I/O port allocation

Many thanks to Mark Rison for providing the original information. Thankyou to Richard Wilson for his discoveries concerning RAM management I/O decoding.

This document will explain the decoding of the I/O ports. The port address is not decoded fully which means a hardware device can be accessed through more than one address, in addition, using some addressess can access more than one element of the hardware at the same time. The CPC IN/OUT design differs from the norm in that port numbers are defined using 16 bits, as opposed to the traditional 8 bits.

• IN r,(C)/OUT (C),r instructions: Bits b15-b8 come from the B register, bits b7-b0 come from "r"
• IN A,(n)/OUT (n),A instructions: Bits b15-b8 come from the A register, bits b7-b0 come from "n"

Listed below are the internal hardware devices and the bit fields to which they respond. In the table:

• "-" means this bit is ignored,
• "0" means the bit must be set to "0" for the hardware device to respond,
• "1" means the bit must be set to "1" for the hardware device to respond.
• "r1" and "r0" mean a bit used to define a register

Hardware device	Read/Write	Port bits
		b15	b14	b13	b12	b11	b10	b9	b8	b7	b6	b5	b4	b3	b2	b1	b0
Gate-Array	Write Only	0	1	-	-	-	-	-	-	-	-	-	-	-	-	-	-
RAM Configuration	Write Only	0	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
CRTC	Read/Write	-	0	-	-	-	-	r1	r0	-	-	-	-	-	-	-	-
ROM select	Write only	-	-	0	-	-	-	-	-	-	-	-	-	-	-	-	-
Printer port	Write only	-	-	-	0	-	-	-	-	-	-	-	-	-	-	-	-
8255 PPI	Read/Write	-	-	-	-	0	-	r1	r0	-	-	-	-	-	-	-	-
Expansion Peripherals	Read/Write	-	-	-	-	-	0	-	-	-	-	-	-	-	-	-	-

Notes:

• The Gate-Array and CRTC can't be selected simultaneously, which would otherwise cause potential display corruption. This fact has been used in demos which perform "horizontal splitting": e.g. "Gozeur's Madness" and "Dreamend Megademo"
• In the case of the CRTC and PPI, bits b9 and b8 are used to define the register to select.
• It is entirely possible to select multiple devices and access these simultaneously (except for the CRTC and Gate Array)
• The "official" ports are defined to eliminate conflict between devices, as follows ("r" defines a bit which is reserved for register selection on the specific device):

  Hardware device	official port address	Port bits
  		b15	b14	b13	b12	b11	b10	b9	b8
  Gate Array	&7F	0	1	1	1	1	1	1	1
  RAM Management	&7F	0	1	1	1	1	1	1	1
  CRTC	&BC-&BF	1	0	1	1	1	1	r	r
  ROM select	&DF	1	1	0	1	1	1	1	1
  Printer port	&EF	1	1	1	0	1	1	1	1
  PPI	&F4-&F7	1	1	1	1	0	1	r	r
  Expansion Peripherals	&F8-&FB	1	1	1	1	1	0	x	x

CRTC:

b9	b8	Function	Read/Write state
0	0	Register select	Write Only
0	1	Register write	Write Only
1	0	(note 1)	-
1	1	(note 1)	-

Notes:

1. These functions are dependant on the model of 6845. Please see the documentation on the 6845 for more information

PPI:

b9	b8	Function	Read/Write state
0	0	Port A	Read/Write
0	1	Port B	Read/Write
1	0	Port C	Read/Write
1	1	Control register	Write only

Expansion Peripherals (including FDC):

If b10 is reset, bits b7-b5 are used to select an expansion peripheral. Again, this is done by resetting the peripheral's bit:

Bit	device
b7	FDC
b6	Reserved (was it ever used?)
b5	Serial port

In the case of the FDC, bits b8 and b0 are used to select the specific mode of operation; all the other bits (b9,b4-b1) are ignored:

b8	b0	Read	Write
0	0	not used	Floppy disc drive motor control
0	1	not used	Floppy disc drive motor control
1	0	Main status register of FDC	Data register of FDC
1	1	Data register of FDC	Data register of FDC

If b10 is reset but none of b7-b5 are reset, user expansion peripherals are selected. The exception is the case where none of b7-b0 are reset (i.e. port &FBFF), which causes the expansion peripherals to reset.