Most of the hardware is incorporated in a single purpose
built chip called a ULA (Uncommitted Logic Array). The CPU
in the Spectrum is the Z80A, which works at double the speed
of the Z80. This speed is measured in megahertz (MHz) and
represents the number of regular pulses generated per second.
In theory the Z80A can work at 4MHz (4 million pulses per
second), but is set at 3.5MHz in side the Spectrum. The number
of pulses needed to carry out each instruction depends on
the complexity of the instruction.
The Spectrum is an 8bit computer with a 16bit address line,
capable of addressing 64k (one k - kilobyte - is 1024 bytes).
The Z80 has an additional 64k of address space that is dedicated
to I/O devices. This is achieved by an additional address
bit called the IORQ (Input Output Request) which selects between
memory and I/O. However, the I/O devices have a very restricted
set of instructions - basically, IN and OUT. Discussed Later.
Instead of address locations, the 64k of I/O devices are
called Ports. For example the ZX Printer is found at Port
251, the Microdrive is found at Ports 254, 247 and 239.