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Why the Z80 processor was so important

It was 50 years ago this month that Zilog released the Z80 microprocessor.

Oh well, I guess there's nothing like a birthday to make you feel old!

Although the 4004, the 8080 and the 8086/8088 get most of the headline coverage as "the most important processors ever made", my vote has to go to the Z80.

This is because, in those early days, just about every non-Apple, non-Commodore, non-Atari 8-bit computer was based on the Z80. These processors were everywhere, in machines from the ZX80/81 and spectrum, TRS80, the Osborne 1, the Epson QX-10 and the myriad of S100 CP/M systems that were the cornerstone of the hobby and small business market of the day.

Without the Z80 and its backwards compatibility with Intel's 8080/8085, I don't think we'd have seen the microcomputer progress at the rate it did during the 1970s and early 1980s.

This is why the Z80, in my opinion, is greatly underappreciated.

What made the Z80 so popular back then?

Well there were many reasons...

Firstly, and this can't be understated, it allowed the use of cheaper DRAM without the complexity of added refresh circuitry.

Back in the 1970s (like today), RAM was an expensive commodity and it came in two forms, static and dynamic.

Static RAM was easy to use -- you just applied power and you could write in data or read out data, that's it. However, that simplicity came at a price. Static RAM was several times more expensive than dynamic RAM and if you were building systems with more than just a few KBytes, that extra price added up really quickly.

Dynamic RAM was cheaper so for a given spend on RAM chips, you got a lot more memory. Unfortunately, dynamic RAM was much harder to use, from a system design perspective. It was the solid-state equivalent of a goldfish. If you wrote data into a dynamic RAM chip it would only remember it for a very short period of time -- milliseconds in fact. If you wanted dynamic RAM to remember things for longer than that (and of course you did), it was necessary to "refresh" each and every memory location every few milliseconds, right before it would otherwise have been forgotten.

Refreshing dynamic RAM required that the address lines be cycled constantly in order that the contents of each row was read into a buffer and written back -- hence the "refresh" term. To do this, Intel's 8080 and 8085 processors required external circuitry to drive the address lines and trigger the internal refresh operation.

The Z80's big feature (for hardware designers) was that it had this refresh circuitry built into it. Unlike the 6502, 6800, 8080, 8085 or any other 8-bit processors of the era, the Z80 made it as simple to use dynamic RAM as it was to use static RAM. That allowed designers to save money and cut complexity which ultimately becomes a win for consumers.

Zilog didn't forget programmers though. They extended the instruction set of the 8080 to add "block" instructions. These were fantastically powerful CPU instructions that allowed entire blocks of memory to be copied or searched with a single line of assembly code instead of having to create itterative loops and conditional tests. In reality, these instructions were not necessarily much faster than separate itterative statements but they were convenient and simplified programming just enough to be really nice.

Then there were the IX, IY and shadow registers that Zilog implemented. These allowed for cool things like indexed addressing and made handling interrupts a breeze and often saved having to push stuff onto the stack when doing so.

In fact, the Z80 had 158 instructions versus the 8080's paltry 78 instructions. That's a huge increase in functionality and all of those instructions made a programmer's life much easier.

There were other hardware benefits to the Z80 over the 8080 as well. These included the ability to run from a single 5V supply instead of also requiring the -5V and 12V supplies that the Intel chip needed. This simplified power supply circuitry.

It also had a much simplified clock signal requirement -- only needing a single phase, versus the overlapping two-phase requirements of Intel's offering. In fact, Intel systems generally had to use a dedicated clock generator chip (the 8224) to create this clock, Z80 systems didn't.

There were other advantages as well but I'm pretty sure that even if you don't understand half the stuff I've just talked about, you can clearly see that the Z80 was a huge advance over the 8080 processor that kicked off the whole microcomputer revolution back in the mid/late 1970s.

Lots of people rave about the 6502 and how it was cheap, fast and cheerful -- making low-cost graphically impressive machines like the Apple II, Atari 400/800, VIC20/C64 possible but I always preferred the Z80 for its obvious virtues. However, I concede that this is likely a Ford vs GM kind of debate that has no real winner.

Ultimately however, the Z80 will always have a place in my heart and I'm still looking (without success) for a microcomputer from that era. An old Amstrad or something would be perfect and allow me to take a trip back down memory lane to simpler times :-)

Happy Birthday Z80, we will miss you.

Carpe Diem folks!

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