6502/Z80 speed comparison (was MITS 2SIO serial chip?)

From: Hans Franke <Hans.Franke_at_mch20.sbs.de>
Date: Thu Dec 20 12:09:56 2001

My favorite topic - I have to come out of hybernation :)

> If you run each of the two at a rate that allows each to use its shortest memory
> cycle as the basic access window to memory, the 6502 will probably outperform
> the Z80 in a task that operates on 8-bit data in an 8-bit way. The more 16 or
> 32-bit things you do, the more it favors the Z80, due to its inherent features
> such as the 16-bit registers and operatons on them without intervening
> instruction fetches.

Well, only to an extend - as usual we have to see the registers as
some cache - and as soon as your datasize outruns the cache size
you're soly dependant on the load and store interface ... and here
the 6502 is not only faster but also better suited if you forget
about memory as an enemy but wellcome it as a huge registerfile.
And the 6502 speeds away again.

> If you compare the rate at which they operate, however, the 6502 has features
> that enable it to do 16-bit things quite handily in memory. Since one can view
> the 6502's page-zero as an extended register set, with instructions dedicated
> for it. Those instructions include indirection into main memory, which make
> 16-bit operations remarkably fast for a CPU that has no 16-bit registers.


> Have a look at it and contemplate that. It's a VERY clever design, intended to
> make for a small, hence, cheap, CPU chip.

Also, if you realy want 'ultimate speed' you have to look at actual Versions.
close Z80 relatives (like 180) are available up to 32 MHz (I have seen only
20s, but well) while actual versions of the classic 6502 and 65816 is able
to do 14 MHz. Now take in account that a clock ratio of 1:3 to 1:4 for 6502
vs. Z80 (*) is a good rule of thump, so a 14 MHz 6502 equals a 42 to 56 MHz
Z80. Quite a racer ...


(*) I remember the the classical 3:1 or 4:1 clock ratio from various tests.
There have been lots of comparions around 1980 about that, and all eded
up around 1:4.

VCF Europa 3.0 am 27./28. April 2002 in Muenchen
Received on Thu Dec 20 2001 - 12:09:56 GMT

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