Why the 4MB limit on RAM?

[6BQ5]

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!

[ParanoidLittleMan]

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.

[ijor]

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.

[hunter44102]

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

[TornadoTJ]

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.

[Bikerbob]

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 April 13, 2019 by Bikerbob

[ParanoidLittleMan]

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.

[LoTonah]

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)?

[ParanoidLittleMan]

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.

[6BQ5]

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?

[zzip]

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.

[ParanoidLittleMan]

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.

[Bikerbob]

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.

[EJRS]

I know it is an old topic but I was searching around as I had the same question.

 

So from what I can tell, and I could well be wrong.

 

The 68000 has 24 address lines so can address 16Mb. From what I can tell it technically has 23 and one more line which seems to be more of a upper/lower memory select (a bank select kind of) but it is essentially another address line – not sure why they did this rather than just label it A0.

 

The MMU on the STE only has 20 address lines (well seems to, the pins are labelled funny), so 4Mb. Interestingly I think the MMU on the standard ST only has 19 lines so can only support 2Mb. From what I can tell this is also the case for several of the custom chips in the Amiga so samples, screen, sprites, etc had be in the lower 2mb (I understand later chipsets “addressed” these limitations) although I think the MMU supported 4mb for the CPU. The STE DMA/Chips hardware could address all 4mb.

 

The data bus is 16bit so only a byte at a time is moved, I am not sure if a move.b only does one read from the BUS or if it was forced to do two reads.

 

The 68030 could address 16Gb so I am guessing the Falcon MMU only had 24 address lines, however, it could only have 14mb and the last 2mb were masked off, I suspect this was done DMA or DSP reasons. I am not sure what the TT was capable of, but I think some of the chips were constrained to the lower 2mb.

 

That said it may be that you could write software for the ST were a move.b $000,£ff8240 had the same result as move.b $000,£ffff8240 (as the upper 8bits were masked off) so it could be for compatibility. This is speculation, I have not done the math/research on this.

 

There were upgrades for both the ST and STE to go beyond their limits and I assume they tapped directly into the CPUs extra address lines and then some bodged external MMU logic. Any memory above 4mb (2Mb on the ST) was classed as TT RAM (Fast RAM) so only programs and their data could be stored here (as the other chips did not have the address lines). Now I am unsure if this was technically “fast RAM” as I understand the purpose of calling it this was because when the CPU was accessing this memory it did not have to share the BUS cycles with the other chips, I wonder if this may not be true in this situation.

 

If anyone wants to point out anything I am happy to learn.

[EJRS]

On 4/13/2019 at 10:52 AM, LoTonah said:

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)?

 

I was surprised when I found out that the 2600 had a palette of 128 colours (and good colours), this was in 1977 and would only be surpassed by the Atari 8bit machines (256 colours, 1978) and then not again until the likes of ST, Amiga, Megadrive, VGA, etc.

 

I say "good" colours as most other 8bit machines not only had less colours but also the colours they had were not nice. C64 was a pastel range which was fine for skin tones but useless for explosions, the Amstrad CPC could do explosions but anything else looked garish. Then think of the likes of CGA and EGA, why did they choose those colours! Is there a technical reason these colors were picked, I am thinking readability on poor CRTs or something or less circuitry if colours kept in a range?

 

In fact, if you limit it to 8bit machines/consoles I think the only ones that matched the 2600 and Atari 8Bit was the SAM Coupé released in 1989 the TurboGrafx-16 (yes 8bit CPU and 512 colours) release 87-89. The Commodore 16 and Plus/4 came close with 121 colours. There may be others, but I am not sure and they were not popular machines, the Fujitsu FM77AV40 (1986) apparently had 262144 colours.

[zzip]

1 hour ago, EJRS said:

I was surprised when I found out that the 2600 had a palette of 128 colours (and good colours), this was in 1977 and would only be surpassed by the Atari 8bit machines (256 colours, 1978) and then not again until the likes of ST, Amiga, Megadrive, VGA, etc.

I think there was another early console that bragged about 256 color palette.   Maybe the Astrocade?  It was one that didn't gain much traction and most systems of that era had an 8 or 16 color palette. 

 

1 hour ago, EJRS said:

Then think of the likes of CGA and EGA, why did they choose those colours! Is there a technical reason these colors were picked, I am thinking readability on poor CRTs or something or less circuitry if colours kept in a range?

Ugh, CGA was awful!   EGA was almost good enough color wise.  Skintones were off I think.  

 

I suspect CGA was intended for Pie Charts/Bar Charts rather than gaming.  PC was positioned as "all business" then.

Edited March 1 by zzip

[theboyfromanotherplanet]

If the 2600 had video ram it probably would have been a mono machine. With VRAM, more colours require more RAM which was expensive - probably a bit less expensive by the time the ST was released. I suppose, the 2600 could utilise so many colours as it didn't rely on expensive VRAM