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8255 Part numbers used in the CPC during its lifetime

  • NEC D8255AC-2
  • NEC D8255AC-5
  • Toshiba TMP8255AP-5

All of these are almost identical in their operation. It is possible to detect each version by writing and then reading from the ppi control i/o port. Each can give a different pattern of values that are read back.

The 8255 in the CPC

The 8255 PPI chip is a general purpose input/output IC. This document will describe its role in the Amstrad CPC, CPC+ and KC compact systems. To understand its full functions please read the datasheet.

In these systems it is connected to the AY-3-8912 Programmable Sound Generator (PSG), keyboard, cassette recorder, the VSYNC of the 6845 CRTC and the "busy" signal from the parallel port.

The PPI is selected when A11 of the I/O port address is set to "0", A9 and A8 then define the PPI function access (as shown below), A15-A12 and A10 should be "1" (to prevent conflicts with other hardware), A7-A0 are don't care. So, resulting Port addresses are:

I/O address A9 A8 Description Read/Write status Used Direction Used for
&F4xx 0 0 Port A Data Read/Write In/Out PSG (Sound/Keyboard/Joystick)
&F5xx 0 1 Port B Data Read/Write In Vsync/Jumpers/PrinterBusy/CasIn/Exp
&F6xx 1 0 Port C Data Read/Write Out KeybRow/CasOut/PSG
&F7xx 1 1 Control Write Only Out Control

In the CPC+, the 8255 is integrated into the ASIC. The "emulation" is not complete and some functionality is not available. Please see the "Extra CPC+ documentation" for more information.

  • Mode 1 (Strobed Input/Output) and Mode 2 (Bi-Directional Bus), as far as I know, are not used by any program, Mode 0 (Basic Input/Output) is always used.

Port Usage

  • NOTE - If you are using the firmware, always return the operating modes and I/O state of the ports used to their settings below. The firmware expects the settings to be the same as given below and may operate incorrectly if they are not.

PPI Port A

Operating system settings:

  • I/O Mode 0,
  • For writing data to PSG all bits must be set to output,
  • for reading data from PSG all bits must be set to input (thereafter, output direction should be restored, for compatibility with the BIOS).
Bit Description Usage
7-0 PSG.DATA PSG Databus (Sound/Keyboard/Joystick)

PPI Port B

Operating system settings:

  • I/O Mode 0,
  • Input
Bit Description Usage in CPC Usage in KC Compact
7 CAS.IN Cassette data input Same as on CPC
6 PRN.BUSY Parallel/Printer port ready signal, "1" = not ready, "0" = Ready Same as on CPC
5 /EXP Expansion Port /EXP pin Same as on CPC
4 LK4 Screen Refresh Rate ("1"=50Hz, "0"=60Hz) Set to "1"=50Hz (but ignored by the KC BIOS, which always uses 50Hz even if LK4 is changed)
3 LK3 3bit Distributor ID. Usually set to 4=Awa, 5=Schneider, or 7=Amstrad, see LK-selectable Brand Names for details. Purpose unknown (set to "1")
2 LK2 Purpose unknown (set to "0")
1 LK1 Expansion Port /TEST pin
0 CRTC VSYNC Vertical Sync ("1"=VSYNC active, "0"=VSYNC inactive) Same as on CPC
  • LK1-4 are links on the mainboard ("0" bits are wired to GND). On CPC464,CPC664,CPC6128 and GX4000 they are labeled LK1-LK4, on the CPC464+ and CPC6128+ they are labeled LK101-LK103 (and LK104, presumably?).
  • Bit5 (/EXP) can be used by a expansion device to report its presence. "1" = device connected, "0" = device not connected. This is not always used by all expansion devices. is it used by any expansions? [in the DDI-1 disc interface, /EXP connects to the ROM bank selection, bank 0 or bank 7]
  • If port B is programmed as an output, you can make a fake vsync visible to the Gate-Array by writing 1 to bit 0. You can then turn it off by writing 0 to bit 0. It is fake in the sense that it is not generated by the CRTC as it normally is. This fake vsync doesn't work on all CPCs. It is not known if it is dependent on CRTC or 8255 or both.

