dave@heurikon.UUCP (Dave Scidmore) (12/24/85)
Lately I have seen some conversation on the net which I feel could be somewhat misleading. The first is that the ST is half the price of the Amiga. This is quite simply not true. First off, not every Amiga sold needs a monitor, this is not true for the Atari whos 70Hz non standard video rate requires that you purchase their monitor. This means that for people who have a TV/monitor or an existing monitor that will accept the Amiga output, an Amiga without monitor vs Atari with monitor comparison would be more accurate. This aside, lets try to compare two machines that are as close to the same as possible. In the interest of making things simple lets compare machines with color monitor and double sided double density disk drive and comparable memory. The Amiga as it is shipped now really has 512K built in, 256K of user RAM and 256 of kickstart RAM. The Atari has 512K of RAM of which 206K is used for the operating system. This leaves a little over 300K for user RAM. This is about comparable to (though slightly better than) a 256K Amiga. The double sided double density floppy lists out at about $100 more than the single sided floppy. If you add this up you get $1099 for the Atari and $1795 for the Amiga. This makes the Amiga about 63% more expensive. This is much closer to 50% than it is to twice as much. Since both machines can be discounted by a dealer any comparison of discounted versus non discounted prices is invalid. The second thing I saw was a comment claiming that the Atari has special drawing hardware. I searched every article I could find on the Atari and while I found articles stating that it did have special display hardware I could not find even one reference to any special drawing hardware. As a matter of fact I ran across an article in Byte specifically stating that blitting was done in software in GEM. The third point of interest is that several people have been stated that the method the Amiga uses to share cycles with the custom chips must slow the system down. Well, according to what I have read at equivalent screen resolutions and equivalent number of color bit planes, and with no drawing taking place the Amiga and Atari share cycles in an almost identical way. In both machines the memory cycles take place in two CPU clock cycle, since the CPU takes four to perform an access the display hardware steals the first two to do its thing while the 6800 is asserting its address and the machine is decoding it. The second cycle is then used by the processor. Both Atari and Commodore claim that this system has little impact on CPU performance. Furthermore, in the Amiga during the horizontal retrace time when the display hardware does not need memory, the disk and sound hardware get to steal display cycles. This means that unlike the ST which must use CPU cycles to transfer data to or from disk the Amiga transfers the data transparently. Two things can make the CPU give up its even cycles on the Amiga, thus degrading performance. The first is if it uses 5 bit planes in low res mode (320 by 200 or 320 by 400) or if it uses 3 or 4 bit planes in high res mode (640 by 200 or 640 by 400). Since the Atari does not support these numbers of color planes you can only degrade performance by using a feature that the Atari does not even have. The other thing that can make the CPU give up its cycles are the copper and the blitter. When the "copper" is running it is usually setting up registers for the CPU. Obviously if the CPU were performing this task it would take it at least as long to do so. When the "blitter" is running it is doing drawing operations (lines, area fills or blt operations). Once a drawing operation has been set up, the blitter only needs to perform the memory accesses needed to do the drawing operation. The Atari on the other hand must not only perform those same accesses but must read the instructions that tells it to do them and must also read and execute instructions to do any ANDing, ORing and shifting needed to perform the operation. Anyone who thinks that the STs 68000 can do drawing as fast as the Amigas hardware has to have rocks in their head. Now if you don't need high speed graphics (or for that matter expandability) then all of the above probably does not interest you. But if you are planning to use good, high speed color graphics or can afford the Amiga then it is the machine for you. Their also seems to be a consensus that all that graphics are good for is games. This too is not true, their are many applications such as CAD or Imageing that need good high speed graphics. I believe that many of these types of applications will prefer the Amiga Dave Scidmore