Spectrum I/O map

The spectrum hardware only partially decodes some of the address lines. 
The video/sound ula is selected when A0 is low.  The Interface 1
microdrive and serial/network interface is selected with A3 and A4.  The
Sinclair printer is selected when A2 is low, but a proper Centronics
interface uses full decoding.  Some Kempston standard joystick interfaces
are selected by A5.  The addresses that would clash with the Kempston
interface are listed in Iitalicsi.

76543210  Hex Dec	Input			Output
11111111  FF  255	User port in		User port out
11111110  FE  254	Spectrum keyboard/Tape	Border/Sound/Tape
11111101  FD  253	Ascii keyboard
11111011  FB  251	Printer	status		Printer output
11111001  F9  249	Ascii keyboard status
11110111  F7  247	Microdrive 		Microdrive
11101111  EF  239	Microdrive		Microdrive
11100111  E7  231	Microdrive		Microdrive
I11011111  DF  223i
I11011101  DD  221i
I11011011  DB  219i
I11011001  D9  217i
10111111  BF  191
10111101  BD  189
10111011  BB  187
10111001  B9  185
I10011111  9F  159i
I10011101  9D  157i
I10011011  9B  155i
I10011001  99  153i
01111111  7F  127
01111101  7D  125
01111011  7B  123
01111001  79  121
I01011111  5F  95i
I01011101  5D  93i
I01011011  5B  91i
I01011001  59  89i
00111111  3F  63
00111101  3D  61
00111011  3B  59
00111001  39  57
00011111  1F  31	Kepston Joystick
00011101  1D  29
00011011  1B  27
00011001  19  25