From: wallmann@backus.pu.informatik.th-darmstadt.de (Natuerlich!)
Subject: Re: Amiga Blitter 40% slower than Atari Blitter (???)
Date: Tue, 6 Oct 1992 18:24:56 GMT
Organization: MSD software vault #9 @THD

In article <u894776.718249257@bruny> u894776@bruny.cc.utas.edu.au (James McCoull) writes:
>wallmann@backus.pu.informatik.th-darmstadt.de (Natuerlich!) writes:
>
>>Let me check out how this might be worst case though.
>>You forgot to run ye olde raytracer in the background. The CPU can't do 
>>anything useful. (Even on this HP Workstation that's the regular state 
>>of affairs 90% of the time). That cuts the 4x factor into 2x. 
>
>Anyone care to explain this bit of logic to me?  How does the 4x factor
>get cut down to 2x? 
Well that's the way the 4x factor was built up, double the bus rate and
you get the 68000 for free too. Read the orignal post by Dave Haynie.

>	So your chipram system has about 3575450 memory slots available.
>Say take 4 clocks per scan line for the memory refresh, and a few other
>things [cant remember the exact details sorry].  Take off another 80 slots
>for the video fetch [320 pixels = 20 short words x 4].  So that leaves us
>with about 140 memory slots per display line, and over 220 memory display
>slots for the vblank area.  So the total of cycles available for the blitter
>and cpu is: 220*(312.5-200)+140*200 ie over 52000 memory slots in chipram.
>That is over 2637500 memory slots per second, each slot capable of transfering
>16 bits [I assume you understand this much no?].
OK agreed so far. Assuming a refresh rate of 60, I'd calculate
8000x4x60/2=960000 cycles -> 2615450 (ignoring refresh)

>Now lets compare with the bog standard Atari STe.  There are no more than
>2000000 memory cycles available per second.  Add the overhead of the
                         ^^^^^ for the CPU/Blitter.
>atari blitter, and its a real chuckle.
What overhead ? Why this sudden turn from precise arithmetic to fuzzy logic ? 
Could it be that with the 30% lossage of the Amiga blitter, both come out to 
be 'even' ??

>[deletions]
>Hmmm tell me how you can add this detection into a two staged pipeline
>blitter with more than two sources?  Your logic functions on the atari blitter
>are so brain dead that its easy to detect - you can just base it on the midword
>mask - with the minterms on the amiga its a bit harder.
Well sometimes you lose, when you add something to the chip. Do I really 
care WHY it's not any faster ? As a user most certainly "no". 

>Anyway if you are really after speed... then you split it up into three
>blits no?
Sure, but we were talking hardware. If you are multitasking and you are doing
split blits, I suspect it might not look so good, when task switches hit
you between blits.

>Now I will state it clearly - I have nothing against STs, but the blitter
>in the STe is an absolute joke.  Even a "hardcore" atari coder I know
>[owns an ST, STe, Amiga1000 [which he hates]] admits that the blitter in
>the STe is a waste of space.
It is not. I'd agree that it's not cleanly implemented in a systems design
way but its certainly not useless. For that you just run the machine once
with and once without the blitter. Heck anything, that equals the fab Amiga
blitter in some (common) operations can't be a waste of space now, can it ?
(har) 

>The STe was an "amiga killer" wasnt it?  The amiga is a games machine [as
>the atari folk say].  Then why is the STe blitter only good for scrolling
>desktop windows, rather than real game work?
Its also used in the MEGA-ST (no-e) series, which I own one of, and which
came earlier. The added STE features, where stereo-DMA sound, 4096 colors
and some improvements on the video system. I don't think you will see me 
arguing that the Atari Blitter is the superior chip but, I'll write 
everytime that someone translates Dave Haynie's 4x improvement calculation 
in best case, to "Well the Amiga Blitter IS 4x better". 

Nat!

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