The Zilog Z80 turns 50: the chip that shaped 8-bit computing

From a terminal to a single chip

The Zilog Z80 was launched in July 1976, fifty years ago, and became one of the most widely used processors ever built: it powered early personal computers, home and hobby machines and countless embedded and industrial devices. Binary compatible with Intel's 8080 and 8085, it helped establish a de facto hardware standard for 8-bit microcomputers — and a software standard built on CP/M and Microsoft BASIC.

Its prehistory starts at Computer Terminal Corporation, which built the Datapoint 2200 terminal around an 8-bit processor assembled from individual TTL chips. Texas Instruments and Intel were contracted to put that CPU on one chip, but neither finished in time: when Intel's part — numbered 1201 and renamed 8008 by marketing — was ready, CTC was already selling terminals built on the TTL design. TI cancelled its version; Intel commercialised the 8008.

The 8080 followed. Federico Faggin pushed for an improved design, but management waited for the market's reaction, and competitors' announcements cost Intel nine months of lead time; Masatoshi Shima was hired away from Busicom for the project. Feedback from customers who had seen the 8008 shaped the new chip, which dropped binary compatibility and gained an external stack in memory, 16-bit addresses, 256 I/O ports and NMOS logic needing three supply voltages.

Zilog and the Z80

Faggin left Intel and founded Zilog with Ralph Ungermann, then settled on an improved 8080 nicknamed “Super 80” after deciding that a microcontroller's margins were too tight for a fabless startup. Exxon put up $500,000; development to working prototypes cost roughly $400,000 and finished on time and under budget, with Shima joining from Intel and Mostek manufacturing the parts.

The Z80 stayed binary compatible with the 8080 while adding index registers IX and IY, bank-switched register pairs for faster interrupt handling, three interrupt modes, bit instructions, BCD arithmetic and self-repeating block operations such as LDIR. Because Intel claimed copyright on the assembly mnemonics, Zilog shipped its own, cleaner syntax. Electrically it needed a single 5V supply and clock, exposed MREQ, IORQ, RD, WR and M1 signals, and refreshed DRAM by itself; a working computer needed little more than a decoder, EPROM, RAM and a UART — the author built one as a teenager in a school photo lab.

What the Z80 left behind

Design work on the 16-bit Z8000 had already begun before the Z80 shipped; it arrived in 1979 and its memory-management design influenced the descriptor tables of the 286's protected mode. Zilog's ties to Exxon also coloured its fate: IBM, wary of the oil giant's computing ambitions, chose Intel's 8088 for its PC. Zilog became a full Exxon subsidiary in 1980, split off again in 1989 and is now owned by Littelfuse. The Z80 lived on for decades in embedded designs, and its clones reached millions of homes — the Sharp LR35902 in the original Game Boy among them — until Zilog discontinued the chip in June 2024.