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.

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

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.

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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.

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.

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.

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

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

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.

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

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

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

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

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.