Are there other examples of CPU architectures mostly compatible with Intel 8080 other than Z80?

Question

When the Zilog Z80 was released, it was marketed as being mostly ISA compatible with Intel 8080, while also providing enhanced features.

Likewise, the CPU of the Nintendo Game Boy is a custom architecture designed by Sharp, (believed by the Game Boy homebrew community to be called SM83) which is mostly compatible with Intel 8080, while also providing some enhancements over it.

Are there any other examples of non-Intel designed CPU architectures which are similarly enhanced, mostly compatible derivatives of the Intel 8080?

My interest is in similar ISAs, as opposed to similar bus layout or timings. To put a threshold, let's say a CPU architecture implementing >90% of the 8080 ISA with binary compatibility. I'm looking to prove or disprove whether there are lesser known 8080 derivatives, other than the well documented cases of Intel's own continuation of the architecture, and the Z80 family of architectures. The goal is to figure out how unique the Sharp SM83xx architecture really was at the time, broadly being a 8080 derivative while also not being a direct descendant of Z80.

Answer 1

Foremost, there are direct continuations, CPU's able to execute 8080 code and (basically) hardware compatible, like the 8085, Intel's answer to the Z80, as it's mostly software and hardware compatible.

Beside Intel's direct extension, there is of course the series of enhancements of Z80 design, from Z800/Z280 all the way to the eZ80, which in some way can be seen as the pinnacle of (upward compatible) 8080/Z80 continuation.

A very special case is the NEC V20/30/... series. While basically 1:1 8086 replacements (with some enhancements) they as well include a mode allowing direct execution of 8080 binaries without any emulation.

Beyond that, it depends on what kind of compatibility this is about.

Hardware wise the whole (early) 8086 world would fit, as its bus system is based on the 8080 design. Adapting a 8088 into an 8080 design is about the same complexity as doing so with a Z80. Then again, early bus systems can be categorized by their bus cycles, as mentioned here. Transition between the basic types can be made with just minor hardware adjustment.

Software-wise (ISA) it gets a bit more diverse. For example the x86 family was designed with 8085 source code compatibility in mind, meaning that a program source can be translated automatically from 8080/85 to 8086 with a high rate of success.

And last, but not least, there is software emulation for compatibility - that one works on essential every CPU.

Pick your favourite modus operandi.

Oh, and beware of false friends on your search. Some CPUs may look like x80 on first sight, but are compatible only in spirit. Examples here are Zilog's Z8 or Z8000. Both use assembler mnemonics with a close resemblance to Z80, but for a completely different CPU architecture.

Answer 2

There were of course clones produced in the COMECON countries. Being clones, they would probably not make a good answer, with the exception of:

• КР580ВМ80А was a Soviet clone that has been developed further into:
• КР580ВМ1 with a frequency 5MHz needed just 5V and added some new commands, had support for bank switching and multiprocessor systems

Answer 3

Obviously what's compatible (or an advancement) to the Z80 also has to be compatible to the 8080.

That includes the Hitachi HD64180, the Zilog Z180 (which is essentially the same thing), the Toshiba 84013 and 015, and to a limited degree, the Toshiba TMP90 (which is only source-code-compatible to the Z80 but uses different opcodes). Rabbit Semiconductor is yet another company that has/had a wide range of CPUs that could be considered Z80/8080 derivatives - They share register names and look to be largely influenced by the Z80/8080 design.

The Sharp PC-1500's CPU, the LH5801 is an interesting one, because it came in disguise - It's officially published instruction set looks a bit like 6800/6500 instructions, but when you dig a bit deeper, it actually uses an extended set of Z80 opcode equivalents.

Answer 4

If Intel 8008 qualifies as answer one can add the Texas Instrument TMS-1795 which was TI's take at the Datapoint 2200 microprocessor. It even came out before Intel's 8008 as explained by Ken Shirriff .

Answer 5

Going backwards, the 8080 is "mostly compatible" with the Intel 8008. They share most of the same registers, and most instructions map between the two. With some changes (or using a cross-assembler), you can assemble the source code of most 8080 programs for an 8008.

There certainly are differences between the two processors. The 8008 had an internal stack and a 14-bit address space, in an 18-pin package. The 8080 had stack in memory with a proper stack pointer, and a 16-bit address space, in a 40-pin package. The 8080 had one additional flag: the auxilliary carry. Most importantly, the opcodes were different, making the executables incompatible.

Computer Terminal Corporation initially contracted Intel to build a microprocessor and gave Intel the desired instruction set. Intel was not able to deliver on time, so CTC created their own design with discrete TTL parts, the Datapoint 2200. Intel was allowed to finish their microprocessor, which was the 8008. The two systems are binary-compatible; therefore, the arguments for the 8008 also hold for the Datapoint 2200.

Answer 6

Missing so far is the roughly Z80-compatible NSC800, from National Semiconductor. (I forget the details but I see a CPU World comment tah it's a "Z80 in 8085 clothing").

Its USP was its CMOS construction and much lower power consumption than any of its rivals (until the Hitachi 64180 came along several years later)