reZet80 PIONEER - I/O map and decoding

The first 128 I/O ports (00-7F) are reserved for internal reZet80 use.
They are divided into 8 regions of 16 ports each:
00-0F, 10-1F, 20-2F, 30-3F, 40-4F, 50-5F, 60-6F and 70-7F.
These 8 regions will be further subdivided on demand.
The remaining 128 I/O ports are free to use: 80-FF.

The 74LS138 demultiplexer provides the following low-active signals:
- /IOSEL_0X (reserved for future use)
- /IOSEL_1X (reserved for future use)
- /IOSEL_2X (reserved for future use)
- /IOSEL_3X (reserved for future use)
- /IOSEL_4X (reserved for future use)
- /IOSEL_5X (alternative keypad)
- /IOSEL_6X (keypad or alternative keypad)
- /IOSEL_7X (6-digit display, TIL311 or 7-segment)

All I/O port addresses are given in hexadecimal notation.

Bill of materials:
- DIP-16 socket
- 74LS138 demultiplexer
- capacitor 100 nF