Z80
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3.5469MHz  (ZX Spectrum +3)

AY sound chip
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Clock is 1.7734MHz
&BFFD	write data (w)
&FFFD	register select (w)
&FFFD	read data (r)

ROM/RAM paging (ZX128)
----------------------

paging after reset:
0000	4000	8000	C000
ROM		Bank5	Bank2	Bank0

OUT		b5		b4		b3		b2		b1		b0
&7FFD	Lock	ROM0	Screen	RAM2	RAM1	RAM0

Lock	Lock to current configuration - can only be cleared by RESET
ROM0	ROM bank number
Screen	Screen select (bank 5 if clear, bank 7 if set)
RAMx	Bit x of RAM bank number [banks 4 to 7 are contended by the ULA]

ROM banks:
	0	2	Editor/BASIC
	1	3	Old ROM