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.
> Speedwise the 1MHz 6502 and 4MHz Z80 were on par.
This is a bit of an exaggeration, the 6502 was efficient but not that efficient. While generally understood that the Z80 took 2x-4x ticks to execute instructions as the 6502, in the real world its larger register set meant properly-written Z80 code could avoid expensive, slow round trips to memory.
Outside of artificial benchmarks real world performance shows that the 6502 is roughly 2x as efficient per clock cycle as the Z80[0], i.e. a 1 MHz 6502 is approximately equivalent to a 2 Mhz Z80.
This is reflected in the computers of the day, i.e. TRS-80s were not being blown out of the water by Commodore PETs.
[0] https://github.com/soegaard/minipascal/blob/master/minipasca...
A few thoughts:
> the complexity of some instructions (e.g. DJNZ)
Well, of course the idea of DJNZ was to implement a very common pattern (decrement a register and jump (normally backwards) if the result was not zero) - this tended to simplify code rather than make it more complex.
> However I couldn't keep all instructions and timings in my head.
I was never really interested in the timings, but I did get to the stage (not by conscious memorisation) of being able to assemble and disassemble Z80 code in my head, with some accuracy.
> I preferred, however, the fact that I/O was memory mapped on the 6502.
Many (most?) Z80 systems used memory mapped I/O. It's down to the hardware designer.
> I did get to the stage (not by conscious memorisation) of being able to assemble and disassemble Z80 code in my head, with some accuracy.
Same here.
I never got any fluency using EXX and the shadow registers - there were so few situations it was worth the effort. I always felt like I must be missing something.
I suspect the shadow registers were there to improve interrupt handling, reserve their use exclusively to an interrupt handler, and you could save time in not having to store registers in memory.
That, or in general for system calls. In particular EX AF,AF' + EXX allowed you to exchange all registers other than AF, to be able to return an error code or status.
> I preferred, however, the fact that I/O was memory mapped on the 6502.
The Z80 could do memory mapped IO as well of course (used at least in some arcade machines), but why waste valuable address space when there's an entire 64 KB of extra address space reserved for IO ;)
I'm a 6502 guy myself, but the Z80 instruction set is also easily holdable in one's memory.
As others said, a 4MHz Z80 is clearly capable of outperforming a 1MHz 6502 as is evidenced by the many ZX Spectrum demos that show off 3D/plotting effects.