Qzero with SRAM extension reaching 40 MHz 68040

[Peter]

The Qzero is available, but for friends only. No time for a public release. There are too many variants and a complete lack of documentation. Mind you, it would be a three-board sandwich for most: QLbase, Qzero and SRAM add-on. A laborious build.

If I decide to release the "superfast" SRAM variant into general public someday, it is more likely to be a completely new board. Sort of a 3-in-1 solution or at least a 2-in-1 solution. We will see. And remember what I wrote here: viewtopic.php?p=60775#p60775

[ql_freak]

Well, that's unfortunately no problem for me. Cause I have (currently) no time to tinker with Vintage Computers. But at least I have 3 BBQLs, GC + Hermes, TC (with level 2 directories!), 512 KByte RAM-expansion plus Kempston Floppy-Disk-Controller, 2 C68, 1 QIMSI AND an EPROM-programmer (up to 64 KByte IMHO). Waiting for retiring :-)

p.s.: And of course a lot of other stuff and original software.

[Mark Swift]

Hehe, very good as it still seems to work fine!

The only issue I have had was a year ago when I powered it on for the first time in a while.
Time appeared to be going backwards. I replaced the timekeeper chip with a M48T02-150PC1

How did you manage to get a full size picture? Still a CRT monitor?

I don't have a working VGA monitor so I use a VGA to AV Adapter although it does cut off a few pixels on the right which I believe you can see on my screen grabs.

If I do need screen grabs I convert to AV and use an AV Video Recorder.
I originally bought this as a system agnostic way to transfer from VHS.

My setups usually end up in a mass of twisted cables and converters.

[Peter]

The only issue I have had was a year ago when I powered it on for the first time in a while.
Time appeared to be going backwards. I replaced the timekeeper chip with a M48T02-150PC1

Yes, it includes a battery and needs to be replaced after about 15 years.
Fortunately, it is still available off the shelf. One can also use the M48T02-70PC1.

I don't have a working VGA monitor so I use a VGA to AV Adapter although it does cut off a few pixels on the right which I believe you can see on my screen grabs.

That's relatively good. For better results, one needs the OSSC. The 1024x512 resolution of the Q60 has become such a pain after multisync monitors went out of business. I use a modded video controller PLD generating 1024x768, but a black bar at the bottom. I have considered a Q60 video card to resolve this, but the effort comes close to a completely new Q60 mainboard.

Back on topic, a '060 still easily beats the Qzero with SRAM extension. And fast parallel SRAM is so expensive, that cost for 16 MB would exceed a 68EC060.

[Nasta]

If I recall correctly, the original 68k has 2 16-bit ALUs, one for data and the other for address calculations. It was not before the 68020 was introduced that this was expanded to 32 bits, and then it's core was re-used as the CPU32 which is essentially a 68020 without cache. It has benefitted in speed from these enhancements. As far as I understand the FPGA core has a full 32 bit ALU and data paths so if that can run at 40MHz with fast bus access, no wonder it can get that performance.
That begs the question - what if a small cache was added to the regular q68 or quimsi? Considering QL code is quite dense and small, I would expect a good speed up even with a very small cache.

[Peter]

Hi Nasta! Good to read from you.

As far as I understand the FPGA core has a full 32 bit ALU and data paths so if that can run at 40MHz with fast bus access, no wonder it can get that performance.

External data bus is only 16 bit. But you were right with your estimate a few years ago.

That begs the question - what if a small cache was added to the regular q68 or quimsi? Considering QL code is quite dense and small, I would expect a good speed up even with a very small cache.

At 40 MHz, I'd need one CPU waitstate for cache based on FPGA block RAM. Results would be nowhere near the SRAM extension.
By the way, did you find a little time to try QIMSI Gold?

All the best, Peter

[Peter]

SRAM is expensive, so I came to consider a "mixed memory" approach with only 4 MB of SRAM. For example, only the lower 1 MB and the upper 3 MB of the main memory would be mapped to fast SRAM, while the rest of 28 MB stays SDRAM. (Video memory is separate on Qzero anyway). This would not require any OS changes and make things affordable. As long as the machine is only used SuperGoldCard-like, it would run full "Q40 speed". Which is more than 3 times faster than a regular Qzero.

Only if more memory is consumed, the Qzero would partially slow down. Which is better than just going out of memory. The disadvantage would be: If more than 4 MB total are consumed, speed depends on the code/data position in memory and is not easily predictable. Any thoughts on this?

[Pr0f]

Any way you can set some sort of "afinity" flag so certain types of code will always go to the faster memory?

[Derek_Stewart]

HI Peter,

Personally, I am not worried by the increased cost of SRAM, but but much more expensive would to have the Qzero use fast ram completely and not use slow ram?

Not knowing exactly what the use of slow ram compared to fast ram, could slow ram be eliminated, for greater speed?

[Tinyfpga]

I'm with Derek on this, but I don't know much about the complexities of using SRAM instead of DRAM. In fact I know very little about computing systems.

SRAM is certainly expensive but is much cheaper than it used to be.
A 32 Mb (4MB?) IssI asynchronous 12ns TSOP, part no:- IS64WV204816BLL-12CTLA3 costs about £40. A pair giving 8MB would still be affordable for those that valued speed above everything else.

Yesterday I played around with my memory hungry QPC setup using 4MB of RAM. I found it perfectly usable, 8MB was ample for my needs. I don't know how QPC uses memory. Does it use it up for display purposes?

The zero has separate video memory so I assume this reduces the need for "main memory" considerably, especially at maximum resolutions.