Best 1970s Microprocessor

[towforce]

In the "history" thread, it was good to see that, nearly 50 years on, people are still interested in the devices that made dedicated chess computers and playing chess software on "home computers" (as we used to call them) possible - the 8 bit microprocessor!

Your choices are:

1. 6502

* brought the price of 8-bit microprocessors down (and forced the others to lower their prices): when it came out, it was much cheaper than the other 8-bit cpus

* relatively simple instruction set (in the age of the portable device, where low power consumption is important, simple instruction sets like the ARM architecture are the best selling SOCs)

* Used in many brand name computers like Apple, Commodore Pet, many games consoles, and many arcade games

2. 8080 . The later 8086/8088 cpus, which, while 16 bit instead of 8 bit, used the same microarchitecture, and were used in the "PC" computer which dominated personal computing for decades

3. Z80. Faster clock speed, was able to do more (hence required fewer support chips, meaning that very low cost computers like the ZX80, ZX81 and ZX Spectrum could be built. Large instruction set, with more sophisticated instructions, meant that programs could be smaller (important in an age in which devices sometimes only had 1kb of RAM, and frequently only 16kb). The Soviet Union liked it - unlicensed clones were manufactured in both East Germany and Romania.

Which was the best? Let's have a vote!

[towforce]

My choice: I know that many of you like the 6502 (e.g. Ed and Thorsten), but I've always thought that the Z80 was ultimately the better CPU for the circumstances of the time.

I appreciate that these CPUs were expensive, and hence low cost was an important consideration - and that the assembly language for the 6502 was probably easier to learn.

[tmokonen]

Isn't the Z80 instruction set just a mostly compatible superset of the 8080 instruction set?

[towforce]

Isn't the Z80 instruction set just a mostly compatible superset of the 8080 instruction set?

Yes - but it's larger (or to put it the opposite way, the 8080 instruction set is a subset of the Z80 instruction set). Presumably this was to ensure that customers wouldn't need to get a new compiler to get started.

[hgm]

Not sure what you mean by 'fewer support chips' for the Z80. My 'matchbox computer' used 6502, and did not need anything more than the CPU, an 8K x 8 memory chip, an 8-bit latch as output port for the 7-segment LED display and one more TTL chip with gates for generating the control signals for that latch and memory, and the clock signal.

Perhaps you are comparing with the 8080, which was of an earlier generation of hardware designs, like the 6800, which often required more primitive interfacing. And didn't have the 8080/Z80 chips a multiplexed data and address bus, which required demultiplexing with an address latch before you could connect it to a static RAM chip?

I did not like the 8080 much: it was full of practically useless registers, not much more helpful than the zero page of 6800/6502. (But that had a capacity of 256 rather than 6). And the Z80 just provided two sets of the (still equally useless) registers.

The 8086 had a completely different architecture.

[smatovic]

The Z80 in Yugo "Galaksija" as self assembly kit from 1983:

https://en.wikipedia.org/wiki/Galaksija_(computer)

I am the plug generation of computer users, we did not solder assemlbly kits by ourselves, so I vote for 6502 in Atari 800 XE.

--
Srdja

[towforce]

Not sure what you mean by 'fewer support chips' for the Z80. My 'matchbox computer' used 6502, and did not need anything more than the CPU, an 8K x 8 memory chip, an 8-bit latch as output port for the 7-segment LED display and one more TTL chip with gates for generating the control signals for that latch and memory, and the clock signal.

Where were programs and data stored? How was the program entered? How did you play chess using the 7-segment LED display?

Presumably you were using expensive SRAM: had you been using cheap DRAM, you'd have needed to refresh it (the Z80 would have done that for you). The Z80 would also have only used a single clock (having multiple clocks can be useful: I remember writing a background program for a PC (in the DOS era) that hooked the interrupt on the 1/18th second clock, did it's processing, then released itself enabling foreground processing to continue).

Maybe you were using a later 6502 with more features?

I did not like the 8080 much: it was full of practically useless registers, not much more helpful than the zero page of 6800/6502. (But that had a capacity of 256 rather than 6). And the Z80 just provided two sets of the (still equally useless) registers.

These 256 "registers" of the 6502 weren't fast: they had to be fetched from RAM. It was really a reserved section of RAM memory.

The 8086 had a completely different architecture.

From link: "Whereas the 8086 was a 16-bit microprocessor, it used the same microarchitecture as Intel's 8-bit microprocessors (8008, 8080, and 8085). This allowed assembly language programs written in 8-bit to seamlessly migrate"

[towforce]

The Z80 in Yugo "Galaksija" as self assembly kit from 1983:

https://en.wikipedia.org/wiki/Galaksija_(computer)

When it came out, the Sinclair ZX80 had an eye-popping price of £99.95 - which was unbelievably cheap at the time - and a great demonstration of how the Z80 CPU could be the basis of a cheap computer. Alternatively, you could buy it in kit form for £79.95 and solder it together yourself.

Maybe a better option for self-soldering would have been its successor, the ZX81, which had only 4 chips in it instead of 21. This gave you a computer, the basic programming language, the ability to display on a TV, the ability to record your work to a cassette player, and, of course, a keyboard for input.

[Bo Persson]

Isn't the Z80 instruction set just a mostly compatible superset of the 8080 instruction set?

Yes, it was designed by the original Intel engineers, who wanted to do an improved 8080 version. Intel didn't approve of that, so they became "former Intel engineers".

[Bo Persson]

The 8086 had a completely different architecture.

From link: "Whereas the 8086 was a 16-bit microprocessor, it used the same microarchitecture as Intel's 8-bit microprocessors (8008, 8080, and 8085). This allowed assembly language programs written in 8-bit to seamlessly migrate"

That's just not true, but more a part of Intel's marketing. Of course the 8086 would be very close to its immediate predecessor the 8085. You can see that in the names, right?

In practice the 8-bit assembly could not be directly reused in the 8086, but had to first be run through a converter program. And the resulting code was nothing like what new 8086 code would look like.

But as it had a 16-bit memory bus and ran at 5 MHz, the converted code was still faster than the original. So it "worked", sort of.