Comparing the Z80 and 6502 to Their Relatives (opens in new tab)

I could hold the whole 6502 instruction set (and their cycles) in my mind while programming, it was that simple.

I acquired a Z-80 softcard for my Apple ][ (for trying out CPM) and was flabbergasted by the expanded register set, the complexity of some instructions (e.g. DJNZ) and the fact it ran at 4MHz vs 1MHz for the 6502 (got a speed demon 65C02 card later). However I couldn't keep all instructions and timings in my head. Speedwise the 1MHz 6502 and 4MHz Z80 were on par. I preferred, however, the fact that I/O was memory mapped on the 6502.

The Z80 spawned the 64180 which was a Z80 with loads of stuff built in (from Wikipedia)

Execution and bus access clock rates up to 10 MHz

Memory Management Unit supporting 512K bytes of memory (one megabyte for the HD64180 packaged in a PLCC)

I/O space of 64K addresses

12 new instructions including 8 bit by 8 bit integer multiply, non-destructive AND and illegal instruction trap vector

Two channel Direct Memory Access Controller (DMAC)

Programmable wait state generator

Programmable DRAM refresh

Two channel Asynchronous Serial Communication Interface (ASCI)

Two channel 16-bit Programmable Reload Timer (PRT)

1-channel Clocked Serial I/O Port (CSI/O)

Programmable Vectored Interrupt Controller

As a consequence it was really popular in the 90s as an embedded processor just when I was starting my career. This lead to me writing thousands of lines of Z80 assembly. You could program it in C but the compiler was useless at making stuff go fast.

One of those things I wrote was an LZ77 decompressor used in a satellite broadcast system. It took me about a week to write it, test it and optimise it. Quite a challenge! I remember optimising it about the LDIR instruction to copy memory.

The compressor was written in C and ran on the PCs of the day.

I come from the same lineage as the author. I did 6502 (doing C64 demos) long before I encountered the Z80. From what I remember, the Z80 offers a vastly superior programming experience. It has more registers. it has 16 bit registers. It has a shadow register set (you can switch between sets, which is handy for interrupt routines, for example) Programming assembly on the Z80 just is less of a fight.

> Less immediately visible to someone working at the assembly language level instead of the machine code one is that relative addressing is much more common on the 6809, meaning that it’s significantly more viable to write position-independent code on it than any of the other chips we’ve looked at here. Only the 8086 comes close, and it achieves it by using its segment registers as a de facto relocation base.

I would love to learn more about this. Does more "position-independent code" mean the linker has much less to do [0], or is there an actual difference in the code base for similar tasks?

[0] https://sourceware.org/binutils/docs/ld/Overview.html

I admit that I did everything with a Z80, except for using one. Because the TRS-80 used a Z80, Radio Shack carried a book on it, published by Howard Sams. It was next on the shelf after the TTL book, so of course I bought it and devoured it without ever touching a Z80.

My impression of the Z80 being clean and simple probably resulted from that book being so clearly written. It gave me a good enough understanding of how micro's work, that lasted until the more modern chips came out with things like pipelining. But I think that learning one of those old 8 bit chips would still be a great place to start for understanding things at a hardware level.

Dynamic memory refresh on chip was clever.

I find programming for the 6502 to be a joy in a way that the Z80 isn't. I'm not quite sure why that is; I guess maybe because 6502 feels so stripped down, the amount of context you have to keep in your head is extremely low?

As someone familiar with z80, 6502, and 68HC11 instruction sets this breaks down the key differences between the "6s" and "8s" archs well. I personally learned z80 first, then found the 6502 a bit limiting after writing z80 for a bit. That said, I still love writing bare metal 6502 for my Atari 2600's handicapped 6507 (missing an address line among a few other oddities). Sometimes limits are a good thing. I certainly would never want to remember all the more complex cisc instruction sets for example.

There's also the weird more than an 8080 but not quite a Z80 CPU in the Gameboy (which Sharp seem to call an SM83 when they used it in their microcontrollers).

"The 6809 saw some success, especially in arcade machines, but it did not steamroll the world the way the 6502 and Z80 did."

Could have mentioned the use of the 6809 in the Radio Shack TRS-80 Color Computer and the Dragon in the UK. Using the TRS-80 tag on something not using a Z-80 never made sense.

TIL that my mind has dynamic memory and enjoys an occasional refresh cycle.

Thank you!

I have never owned a computer before like 2002.

In 2025 I started programming 6502 assembly just for fun as intellectual exercise (i did TINY bit of x86 asm in the past) and MY GOD: this is so easy and so valuable to learn!

Programming 6502 seems simpler than learning lets say JS framework or to learn just about anything modern.

Its super fun, super easy and very rewarding.

I ended up designing my own ultra RISC, stack based and uniform 32-bit fixed-length data size (all instructions and data have exactly the same size) with mimo and other cool features. 6502 on steroids

I felt competent first time for long time as jobless programmer doing that :)

I'll just leave this here

https://www.llvm-mos.com