Re: Crosspost: Did the cpu influence the display?

[A936@hotmail.com]

Peter van Merkerk wrote:

> > So on the 65xx side you needed more support
> > chips to do things that the cpu could not.

> The Z80 did have feature that 6502 did not; the Z80 did have support for
> DRAM refresh so it could be used with DRAM memory with minimal support
> circuitry. On the C64 the video chip (VIC-II) is responsible for the
> DRAM refresh

> > For example on a 65xx would it be possible to drive the display as is
> > done on the zx80 (not the ts1000 or zx81 which used a custom chip) and
> > the cpu with only a few off the shelf logic chips?

> The ZX80 to advantage of the DRAM refresh and some other Z80 specific
> "features" to implement a very simple (but clever) display system. Some
> aspects like executing a display file might be possible on a 6502 based
> system, other aspects would require another solution.

The poster is likely referring to the video system used in the
zx81/zx80
which makes special use of the z80 dram refresh features to get
the z80 to generate the video display, and no this can't be done on
any other 8 bit uP.  The Atari 2600, as you (Peter) point out also has
simple display logic, but it still uses the TIA chip to general
horizontal
video timing, whereas in the zx80/zx81 the z80 uP does it all --
vertical
timing, horizontal timing and fetching of data to output to TV.  The
only
thing missing is the modulator and parallel -> serial conversion.  This
was likely the key reason the z80 was chosen for the zx80/zx81, whose
primary purpose was to get a useable computer as cheap as possible.

The dram refresh feature and the bus timing in the z80 did make it
easier to build a system with less glue logic than other cpus,
but early on that cost was easily swallowed by the price difference
between say the 6502 and z80, being the most dramatic example.

> On the other hand the 6502 had a feature that allowed display chips to
> use the same memory as the CPU without slowing the CPU down (at least
> most of the time). The 6502 accessed memory only one phase of the clock,
> the other phase could be used by the display chip without the CPU
> noticing it. Of course the display chip had to be specificly designed to
> be matched with the 6502, which may explain why 6502 based system often
> use custom designed chips, whereas many Z80 based systems used
> of-the-shelf solutions.

You can do hidden cycle dma access on a z80 too.  Ignore the
dram refresh feature and you have 2 cycles of free access for
every instruction byte fetched.  You could still use the z80
refresh signals to simplify any dram refresh hw you would
need to add.