26 Facts About Zilog Z80


The Zilog Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975.

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Early Z80s were manufactured by Synertek and Mostek, before Zilog had its own manufacturing factory ready, in late 1976.

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These companies were chosen because they could do the ion implantation needed to create the depletion-mode MOSFETs that the Zilog Z80 design used as load transistors in order to cope with a single 5-volt power supply.

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Masatoshi Shima designed most of the microarchitecture as well as the gate and transistor levels of the Zilog Z80 CPU, assisted by a small number of engineers and layout people.

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Zilog Z80 took over from the 8080 and its offspring, the 8085, in the processor market and became one of the most popular and widely used 8-bit CPUs.

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Programming model and register set of the Zilog Z80 are fairly conventional, ultimately based on the register structure of the Datapoint 2200.

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The Zilog Z80 was designed as an extension of the Intel 8080, created by the same engineers, which in turn was an extension of the 8008.

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The 16-bit IX and IY registers in the Zilog Z80 are primarily intended as base address-registers, where a particular instruction supplies a constant offset that is added to the previous values, but they are usable as 16-bit accumulators, among other things.

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The Zilog Z80 introduced a new signed overflow flag and complemented the fairly simple 16-bit arithmetics of the 8080 with dedicated instructions for signed 16-bit arithmetics.

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Instructions LD A, R and LD A, I affect the Zilog Z80 flags register, unlike all the other LD instructions.

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The Zilog Z80 syntax uses parentheses around an expression to indicate that the value should be used as a memory address, while the 8086 syntax uses brackets instead of ordinary parentheses for this purpose.

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Zilog Z80 uses 252 out of the available 256 codes as single byte opcodes; the four remaining codes are used extensively as opcode prefixes: CB and ED enable extra instructions, and DD or FD select IX+d or IY+d respectively (in some cases without displacement d) in place of HL.

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Arithmetic instructions on the Zilog Z80 set it to indicate overflow rather than parity, while bitwise instructions still use it as a parity flag.

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Zilog Z80 has six new LD instructions that can load the DE, BC, and SP register pairs from memory, and load memory from these three register pairs—unlike the 8080.

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However, it was not until the fully pipelined eZ80 was launched in 2001 that those instructions finally became approximately as cycle-efficient as it is technically possible to make them, i e given the Z80 encodings combined with the capability to do an 8-bit read or write every clock cycle.

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Zilog Z80 published the opcodes and related mnemonics for the intended functions, but did not document the fact that every opcode that allowed manipulation of the H and L registers was equally valid for the 8 bit portions of the IX and IY registers.

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However, as they are not part of the formal definition of the instruction set, different implementations of the Zilog Z80 are not guaranteed to work the same way for every undocumented opcode.

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Furthermore, the Zilog Z80 has a single instruction that will execute the entire loop.

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One central example of this is that, for opcode fetch, the Zilog Z80 combines two full clock cycles into a memory access period.

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However, this relation has slowly changed during the last decades, particularly regarding SRAM; cacheless, single-cycle designs such as the eZilog Z80 have therefore become much more meaningful recently.

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Also, several clones of Zilog Z80 were created in the Soviet Union, notable ones being the T34BM1, called ??1858??1.

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The Zilog Z80-derived Z8S180 found its way into an early pen-operated personal digital assistant, the Amstrad PenPad PDA600 in 1993.

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Zilog Z80 has long been a popular microprocessor in embedded systems and microcontroller cores, where it remains in widespread use today.

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Applications of the Zilog Z80 include uses in consumer electronics, industrial products, and electronic musical instruments.

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For example, Zilog Z80 was used in the groundbreaking music synthesizer Prophet-5, as well as in the first MIDI synthesizer Prophet 600.

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Zilog Z80 was used in the Sega Master System and Sega Game Gear consoles.

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