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

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu Dec 20 10:42:05 2001

There are lots of things that you could compare, but the first things you've got
to leave out are the ones that aren't a Z80, which immediately deletes the Z180,
and Z280. The Z80 is not around any more than the 6502 is around. There are
derivatives of the 6502 still in production, but they are few. There are
derivative of the Z80 being produced as well. None of the former are 6502's,
and none of the latter are Z80's.

Your're right, in that they use their time differently. Because of the vast
architectural differences, one has to be cautious about chosing a comparison.
The first basis, before chosing a set of instructions for a comparision,
however, is selection of an appropriate timebase. The Z80 uses its clock
differently than the 6502 does. The Z80 never uses fewer than 2 clock ticks for
a complete memory cycle, ant 2 ticks is only the lenght of the M1 cycle. That's
followed by the refresh cycle, during which, if I understand the process
correctly, the Z80 decodes that opcode. "Normal" memory cycles are three
clock-ticks in length, though only thwo of them are actually used during the
actual access of memory. The memory access portion of the M1 cycle is
essentially a

The 6502 uses one clock tick for each CPU cycle. External hardware determines
how long the memory access is, though the period beginning with valid addresses
and read/write begins just under half-way into phase-1 and continues throughout
phase-2, each of which is actually more-or-less half a clock cycle long. If you
don't attempt to do transparent memory sharing along the phase-1/phase-2
boundary, a memory cycle can be as long as 1-1/2 clock ticks.

All of these constraints are based on the addition of no wait-states. The Z80
has excellent support for wait-state insertion during any cycle other than the
refresh cycle. The early NMOS 6502 supports wait state insertion on read cycles
only. There are ways of getting around this, but the Z80 certainly makes it
easier than does the 6502. CMOS 6502's support wait states on both read and
write cycles.

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.

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.


----- Original Message -----
From: "Allison" <ajp166_at_bellatlantic.net>
To: <classiccmp_at_classiccmp.org>
Sent: Thursday, December 20, 2001 6:55 AM
Subject: Re: 6502/Z80 speed comparison (was MITS 2SIO serial chip?)

> Z80 uses it's time differently... Then again how many instuctions
> would it take to do a 16bit add (result in register or convenient place).
> The fact that both are still viable suggests they have adaquate
> speed and a rich enough instruction set to do many tasks.
> Last item, z80, Z180 and Z280 do not have the same timing.
> For example the Z280 can be run at a bus speed slower than
> the CPU speed and with the MMU and cache running in burst
> mode you get a very different bus utilization model.
> Generally the only things that count is:
> Can the cpu do the task?
> What cpu are you familiar with?
> What is the total cost to implement the task (firmware/software counts)?
> Politcial impacts (company prefers, owns, has, used before).
> Do I think z80 is better than 6502? Yes, I'm biased. Is 6502 a good cpu?
> I think so, it certainly beat the 6800 and a lot of others in the 8bit
> space.
> Would I design with it? No, lack of experience, no on hand software base
> for it, limited tools to work with it. Would I consider it, likely.
> I have 6502, 6800, 1802, SC/MP, SC/MPII, ti9900, 8048/9/874x, 8080,
> 8085, z80, Z180 Z280, 6809 and T-11 to pick from. For a new design
> (personal) of some size say to run an OS then Z280 or T-11 for single chip
> I have 8748, 8749 and 8751s around. For simple controllers 8085 is easy
> to use if it grows out of the 8749. Then again I also have upd78pg11s too.
Why do you see the 8085 as growing out of the 8049? The 8085 was quite mature
when the 8049 came out, so they'r probably not from a common heritage, and the
instruction sets are so totally different that one might believe they're even
from a different manufacturer. The one that Intel claimed grew out of the
8048/49 was the 8051.

Of all these old-timers, that's the only one that still has life, and it has
LIFE in a big way! There are many variants, with the same common instruction
set, but many different hardware features. The Dallas 89C420 runs at, 50 MIPS,
which certainly breathes life into the line. There are bit-banging things that
you can do with that one that few of the others above could handle in less than
10x the time, yet the architecture supports a full 64K of data space and a full
64K of code space, with memory mapped I/O in the data space.
> >>
> >>
> >
Received on Thu Dec 20 2001 - 10:42:05 GMT

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