  • For more info on LK1-LK4 (and further LKs) see LK Links

PPI Port C

Operating system settings:

  • upper: I/O Mode 0, lower: I/O mode 0,
  • upper: output, lower: output
Bit Description Usage
7 PSG BDIR PSG function selection
5 Cassette Write data Cassette Out (sometimes also used as Printer Bit7, see 8bit Printer Ports)
4 Cassette Motor Control set bit to "1" for motor on, or "0" for motor off
3 Keyboard line Select keyboard line to be scanned (0-15)

PSG function selection:

Bit 7 Bit 6 Function
0 0 Inactive
0 1 Read from selected PSG register
1 0 Write to selected PSG register
1 1 Select PSG register

PPI Control

This register has two different functions depending on bit7 of the data written to this register.

PPI Control with Bit7=1

If Bit 7 is "1" then the other bits will initialize Port A-B as Input or Output:

 Bit 0    IO-Cl    Direction for Port C, lower bits (always 0=Output in CPC)
 Bit 1    IO-B     Direction for Port B             (always 1=Input in CPC)
 Bit 2    MS0      Mode for Port B and Port Cl      (always zero in CPC)
 Bit 3    IO-Ch    Direction for Port C, upper bits (always 0=Output in CPC)
 Bit 4    IO-A     Direction for Port A             (0=Output, 1=Input)
 Bit 5,6  MS0,MS1  Mode for Port A and Port Ch      (always zero in CPC)
 Bit 7    SF       Must be "1" to setup the above bits
  • CAUTION: Writing to PIO Control Register (with Bit7 set), automatically resets PIO Ports A,B,C to 00h each!
  • In the CPC only Bit 4 is of interest, all other bits are always having the same value. In order to write to the PSG sound registers, a value of 82h must be written to this register. In order to read from the keyboard (through PSG register 0Eh), a value of 92h must be written to this register.

PPI Control with Bit7=0

Otherwise, if Bit 7 is "0" then the register is used to set or clear a single bit in Port C:

 Bit 0    B        New value for the specified bit (0=Clear, 1=Set)
 Bit 1-3  N0,N1,N2 Specifies the number of a bit (0-7) in Port C
 Bit 4-6  -        Not Used
 Bit 7    SF       Must be "0" in this case

Programming Examples

1. Using the control byte

  • Setting bit 7 of port C to 1,
            LD B,&F7                ;8255 Control port
            LD A,%00001111          ;Bit Set/reset function
            OUT (C),A               ;Send it to 8255
  • Set port A to input, operating in mode 0, port B to output, operating in mode 0 and port C to input, operating in mode 0.
            LD B,&F7                ;8255 Control port
            LD A,%10011001          ;Configuration function
            OUT (C),A               ;Send it to 8255

2. Using port A/B/C,

In this example, port A is set to output, port B is set to input, and port C is set to output, and they are all operating in mode 0.

We will only be using port A for these examples.

  • Reading from port A,
                                    ;Set port A to input

            LD B,&F7                ;8255 Control port
            LD A,%10010010          ;Configuration function
            OUT (C),A               ;Send to 8255

            LD B,&F4                ;Port A port address
            IN E,(C)                ;Get byte from port

                                    ;Register E holds value from port

                                    ;Return port I/O status and operating modes
                                    ;to previous settings.

            LD B,&F7                ;8255 Control port
            LD A,%10000010          ;Configuration function
            OUT (C),A               ;Send to 8255
  • Writing to port A,
                                    ;Set port A to output

                                    ;(Note the next few lines are not necessary
                                    ;as port A is already acting as output, however
                                    ;it is given here just to make the example
                                    ;more understandable)

            LD B,&F7                ;8255 Control port
            LD A,%10000010          ;Configuration function
            OUT (C),A               ;Send to 8255

            LD B,&F4                ;port A port address

                                    ;Register E holds value to put into port

            LD E,&FF                ;Data to put into port
            OUT (C),A               ;Send to port A

                                    ;Return port I/O status and operating modes
                                    ;to previous settings.

            LD B,&F7                ;8255 Control port
            LD A,%10000010          ;Configuration function
            OUT (C),A


8255 ppi 1.jpg

8255 ppi 2.jpg



Wikipedia about the 8255ppi