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Computing (FOLDOC) dictionary
Zilog Z80
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processor An 8-bit microprocessor. It was released in
July 1976 with a 2.5 MHz clock rate. The Z80 was a much
improved Intel 8080 (as was the Intel 8085). It also used
8-bit data and 16-bit addressing, and could execute all of the
8080 op codes as well as 80 new ones, instructions that
included 1, 4, 8 and 16-bit operations and even block move and
block I/O instructions. The register set was doubled, with
two banks of registers (including A and F) that could be
switched between. This allowed fast operating system or
interrupt context switches. It features 3 types of
interrupt mode.
The Z80 also added two index registers (IX and IY) and
relocatable vectored interrupts (via the 8-bit IV register).
Like many processors (including the 8085), the Z80 featured
many undocumented op codes. Chip area near the edge was used
for added instructions, but fabrication made the failure of
these high. Instructions that often failed were just not
documented, increasing chip yield. Later fabrication made
these more reliable.
The thing that really made the Z80 popular was the memory
interface - the CPU generated it's own RAM refresh
signals, which meant easier design and lower system cost.
That and its 8080 compatibility and CP/M, the first
standard microprocessor operating system, made it the
first choice of many systems.
In addition to the original Z80 (2.5 MHz) there are the ZilogZ80A (4 MHz), Zilog Z80B (6MHz) and Zilog Z80H (8 MHz)
versions. The popular Hitachi HD64180 processor family adds
peripherals and an MMU to the Z80.