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All CPC models were based on a [[Zilog ]] [[Z80 ]] processor clocked at 4 MHz. Because a common pool of RAM is shared with the video circuits, the Z80 may only make a memory access once every four cycles, which has the effect of rounding all instruction cycle lengths up to the next multiple of four. For details, see [http://www.grimware.org/doku.php/documentations/devices/z80 a nearly complete list of instructions with timings (number of cycles) for each.]