface 8253 Timer counter 2 | Read | Write |
| #FBDF | ? | Amstrad Serial Interface 8253 Timer Modus Select | - | Write |
- The three 765 FDC floppy ports are contained in CPC 664/6128/Plus and DDI-1 only.
- The Amstrad Serial Interface ports are pre-defined as shown above in the AMSDOS ROM. However, neither the CPC 664/6128/Plus nor DDI-1 do actually contain the corresponding RS232 hardware.