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Comment by qwertyuiop924 | Hacker News Reader

...well that helps explain the famous 4x speed difference between the Z80 and the 6502.

The 6502's two-phase clock throws a wrench in direct clock speed comparisons, no?

>The 6502's two-phase clock throws a wrench in direct clock speed comparisons, no?

Not really? The two phases are more or less just inverted versions of each other[1], but 1 clock cycle, for the purposes of counting cycles-per-instruction, is the same time (I think) whether you're measuring phase-1-high to phase-1-high or phase-2-low to phase-2-low.

[1]http://lateblt.livejournal.com/88105.html

The "two-phase clock" is just the alternate halves of one square wave. The 6502 accesses memory during one half and processes during the other. Only one clock signla is needed, and one instruction will be completed at the end of every complete clock cycle.

Fun fact: you can run a dual core 6502 system by hooking up the two processors to the same memory bus, one using an inverted clock. (Slightly more complicated than that, but only slightly.)

The Commodore Pet disk drives used this trick; one processor was the application processor, which listened for IEEE488 commands and handled the disk format; the other processor handled the low-level stuff (we'd call it a DSP today).

Yes, the Pet disk drive did have twice the number of processors as the computer it was attached to.

http://www.6502.org/users/andre/petindex/drives/arch/index.h...

And then you got the Amiga that apparently took that a step further by having the chipset act independently of the cpu, with a but of circuitry sitting in the middle to make sure they didn't both access the shared ram at the same time.

It's immensely charming that you are keeping lit the flame of this three decade old argument.

Famous? Are you referring to performance or just clock frequency? The Z80 takes more cycles to do the same operation as the 6502, but it also (typically) had a higher max clock frequency, mostly making up for it.

I was referring to the fact that a 1MHz 6502 could match a 4MHz Z80.

>but it also (typically) had a higher max clock frequency, mostly making up for it.

That used to be true. It is no longer true nowadays, so 65C02 beats the Z80 speed-wise.

Except Zilog came out with successors to the original Z80 that were far more cycle efficient. For example the eZ80, which can clock up to 50mhz.

Not exactly "more cycle efficient", just more modern manufacturing processes. There are also 6502s available for up to 200 MHz. They are not easily available to hobbyists, though - the modern DIP modules are only guaranteed up to 20MHZ, but they can be run at 25MHz.

It got a few more upgrades than just better miniaturisation:

> The eZ80 has a 3-stages pipeline. Available at up to 50 MHz (2004), the performance is comparable to a Z80 clocked at 150 MHz if fast memory is used (i.e. no wait states for opcode fetches, for data, or for I/O) or even higher in some applications (a 16-bit addition is 11 times as fast as in the original). The eZ80 also supports direct continuous addressing of 16 MB of memory without a memory management unit, by extending most registers (HL, BC, DE, IX, IY, SP, and PC) from 16 to 24 bits. In order to do so, the CPU works in a Z80-compatible mode or a full 24-bit address mode.

https://en.wikipedia.org/wiki/Zilog_eZ80

Note the pipelining, and the 24 bit ALU

No I believe they actually made them more cycle efficient. And the eZ80 is available to hobbyists at the high clock rates if you're willing to do SMT.

Pardon my ignorance on the topic, but I've often wondered if it's possible to take one of these older CPU designs and "just" increase the address space and make the manufacturing process smaller (but otherwise leave the instruction set and architecture as-is) to create a modern bit version at a high clock rate for embedded or mobile use? Or is there a lot of "devil in the details" that has prevented such a thing from existing?

It seems to me like upgrading one of these existing designs would be much cheaper R&D wise that you'd be able to compete very well on price for the IoT or other markets.

While it might (I don't know) be possible to make the manufacturing process smaller, you couldn't increase the address space without effectively rewriting the entire ISA. You could use bank switching or an MMU to increase the effective address space (allowing the machine to access more RAM than it can physically address), which was done even back in the day, on pretty much every machine.

>you'd be able to compete very well on price for the IoT or other markets.

Why do you think that the Z80 and the 6502 (and their derivatives) are still on the market? :-D

If a 1 MHz 6502 could match a 4MHz Z80 then that would mean a C64 could match a ZX Spectrum (which was clocked at 3.5 MHz) for performance in not sprite blitting scenarios.

Which it couldn't.

The spectrum smashed the C64 for speed of numerical operations, that's why the specrrum had lots of wireframe and solid filled 3d games and the C64 didn't.

Like, Carrier Command on the Spectrum vs the abomination that appeared under the Carrier Command logo that appeared on the Commodore.

It's been 30 years man, let it go.

Never!

sure http://www.hardcoregaming101.net/freescape/freescape.htm

nothing like a rose tinted glasses and fanboizm when it comes to membering those fluid 60fps full color FPS Spectrum games.

Sorry, what were you saying? I fell asleep waiting for the C64 version of Total Eclipse to render a new frame

See http://www.ffd2.com/fridge/speccy/score

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