🚀 go-pugleaf

In article <8805222049.AA02208@cory.Berkeley.EDU>, dillon@CORY.BERKELEY.EDU (Matt Dillon) writes:
> :What I said was that the ST 68000 is usurped for video refresh - this
> :is of course incorrect. What I meant to say was that ths ST's 68000
> :is usurped to move information around on the screen (including any
> :text). I was aware of this all along, guess I slipped 'cos I was
> :a little peeved at the tone of the original message - my apologies
> :to all.
> 
> 	Correct me if I'm wrong, but my understanding was that in color
> mode video refresh does not take cycles from the 68000, but in monochrome 
> mode video refresh DOES.
> 
> 						-Matt

Yes you are wrong.  In fact your postings show you have little idea of how
the ST really works, and I'm being to think you don't really understand what
happen in the Amiga either.  Anyway here is the facts.

Unlike the Amiga the ST screen size is always 32k bytes.  This of means that
as you increase the resolution the bits/per pixcel decrease.  Hence while you
can normally get only 16 colors in low resolution, there are only 4 colors
in medium and the monochrome mode of has the highest resolution.  There is
no interlacing and no overscan (two things the ST should have, these are 
outstanding features on the Amiga).  Anyway no matter what screen resolution
you use the 68000 doesn't see any differences in memory access.  

Now we come to the part that most Amiga people are really goofed up about.
Under normal conditions the 68000 will at most access memory every other cycle.
There are a few exceptions, but they don't happen very often.  So as it turns
out at least half of possible memory cycles are used by the 68000.  The Amiga
and the ST both have extra hardware to use the memory extra cycles.  The Amiga
has more hardware doing that, but the ST actually has hardware doing some of
the same things that the Amiga does.

The Video Shifter in the ST uses the bulk of the excess memory cycles.  It 
simply moves the memory out the display hardware.  It doesn't do this during
vertial retrace so the are still left over cycles.  These get used by the DMA
stuff and the DMA can steal cycles from the CPU.  Normally only a few cycles 
can be lost to the 68000 without DMA.  That is the few times a the 68000 and
the video shifter get out of sync the 68000 has to wait a cycle to get it 
back in sync so that the Video Shifter and 68000 are using alternate cycles.

The Amiga is far more complex.  During "normal operation" the Amiga will lose
more cycles than the ST and it has to stay in sync with 4 things instead of
2.  Still the idea is the same - alternate cycles go to 68000 with the others
going to support hardware.  The Amiga has more flexibity in graphics modes,
but that amount of cycles the 68000 gets varies with the modes you using.
The Amiga graphics modes are very flexible from about 320 x 200 1 bit plane
to about 704 x 452 4 bit planes interlaced.  Or another way to think of it
is 8k of memory to around 150k of memory.  At some point, I think about
about 40k of graphics memory the graphics refresh starts cutting into the
68000 cycles.  This is easy to test, but I haven't done it for some time, 
just write a basic program and time it in the various display modes.

Things like blitters and the copper do help out but are not always useful
for every program.  The copper in my mind is much neater than the blitter  
and is much more unused than the blitter.  Anyway the Amiga pays for it's
flexiblity in general speed.  Some things that lend then selfs to the extra
hardware the Amiga has do fine. (windows are a good example of this)  Other
things can slow down depending on how much flexiblity of the Amiga is being
used.

I wish the ST had all the flexiblity of the Amiga.  Still there is always a
price to pay for flexiblity.  In compareable mode the Amiga and ST run 
neck and neck in speed. (often the ST has a speed advantage)  In modes where
more graphics specail stuff in being use the basic speed of the Amiga can
really slip, but hey at least you can do those modes!

If you want something to be proud of, think sound.  The basic sound on the
Amiga is great - best I've ever seen.  And enjoy the flexiblity the machine
has.  A lot can be done with it.  

                            Wayne Knapp