The Odd Story of Factory-Downgraded 486s

Counterfeits CPU were very common in the mid-90s. The worst period was between 1993 (just after the launch of the Intel 486 DX2) and 1998 (when the Pentium II started to be multiplier-locked). It was extremely easy for tricksters to remove the original marking and reprint another one with a higher frequency rating. Many DX4-75 were remarked to DX4-100, and even more Pentium 133/150 were remarked as Pentium 166 or 200s.

Genuine factory-remarked CPUs also exist, but they’re generally uncommon. The most well-known example is the double-sigma (ΣΣ) sign added on early 386s after they had been tested bug-free from the infamous 32-bit multiplier bug. Some rare Intel 486 SX were also later remarked with a higher speed grade. Here are two of them:

As for all factory-remarks, the addition is quite obvious. Intel probably binned twice these CPUs again at the request of a big customer (IBM?) and added the second rating later. Today’s story about factory-remarks is much more unusual because it concerns standard models.

Am486DX4-100SV8B (remarked 5×86)

After I published this analysis some weeks ago, a reader told me he had a strange Am486DX4-100 that seemed to be a AMD 5×86. After a careful look at the printings that looked 100% genuine at first sight, he was kind enough to lend it to me for further investigation with the UCA. Here it is:

The “9626” date code tells us it was manufactured in late June or early July 1996, which is quite late for a Am486DX4. I immediately noticed the 25544 package code, only used for the 350 nm die. This die was the basis of all Am486DX5 and Am5x86. The “C” stepping was also unusual as the Am5x86 is based on the A-step (from November 1995) or B-Step (from March 1997). A “C” Stepping build in 1996 is incoherent with the 5×86 line, but very coherent with the 486DX4 (later 486DX4 in the latest “C” Stepping were built on the 25498 package in May/June 1996). So it was time for a test on the Universal Chip Analyzer:


WOW! There is no doubt: this CPU is really based on the standard 350 nm die with a fully enabled 16 KB Write-Back L1 cache and a working 4x multiplier. Actually, it can even be overclocked easily to 133 MHz. All specs, including power consumption and CPUID (0x4F4), make it indistinguishable from an AMD 5×86. This CPU can of course also work with a 3x multiplier like an AMD 486DX4-100 (CPUID drops to 0x494).

After some research, it seems that all CPUs based on the 25544/C package are marked as 486DX4-100SV8B while being really DX5 SV16B (5×86). AMD produced them for quite some time between February 1996 and March 1997. They probably stopped the production of the old 500 nm die in early ’96 but still had some demand from customers for DX4s, so they just used the new 350 nm die and marked these CPUs as DX4-100s. As long as you use the default x3 multiplier, they behave exactly like the old one … except for the cache size.

Has Intel also done such weird things? I could have sworn no way. I was wrong…

Intel 486DX2-66 SK080 (remarked DX4)

The same reader also sends me a DX2-66 that could be “really a DX4-100”. That sounded odd and really unlikely to me because Intel has a strict policy on S-Spec. Intel DX4s also have a specific CPUID to help distinguish them from DX2s by software. Unlike AMD 486s, this CPUID does NOT change with the multiplier used, so it’s strange to have a DX2 with a DX4’s CPUID. Here is the original CPU:

Everything looks genuine here. SK080 is one of the least common S-Spec for Intel DX2s. The only other S-Spec beginning with “SK” is the extremely rare SK058. The SK080 is a 3.3V SL-Enhanced part which seems to have been produced only between WW18’94 (May 1994) and WW48’94 (November 1994). Let’s plug in into the UCA:

Awesome! This is really a DX4 factory-downgraded to DX2-66. The 0x480 CPUID leaves no doubt about the original die used here. The usual power consumption and the ability to work fine at 3.3V at 100 MHz let me think it’s probably a DX4-100. With the multiplier set at 2x, the SK080 also works at 2×33 MHz as expected for a CPU marked as a DX2-66. To be 100% sure, I was able to find another sample to confirm these findings.

