They fooled so many: Fake cache chips!

Most of use have heard of fake cache chips. Today, we worry most about buying slow cache chips remarked as faster models for our retro machines. However, there was a time when dubious business people went the extra mile to sell fake chips! I found one of those boards at the scrapyard. If it is true, then the buyer of this board may have never known that this board never had any second level cache! Let's find out what is going on with those chips!

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▬▬▬▬ *Timestamps* ▬▬▬▬▬▬▬▬▬▬▬▬▬▬
00:00 BEK-tronic BEK-V429S
02:25 Article claiming my chips are fake
04:54 Original BIOS (2.01r) no cache
08:32 Preparing second BIOS chip
11:00 BIOS 1.2 no cache
12:18 BIOS 1.2 with fake cache chips
15:16 Testing cache chips with programmer
19:41 BIOS 1.2 with working cache chips
22:21 BIOS 2.01r with working cache chips
25:09 Cache Check
25:49 Testing cache chips with multimeter
28:00 Conclusion

[vincentpremel]

Back at the time, a french magazine took a suspicious motheboard to the nearest dentist, to make a X-ray pic of the fake chips : thoses were empty.

[lxforever]

“None” and “256K” are both exactly 4 characters long. The simplest hex edit in the world.

[ascender]

I actually purchased a MB with these fake cache chips on it back in 1995. At the time, I was really obsessed with measuring performance, so when I upgraded from a 486 DX2-66 to a DX4-100 and saw no increase in performance, I started investigating. This was difficult at the time, but I finally read something about this and was able to confirm the scam. When I returned the MB to the retailer and told them the story, the guy smirked and said, "Hm...well, you're smarter than the average bear." :-D He all but admitted to the scam and replaced the board with a genuine board. I was able to confirm that the cache was good and I saw an appropriate perf boost after that. This was an ordeal, because this was not widely known at the time. The "internet" in 1995 was of course not what it is today. I had to smile when I saw this video in my recommendations. Yep...been there, done that. :) Thanks for the video!

[therealjammit]

A quick way to tell if there's any silicon inside a chip is to set your DMM to diode check mode and measure between Vcc and Gnd. With the black meter lead on the Vcc pin and the red meter lead on the Gnd pin you should be able to measure around 0.6 volts on an actual chip. Every chip has a reverse biased diode from Vcc to Gnd that's unintentionally made during doping.

[DarkZenith]

Top of the article is dated 95 not 2000. The way back image date is 2000. Thanks for another video!

[rojovision]

That's a pretty wild discovery. I've heard of fake cache chips, but a BIOS apparently designed to lie to you about it is something else entirely.

[StrikerX]

I have disassembled both BIOSes and essentially what they're doing is making the function that checks for the cache size to always return a value that says it has 256K of cache.

On the v1.2 BIOS, the routine that prints the cache size is at address 2406h. On address 2420h there's a call to a subroutine at A8A0h which does some magic with the CMOS registers and returns some value in AL (presumably reading the cache size from the motherboard).

On the modified v2.01r BIOS this routine is located at 24E3h and the call is at 24FDh. Note that this time around, this particular call jumps to an intermediate routine that calls the original one at 9E23h, which does the same exact magic with the CMOS registers and returns, then it checks if AL is 5, and if it is not, sets AL to 5. So presumably AL=5 means 256K of cache. (A bit inefficient tbh, you could just set AL to 5 and return.)

The easiest way to fix this (assuming there's no checksum validation) is to patch the bytes at 24FEh and 24FFh from 00 BA to 23 79, which will bypass the sneaky modified call that forces the cache size to 256K.
You could also play around with the modified BIOS and make it return different values for AL by patching bytes DF04h and DF08h to some other value to see if it reports other cache sizes. They must both match and the other patch must be undone.

[VladoT]

The pinnacle of fake cache chips is a series of Socket 7 motherboards that we got here and had 4 black rectangle (QFP) chips with embossed "Write Back" on their plastic housing and the housing was just plain PVC with pins. Hilarious.

