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From: hah@isum.intel.com (Hans Hansen)
Newsgroups: comp.sys.amiga
Subject: Re: Help: definitions and terminology
Message-ID: <581@omepd>
Date: Mon, 20-Apr-87 02:13:56 EST
Article-I.D.: omepd.581
Posted: Mon Apr 20 02:13:56 1987
Date-Received: Tue, 21-Apr-87 01:10:30 EST
References: <8704192056.AA07352@cory.Berkeley.EDU>
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Reply-To: hah@isum.UUCP (Hans Hansen)
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Organization: Intel Corp., Hillsboro
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In article <8704192056.AA07352@cory.Berkeley.EDU> dillon@CORY.BERKELEY.EDU (Matt Dillon) writes:
>>Amy vid chip
>
>	The Amiga's Video chip.

Err-ah, naw.  What it/they is/are and where it/they can be found.
A sound chip developed by Atari.  Also a sound chip in the new IBM PCs.

>>video gate array
>
>	???

The device that the Atari ST uses to produce its video.

>>copper
>
>  To do all the stuff the Copper does in the 68000 would take
>considerably more time (10%+).

Anybody want to guess at maybe 100-200%.  Lets consider what is done by
the copper, (Agnus), and also understand that the copper is the heart of
the three chip co-processor call the "Custom chips".  When you consider that
these three chips handle all of the serial IO thru the RS-232 port, the
floppy drive IO and the blitter decodes the trackdisk, the video screens
and sprites, four audio channels, plus the interrupts for the system, I
think that you will have to agree that you would need at least two more
68ks to do the work and still have the same quality of overall performance
that the Amiga custom chips deliver.  Dont forget the lost processing
time that the 68k could not do if it were handling all of these other
functions.

>
>>chipmem
>
>	Chipmem refers to the area of memory that the custom chips can access
>with their DMA.  Currently, the custom chips have only enough address lines to
>access the lower 512K of memory.  Any custom chips which steal processor
>cycles only do so when the processor is attempting to access this area of
>memory.

Close but Chipmem actually refers to the first 2MegBytes of RAM in the
Amiga's memory map.  The existing copper, due to a lack of pins, can only
address 512k of memory.

>>fastmem
>
>	FastMem refers to any expansion memory you might add to your 
>Amiga beyond the first 256K expansion you stick in the front of your machine.
>That is, any RAM beyond the 512K mark is fast memory.

Close again but..  Fast mem refers to the memory above the chip mem area
of the memory map, or any RAM at or above 0x00200000.

>
>	The term 'fastmem' comes from the fact that the 68000 can run at
>full speed when referencing FAST mem no matter how many cycles the custom
>chips are stealing from CHIP memory.

Two problems here.  First the custom chips don't "steal" memory cycles,
instead they utilize the bandwidth more effecently than the 68k and are
therefore told to take more of the chip memory bandwidth to get the
job done.  (I know it's splitting hairs but how can you steal something
that you are ordered to take?).  Secondly the reason the 68k can run
faster when not addressing chip mem is that the chip mem area is seperated
from the rest of the 68ks address map thru a DMA port that fully isolates
the custom chips and their RAM from the rest of the system.  Only by
addressing the chip RAM area of the Amiga's memory map and then thru
an arbitration process within two PALs is the 68k allowed access to the
chip mem area.

>
>>graphmem
>
>	Graphics memory?  Generic term meaning the memory available to
>some graphics device.  On the Amiga, this is the lower 512K portion of memory.

I believe this is a non-Amiga term to describe the assigned video RAM
area of their systems.

>>Agnus, fat agnus, paula, portia, diane, etc.
>
>	The Amiga's custom chips, with some hardware terms sprinkled in.

Never heard of a "fat agnus" or the "diane", and as far as I know neither
are C-A chips.

>>What is bandwidth (I need a really exact definition of this one, it's all
>>kind of fuzzy in my head, must not be enough bandwidth, eh?), and how does it
>>relate to graphics display?
>
>	There are two kinds of 'bandwidth'.  One refers to the 'bandwidth
>of your monitor' (basically the resolution it can support).  The other
>refers to MEMORY bandwidth... how many bytes/sec can be transfered from
>main memory.  An Amiga 1000 has a total memory bandwidth of about 7MBytes/sec.
>(memory access every two cycles of a 7.14Mhz 68000 = 3.57M access/sec, @
>16 bits / access = 7MBytes/sec).  (correct me if I'm wrong).

PArtially correct, the 68k's bandwidth is 7.14MByte however the bandwidth
of the Amiga is really 14.28MBytes.  The custom chips run at twice the
data rate of the 68k!  This is the main reason that the 68020 turbo, 14.28MHz
processors work so well, (well maybe just a little help from the forsight
of Dale Luck and C-A).

>>TurboAmiga
>
>	Coined by ASDG, I believe.  Refers to an Amiga with a 68020 in it,
>usually with some 32-bit wide memory running at 16Mhz.

Try CSA and 14.28MHz.

>>the AT&T blitter beast
>
>	No idea.

Me neither.

>>other blitters (how about a knowledgeable comparison?)
>
>	the Atari ST will soon have a blitter, but rumor is that it will
>	be a very simplistic one.

Until I can buy one off a dealers shelf I will consider this just a big
fat piece of vapor ware.  If Atari ever delivers any blitters they will
probably be in their workstations and not the ST line, too many machines
to retro fit and the fact that not all STs will be retrofitted would
cause quite a bit of selling in the Atari arena.

>
>					-Matt

	For the most part Matt's answers are correct, and I would not
have added to his answer except I was just tired of seeing people say
that the custom chips steal memory cycles!  The DMA circuit in the
Amiga is very efficient and using the copper to do tasks is the most
efficient use of the chip memorys bandwidth.  The fact that the 68k
can be blocked from memory access to the chip ram during heavy usage
of the blitter, just says to me that the custom chips are doing something
that would take the 68000 4 to 20 times as long to do.  One related
thing that is mostly over looked and that is that if the 68k really
needs to take back the chip memory accesses that it has given up it
can.  There is one "hole" on every scan line left open for just the
purpose of turning off blitter_nasty_mode.


	I really wish that C-A would write all of this up and use
it in their ads, or at least give it to their dealers.


				Hans