Zilog Z08470 - Manual (Page 149)

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UM008101-0601 Direct Memory Access
Figure 49c depicts a one-of-eight TTL decoder which provides chip enable
signals for eight different peripheral devices. Address bits A0 and A1 are
often used directly by peripherals such as the Z80 SIO, PIO, and CTC, and
so are not decoded here. Additional decoders can be added when more
peripheral devices are present.
IORQ and M1 are internally gated with CE in Z80 peripheral devices and
need not be terms in CE. However, gating chip-enable signals with these
control lines do no harm and may produce less-ambiguous logic sequences
for circuit-level debugging, as seen in Figure 49c.