[stamasd]

I have a 386 motherboard that came with fake cache chips. Not only that but they weren't socketed - they were soldered on the board. And had a doctored BIOS of course that showed "cache" on POST. I unsoldered those chips, installed sockets and real SRAM in them and it does have real cache now. I was lucky enough to find on ebay a number of years ago a large lot of SRAM cache chips that were NOS in tubes from a surplus lot so I have plenty of those. They are not the faster 15ns variety but rather 20ns, and still they work fine as cache in all motherboards I used them in.

[TonyLing]

I remember a board which had soldered in 'cache chips' but if you were to follow the PCB tracks, they went nowhere.

[argoneum]

BIOS with double characters comes from 286 era, when there were two 8b chips (EVEN and ODD, or HI and LO) for a total 16b-wide ROM function. There was copyright repeated on both chips, hence the double characters. 486 supports bus sizing, so it can read 8b ROM (almost) directly, yet apparently the characters remained as they were before. There is an option in setup to turn on ROM's shadow (copy it into RAM), which speeds things up (32b access). When it's set ROM chip can be removed after boot with no effect, but don't try it 😉 Later BIOSes were compressed (more advanced), and they decompressed into RAM after power on, so those could be hot-swapped for flashing. Did that when computers were expensive, and programmers weren't actually cheap to get either.

[danielktdoranie]

Old guy here again, I vaguely recall a scandal regarding fake cache chips back in the 90s

[advil]

Just out of high school back in the 90s when I was working for my first employer, before I started my own business, I built so many of these 486 and early Pentium systems. You memorize the jumper configuration for the boards pretty quickly. I wonder if a bunch of them had fake cache and none of us youngsters on the build crew ever realized it? We didn't have the means of verifying it. And frankly, back in those days, there wasn't much reason to suspect. I mean, as you show, the BIOS even happily displays cache that isn't there. It was so long ago now I can't remember the specifics anymore but we did do some stability tests and benchmarks and we were all PC performance junkies. You'd think we would have picked up on the cache being totally non-functional. But it was a different time, and we were all pretty green.

[M_McFly]

This brings back memories. Also of the time when they used to put parity generators on the RAM PCBs instead of real RAM parity chips.

[codingwithculp]

OMG.

A few years ago I put together a Tomato 4DPS 486 system with a Am5x86-P100 that I stably overclocked to 200Mhz.

If I remember correctly, the board could hold a whopping 512 K of cache. So I went and searched for cache chips and bought a bunch from few different sellers and sources.

All of them were fake.

I finally gave up on it.

[ThBeowulf]

I would err on the side of preservation. Submit it and see if it can be tagged as modified to show invalid cache levels.

I'd love to see a hex diff of v1.2 and your v2.01 BIOS.

[SianaGearz]

Haha Georg Schnurer from c't is an absolute legend, always worth a read! Still active, still core team, almost 30 years after this article. Also generally seen in c't videos, he's the cheerful guy.

How much could they have saved by shipping fake chips? A couple dollars? 256k SRAM shouldn't be so expensive by mid 90s!

[tony]

I came to know about motherboards with fake cache when I repaired a PC-Chips board. Mine had genuine cache but I wasn't aware that "fake cache" was an option!
It amazes me, the picture you showed shows an IC coming from manufacturing, they just didn't add the actual die which is probably the expensive bit! Fantastic!
And someone edited the BIOS from "None" to "256 KB", so simple and so hilarious!

My first PC, a 386/DX 25 had no cache. I couldn't afford it. Imagine I paid big money (for a teenager in 1992) for cache and not even realise that the long awaited cache was just a piece of LEGO.

I'd share the BIOS anyways, with a note explaining what it does :) For historic purposes :)

[rigues]

Nice video. One of the first things I remember about starting university (Computer Science) here in Brazil in 96 was a warning on the student newsletter about motherboards with fake cache chips.

[RWBHere]

I've owned 486 and Pentium boards with soldered on board 'cache chips' with Copper traces going nowhere. A whole corner of the board was doing nothing electrically.