The UCA 386 Adapter now supports AMD & IBM 386s

 IBM 386

As like all previous microprocessors, Intel licensed the i386 design to third parties. AMD was the only one legally allowed to sell Intel-based 386s to customers (as bare CPUs), but IBM was granted the right to produce some Intel 386s for its own use. They don’t look like a standard ceramic CPUs : IBM used a plastic substrate and a metal cover to protect the die and help with thermal dissipation. Here is how they look like.

IBM 386If the packaging is different, the internal die is the same as on Intel 386s. 7 different IBM part-numbers are actually known: 23F7189 (?? MHz), 32G6633 (25 MHz), 51F0352 (20 MHz), 51F1783 (20? MHz), 51F1784 (20 MHz), 51F1797 (25 MHz) and 63F7615 (25 MHz).I was able to put my hand on a 51F1784 and the later 63F7615. I tested both on the Universal Chip Analyzer. There is no “Pin 1” mark so I had to guess where is pin 1. Fortunately, the UCA has  strong over-current and short-circuit protection. Let’s start with the 63F7615 :

This one is able to work fine up to 33 MHz, with a CPUID set at 0x305, similar to Intel 386 based on the D0-stepping. I don’t know for sure the real rated frequency, but it’s probably only 25 MHz. The other one (51F1784ESD) is not able to work at 33 MHz and not even at 25 MHz. The actual (early) UCA firmware only has 16/25/33/40 MHz, so I can only confirm that that my 51F1784ESD works at 16 MHz but not at 25 MHz. According to various sources, it’s probably rated at 20 MHz.

AMD Am386

AMD also produced a lot of Am386DX at 20, 25, 33 and 40 MHz. While the microcode is 100% from Intel, the manufacturing process is different and they had lower power consumption (thanks to the 0.8µm process used by AMD instead of Intel’s 1µm CMOS-IV on the latest i386s).

Let’s start with the standard Am386 DX/DXL. I tested one Am386 DX/DXL-25 “B-Step”, one Am386 DX/DXL-33 “D-Step” and another Am386 DX/DXL-40 “C-Step”. All came in the 23936 package from Kyocera.

The UCA tool is not yet able to detect them as AMD, but I’m working on a new algorithm based on power consumption to distinguish them from Intel 386s. The B-Step identifies itself as 0x305, the same CPUID used on Intel’s 386 D0-Step. The DXL-25 was able to work up to 33 MHz. Both C- and D-Step have a CPUID set at 0x308, like the later Intel 386s (D1 step and up).

The last CPU to try was the Am386DE-33, an uncommon embedded model. Like the Am386DXL, it uses a fully-static design, meaning it can be clocked down to DC (0 Hz) while retaining all its internal registers content. The biggest difference between the usual Am386DX/DXL and the Am386DE is the disabled Paging Unit in protected mode on the latter. Bit 31 of CR0 (used to enable paging) is reserved on Am386DE. Another difference only available on the Am386DE is the ability to work at its rated frequency with a much lower voltage (down to 3.0V). And it works fine:

At 3.3V, the power needed drops by a huge margin, from 1.1 Watt to as low as 461 mW (0.46W). That’s a -60% power reduction!

[Guide] Am486 Die & Packaging

After weeks spent to test A LOT of AMD 486 with the Universal Chip Analyzer, messing with a gas torch  to decap some of them and speaking with a former AMD engineer that worked on them back in the 90s, I’m happy to publish here all the information I was able to get! Here it is:

The Ultimate AMD 486 Die & Packaging Guide


PS: If you have any more information about AMD 486, please leave a comment. Thanks!

The Universal Chip Analyzer now supports Intel 386 !

