The Gate-Array generates the "READY" signal which is connected to the "/WAIT" input signal of the CPU. This signal is used to stop the CPU accessing RAM while the video-hardware is accessing it.
In fact, the Gate Array allows the Z80 to access the RAM in only 1 out of every 4 cycles. As a result, all instruction timings are stretched so that they are all multiples of a microsecond (1µs), and this gives an effective CPU clock of 3.3Mhz.
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