48k Spectrum
Technical Information

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.


Expansion Connector
Built in I/O
ULA Output
ULA Input
System Variables
ZX Basic
I/O Streams
Video Display
Tape, Sound, Printer
Interface 1
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