Experimental DRAM challenge program

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Joined: Apr 1 2020 - 16:46
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Experimental DRAM challenge program

Hi Apple-1 owners -

this is a little experimental RAM diagnostics program I wrote and I'd like to kindly ask you to run it on your Apple-1, original or clone does not matter, although originals would tell us more, having the elusive original chip sets.

The program expects 4K of RAM at $E000-$EFFF however, which it challenges.

If everything is OK, it just produces dots at a rate of one per 1.6 seconds, but if the issue it looks for is found, it produces a message in the form XX@HHLL in which XX is the syndrome, and HHLL is the address, and then the program continues.


0280: A9 00 85 00 A8 49 FF A2

: E0 86 01 91 00 C8 D0 FB

: E8 E0 F0 90 F4 49 FF 8D

: 3F E0 8D C0 EF 8D 6A E5

: 8D 95 EA CA D0 F1 88 D0

: EE 49 FF 8D 3F E0 8D C0

: EF 8D 6A E5 8D 95 EA A2

: E0 86 01 D1 00 D0 11 C8

: D0 F9 E8 E0 F0 90 F2 AA

: A9 AE 20 EF FF 8A B0 B5

: 51 00 20 DC FF A9 C0 20

: EF FF 8A 20 DC FF 98 20

: DC FF A9 A0 20 EF FF 4C

: 80 02



If you change the last line (80 02) to 1A FF it returns to the monitor in case of an error found, which may be preferred if you let run it over more than 25 minutes unattended - you might miss an error message otherwise.

On most Apple-1 expect only dots. And I hope that actually, NO Apple-1 out there ever produces any error message.But if we find at least one, my hypothesis I seek to test with this experiment is verified, and we need to talk.

You are invited to contribute to the greatest experiment ever done on the Apple-1, and if you participate, please comment on this thread, type of machine (original or clone, which type of PCB), type, manufacturer and date code of ICs in locations B5 and B6, and type, manufacturer and date code of the DRAMs in Bank W, and which error messages encountered, and how many, after which runtime, if any. Please also send a short comment if you only see dots - which is fine, you should have a robust machine then.



Last seen: 1 day 18 min ago
Joined: Jun 18 2010 - 13:54
Posts: 288
I don't have an Apple-1, but

I don't have an Apple-1, but I'm curious about your program here. Is it expected to test the ONLY RAM in a 4K system? If so, then your JSR's are sure to fail...

Last seen: 18 hours 1 min ago
Joined: Apr 1 2020 - 16:46
Posts: 52
Be assured the program works on an Apple-1...

... what eludes me is why you think the JSRs will fail ?  Please explain ?


The program will run on any Apple-1 with memory in the low 4K bank ($0000-$0FFF) and memory in the high 4K bank ($E000-$EFFF), which it challenges, because it is the bank that has the worst signals on the multiplexed address bus to the DRAMs.  This mostly is due to a known layout issue. All Apple-1 should have these poor signals, originals or clones. No other 6502 based computer should be affected. The open question is whether the bad signals ever cause the DRAM to write to a wrong location, or not.


On my own build I can't get any error message from this program unless I pull one of the DRAMs in the high bank out.  Plenty of power supply bypass capacitors were put in, which may mitigate the problem, and I don't want to remove them again.




Last seen: 1 day 18 min ago
Joined: Jun 18 2010 - 13:54
Posts: 288
Sorry, I read your post too

Sorry, I read your post too quickly. I thought you were trying to test the low bank at $0000-0FFF.

Last seen: 18 hours 1 min ago
Joined: Apr 1 2020 - 16:46
Posts: 52
Anyone out there willing to participate ?

Hi Apple-1 (original or clone) owners on this list:


So far nobody has given any feedback about the results running the above diagnostics program.


Shame on you !


I continued working on the subject of these multiplexed address line ringing and I have some evidence now the problem is real and does cause DRAM errors.


If you don't trust my program then try Mike Willegal's DRAM test program on your $EOOO-$EFFF bank.


I bet you may see the occasional error message at location $E237: binary 1110 0010 0011 0111 so:


Multiplexed    row   address:  110111

Multiplexed column address:  001000


See a certain bit pattern here ? With lots of multiplexed address bits toggling ? Do you still think it's a hoax ?


So please participate in my experiment !


Of course your results will depend on your CPU type, DRAM type, pulse width of the CAS timing oneshot, but the problem definitely is out there in the field. And this means: it's lurking in your Apple-1 build.


Last seen: 13 hours 22 min ago
Joined: Feb 25 2014 - 23:19
Posts: 53
Can you relocate the code?

If you post this code relocated at $F000, I can include it on my PROM replacement adapter board...

fingerz's picture
Last seen: 4 days 10 hours ago
Joined: Aug 10 2017 - 13:40
Posts: 121
I'll try it on my Apple-1 and
I'll try it on my Apple-1 and Mimeo-1 next week...just haven't had time to set them up recently.
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