Z80

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  • This article is about the CPU chip used in the Spectrum. For the emulator snapshot format see Z80 format

The Zilog Z80A is the CPU used in the ZX80, ZX81 and all ZX Spectrum models, as well as most Spectrum clones and several other 8-bit micros. In the Spectrum, the Z80 is clocked at around 3.5MHz (for precise values see individual model pages).

Instruction Set


OUT (C),0 / OUT (C),255 Instruction

In 1996, Simon Cooke noted in a Usenet posting that the undocumented instruction usually referred to as 'OUT (C),0' behaves instead as 'OUT (C),255' on CMOS Z80s.

Later, in 2004, Colin Piggot rediscovered this with his own SAM Coupé, when running a demo for SCPDU 6, coincidentally written by Simon Cooke. With the CMOS Z80, a white background colour was set over part of the screen in the demo. This was described in SAM Revival Issue 9 (March/April 2004).

In 2008, this was once again rediscovered by the MSX community.

Bits 3 and 5 of the F Register

Woody confirmed that bits 3 and 5 of the flags are copied on all BIT instructions on both a Zilog Z80 and an NEC clone (stated on the link above as being "unverified").

Stuart Brady discovered that how bits 3 and 5 of the flags are set after the SCF and CCF instructions actually depends on the variant of Z80 in use. On a Zilog Z80, bits 3 and 5 of the flags were set by ORing their previous values with that of A. On an NEC clone, bit 3 was unaffected, while bit 5 appears to be set by ANDing the previous value with something unknown. Previously, Ian Greenway tested this and found that bits 3 and 5 seemed simply to be copied from A, although it is not known which CPU these tests were run on. For more details, see here, here and a more recent investigation here.

Patrik Rak however later discovered that the way how the flags 5 and 3 are affected after SCF/CCF actually depends on the previous instruction completed. In case of genuine Zilog CPU, if an instruction modifies the flags, the immediately following SCF/CCF does move of bits 5 and 3 from A to F, whereas if an instruction doesn't modify the flags (and after interrupt), the SCF/CCF does OR of bits 5 and 3 from A to F. In case of NEC and other clones, it is similar, except that instead of OR it does AND with some unknown value, making the result unreliable.

Bus

The Z80 has a 16-bit address bus and an 8-bit data bus. The control bus pins (all of which are active low) are:

Pin Description
/MREQ Memory Request
/IORQ I/O Request
/RD Read
/WR Write
/RFSH DRAM Refresh
/M1 Opcode Fetch Cycle
/HALT CPU Halted
/WAIT CPU Wait
/INT Maskable Interrupt
/NMI Non-Maskable Interrupt
/BUSRQ Bus (DMA) request
/BUSACK Bus (DMA) acknowledgement

Replacements

Z80 chips are still available new, due largely to their continued popularity for embedded systems. Component suppliers such as Farnell and Rapid usually have them in stock.

Article license information

This article uses material from the "Z80" article on the ZX Spectrum technical information wiki at Wikia and is released under the Creative Commons Attribution-Share Alike License.