Another milestone has been reached. A new iconic CPU family is now supported by the UCA: the Intel 80386, the very first 32-bit x86 microprocessor! The i386 was originally released in 1985 at 12.5 MHz and 16 MHz. It added a 3-stage instruction pipeline and an on-chip MMU (Memory Management Unit) able to address up to 4 GB of RAM. A giant capacity for that time. The original Intel 386 – then renamed 386DX – comes in a PGA132 package and its frequency was later upgraded to 20 MHz, 25 MHz and finally 33 MHz. Later clones from AMD & Cyrix – not yet supported by the UCA – were also released at frequencies up to 40 MHz.

The design of the UCA 386 Adapter was challenging because of the high frequencies involved. While all 486s work with a standard clock input, 386s require a clock-doubled signal with a strong voltage swing (CMOS) between 0 and +5V. Generating frequencies up to 80 MHz (for a 386DX-40) with these requirement was not possible with the UCA architecture, so the UCA 386 Adapter includes an external clock-doubling PLL. For the first try, I used a NB3N511 clock multiplier from On Semiconductor but I was unable to reliably run 386s at more than 20 MHz. The UCA is based on a FPGA and timings are crucial.  With the NB3N511, I was unable to successfully match timings because of the lack of phase synchronization between the input clock generated by the FPGA and the doubled frequency fed to the CPU. So I needed another PLL, with 0-delay between input and output.

After some research, I gave the ICS570A a try and it worked perfectly fine. I was able to sync the internal FPGA logic to the clock-doubled CPU signal up to 40/80 MHz, without having direct access to that signal. Some high-quality ceramic decoupling caps were also mandatory to achieve the highest frequencies. I had a bad surprise while looking at ZIF socket for the 386 Adapter: for some reasons, PGA132 ZIF Socket are extremely expensive and I was unable to source them at a decent price. I bought some at ~$25 each, but if you can help me find more at a lower price, please drop me an email!

The 4 standard test frequencies for 386 are set to 12.5 MHz, 25 MHz, 33 MHz and 40 MHz. Right now, only Intel 386 are supported, but I’m working on AMD, Cyrix, etc. clones and I’m confident the UCA will support them all very soon. Here are the i386 I have for testing:

  1. The first one is an early 386 clocked at 16 MHz and produced in 1986. The ΣΣ sign engraved shows that it has been tested free of the infamous 32-bit multiplier bug (more on this on a later post). According to this source, the S40344 S-Spec is a B1 stepping. This is confirmed by the CPUID displayed by the UCA Analyzer tool: 0x303. At 12.5 MHz, this CPU requires about 188 mA at 5V (a bit less than 1W).

  1. The second one is very similar to the first one, except the rated speed at 20 MHz. Still the same B1-Stepping, same CPUID, same ΣΣ, same power consumption, same everything. It also can’t be overclocked at 25 MHz.
  2. The third one is also rated at 20 MHz but don’t have any S-Spec. It has been assembled in November 1988, more than one year after the previous one. The CPUID is different at 0x305, which indicates a D0 stepping. Surprisingly, the power consumption is 10% higher, at ~212 mA for 5V at 12.5 MHz. Maybe Intel added some logic to solve the numerous erratas in previous stepping, maybe it’s just sample variation. Anyway, the D0 stepping still uses the CHMOS III (1.5 µm) process. This chip can be overclocked at 25 MHz

  1. The fourth one is a 16 MHz marked SX236 and build with the more advanced CHMOS IV process. The CPUID is 0x308, which is used by Intel for D1, D2 and E Stepping. The last stepping is usually marked on the chip, so we can guess it’s a D1 or D2 stepping. Power consumption drops from 212 mA to 147 mA, thanks to the 1 µm process (instead of 1.5 µm). Unfortunately, it can’t work at 25 MHz.
  2. The fifth one is a A80386DX-33 made in 1992 with s-spec SX366. It’s the faster clock speed Intel released for a 386DX. The CPUID is still set at 0x308 but the stepping is clearly more advanced: the power consumption drops to 126 mA while using the same CHMOS IV process than the previous one. This particular CPU requires 126 mA at 12.5 MHz, 206 mA at 25 MHz and up to 261 mA at 33 MHz. It can’t be overclocked to 40 MHz.

