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Why the 4MB limit on RAM?


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#1 6BQ5 OFFLINE  

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Posted Wed Jan 16, 2019 6:59 PM

Could someone explain to me where the 4 MB limit on RAM in the ST came from? I thought the 68000 processor had 24 address lines for accessing 8-bit memory. Given 24 address lines the ST should be able to address 16 MB of RAM.

I understand there are aftermarket modifications available to go beyond the 4 MB limit but those arent the focus of my question.

I also understand that 4 MB was a lot of memory for most users back in the 80s. My 520 STfm had 1 MB and I do t remember ever bumping into software I couldnt run.

Thanks!

#2 ParanoidLittleMan ONLINE  

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Posted Thu Jan 17, 2019 1:12 AM

ST memory is 16 bit wide. Address lines are actually 23 - no A0, instead it there is UDS and LDS

Then, not possible to assign whole address range to RAM, since ROM and peripherals need own address space.

 

Surely, 4 MB was a lot in 1985. And that would be main reason. More accessible space would need more logic and more pins on MMU chip = higher price.



#3 ijor OFFLINE  

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Posted Thu Jan 17, 2019 7:32 AM

I agree with Pera, 16MB is not really possible, or at lest not so simple, because the total address space is not only for RAM.

 

8 MB (or even something like 12MB if you want) would have been possible in theory, but MMU is designed for 21 RAM address bits only. GLUE also is hardwired for 4MB ram maximum, but that would have been easier to modify.



#4 hunter44102 OFFLINE  

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Posted Sun Jan 20, 2019 9:26 PM

On a similar topic how did the Falcon get to 14MB (upgrades) from 4MB? 



#5 TornadoTJ OFFLINE  

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Posted Wed Jan 30, 2019 12:33 PM

The Falcon is a 68030 and a different architecture. The TT/030 didn't stop at 4 MB either, with additional memory going in Fast RAM. You had a utility to tell the applications whether they were to load in ST RAM or Fast RAM.



#6 Bikerbob OFFLINE  

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Posted Fri Apr 12, 2019 11:25 PM

I still dont really understand.. how does a computer designed in 1976 ( the atari 8-bits) work with 4mb expansions and they were designed with 48K. Was the design of the ST so handicapped that it would really require a redesign of many of the chips to address 8mb ?? 8mb would make this machine so much more usable when it comes to Mint or Magic. 

 

James


Edited by Bikerbob, Fri Apr 12, 2019 11:25 PM.


#7 ParanoidLittleMan ONLINE  

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Posted Sat Apr 13, 2019 3:08 AM

It is possible to expand ST to 12 MB linearly addressable RAM, and such expansions exist.  The key is in linear addressing. 8-bit CPUs can access max 64 KB linearly, so all above is with some bank switching, what needs special programming in segments. Should first do some research before claims like that.



#8 LoTonah OFFLINE  

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Posted Sat Apr 13, 2019 3:52 AM

Well, for a similar but somewhat different question: why can't the ST use bank-switching to increase memory?  I always find it strange when an older design has more flexibility than something newer (ie. how the Atari 2600 has more colours available than say, the ColecoVision, when the TI chipset in the Coleco came out after the Atari came out)?



#9 ParanoidLittleMan ONLINE  

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Posted Sat Apr 13, 2019 4:19 AM

Who said that Atari ST can't use bank switching ?

It is possible with such add-on, but there are good reasons why linear memory addressing is preferred. 

With fast mass storage there is no real need for more than 4 MB. And that's is what is supported with later CPU designs - at 68010 there is Virtual memory support.



#10 6BQ5 OFFLINE  

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Posted Sat Apr 13, 2019 10:28 AM

The modifications and add-on boards could be made and installed ... but, you may also need to update TOS to recognize and support the extra RAM. Also, what application would need it or try to access it?

Would the STe even have enough room for extra memory sticks?

#11 zzip OFFLINE  

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Posted Mon Apr 15, 2019 9:22 AM

Could someone explain to me where the 4 MB limit on RAM in the ST came from? I thought the 68000 processor had 24 address lines for accessing 8-bit memory. Given 24 address lines the ST should be able to address 16 MB of RAM.

I understand there are aftermarket modifications available to go beyond the 4 MB limit but those arent the focus of my question.

I also understand that 4 MB was a lot of memory for most users back in the 80s. My 520 STfm had 1 MB and I do t remember ever bumping into software I couldnt run.

Thanks!

 

I believe it was due to the supporting chips of the ST,  not the 68000 itself.  



#12 ParanoidLittleMan ONLINE  

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Posted Tue Apr 16, 2019 5:59 AM

People forget some things:  in 1985 RAM was not cheap. Not cheap MAC had only 128 KB RAM - released in 1984.  Atari ST was planned with 256 KB RAM, but they realized that TOS, GEM need more.

In any case, chipset in ST can address 8x more than 512 KB, what is not bad at all. How many people had 4 MB in their ST, STEs in 1991 ? Not much, because it still costed a lot. Win 95 could work with 4 MB, 4 years later. I had 2.5 MB in 1990, and that was more than enough.

One of the reasons why Motorola MC68000 was so popular in 80-es was linear addressing. Unlike other popular 16-bit CPU Intel 286 . Only with 386 Intel could really compete.

Atari supported linear addressing well in HW and TOS, and that was great thing, much better than how early Macintosh solved it.



#13 Bikerbob OFFLINE  

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Posted Tue Apr 16, 2019 8:50 AM

I understand your responses, but everyone keeps talking about back then.. in 1991 in.. whenever.. that was good then..  We are talking about now.  There was nothing in 1980s on an Atari 8bit that used more than 64k ram.. but now we have 4mb.. most would argue even with all the demos etc..etc.. 1mb is tons .. and I am sure they are right because there is only so much you can program NOW.. that could use that ram.

 

With the ST line.. we have Networking .. we have graphics cards.. we have usb and VME etc..etc.. NONE if it was back then.. back then we did not need more than 4mb.

 

NOW - If I want to run my MEGA STE with NVDI 5. MagiC and some fonts.. maybe run papyrus or Word or try and use CAB or Highwire etc.. OH change the colours.. USE my ET4000 NOVA card .. I am out of 4mb in a sec.    I have seen boosters, and even the MonSTer card.. most of which do not work with the Mega STE.. it was the most advanced ST.. but seems to be the hardest to add on to.

 

I personally dont even care if I am faster than 16mhz..   OR I wish someone had never written NVDI.. or made a video card.. etc.. its like owning a high power race car (I know not by modern standards) and not being able to use it.. its frustrating..   IF banking the ram allowed me to run this stuff.. then I am all for it. I really dont care how.. I would just like to be able to do it.






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