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Z80 Mark III (continued)

This is the Z80 Mark III Central Processing Unit (CPU) board. The Z80 CPU is the 80-pin chip at the centre.

The clock circuit is at the top of the board. The 8 MHz crystal oscillator is divided down to 4 MHz and to 2 MHz by two 74LS74 flip-flop chips. The green and white switch labeled 1A and 2B chooses either the 2 MHz or the 4 MHz line to clock the CPU.

The second switch, labeled FA chooses whether the first address accessed at restart is at hex 0000 or at hex F000. The other chip, including the chips on the bottom row, are buffer and support chips.