  1. The last one is much newer than the others. It was manufactured in 2000 and uses the E-Stepping, as stated by the last letter of the lot code. Aside from this, all specs look identical to the SX366. Measures are also the same and it doesn’t work at 40 MHz.

Stay tuned for more exciting news about the UCA!

[UCA CPU Analysis] Prototype UMC Green CPU U5S-SUPER33

While sorting some new Engineering Samples I received lately, I exhumed some prototypes from my collection. They came without missing pins, so they are good candidates for an advanced investigation with the Universal Chip Analyzer.

Let’s begin with the first one, a UMC Green CPU U5S-SUPER33

It’s marked “Confidential” on the last line, which means it’s an engineering sample. The date code is quite early: 9416. It was manufactured on the third week of April 1994. This CPU is not one of the very first samples of the whole U5S line regardless of the frequency, but probably a prototype for the specific 33 MHz version. Also notice the famous “Not for U.S. sale or import” line, written here because UMC was afraid – and rightly so – of the legal consequences of infringing Intel’s ‘338 patent.

Let’s try it on the Universal Chip Analyzer:

The prototype works fine up to 33 MHz. One of the first interesting points to check is the support of the CPUID instruction on such an early prototype. A few weeks ago, I was chatting with mtx500 (another well-known and very technical-aware CPU collector) about the way to detect UMC CPUs. He told me he uses the SALC/FS method to distinguish UMCs. The idea is to use the undocumented Intel opcode 0xD6 “SALC” (Set AL on Carry) instruction with the 0x64 “FS:” prefix. Only on a UMC, the combined 0xD6 0x64 opcodes return the “magic” constant 0x0AB6B1B07 in the EAX register.

I was wondering why to use this method when the CPUID instruction is supported? Mtx500 told me that early U5S might not support the CPUID instruction, so I was impatient to try on an early U5S like this one. It looks like the CPUID instruction is well supported, with the expected “UMC UMC UMC” string reported as well as the usual 0x423 family/model/revision on U5S(X). At first sight, this prototype looks strictly identical to the retail version. I ran some benchmarks to compare with later U5S and the cycle count of the test instruction flow is exactly the same.

However, on closer inspection, I found a noteworthy difference: power consumption. I ran the same INT benchmark on my 4 U5S with the voltage set at 5.11V exactly on all of them. The UCA is quite precise at measuring power consumption. All of them were tested at a fixed 33.3 MHz frequency, no matter their rated maximum speed.

The results are quite interesting. As you can see, all my retail U5S consume (almost) the same current: about 308 mA at 33 MHz while running my benchmark code. For an unknown reason, the U5SD is a bit higher at 321 mA, but the difference in power is only 50 mW. In the opposite, the prototype U5S-33 need MUCH more power to process the exact same code in the same time at the same frequency with the same voltage: 421 mA, which translate to about 2.15 watts.

The most obvious explanation is a switch in manufacturing process between the early prototype and the commercial revision. UMC was, at that time, one of the two major IC manufacturers in Taiwan, along with TSMC. We can consider they can switch easily between processes. On the very first U5S datasheet published in 1993, UMC indicates the U5S is built using a 0.6 µm CMOS process. This is consistent with the power consumption seen on the sample. I was able to find a table of the manufacturing process evolution in Taiwan in the 90s.

UMC and TSMC switched from 0.6 µm in 1993 to 0.4 µm in 1994. According to this table, it seems likely that the prototype is build using the ’93 process (0.6 µm) while UMC switched to the ’94 process (0.4 µm) for their retail mass-volume production. The retail U5S tested here need about 27% less power than the prototype. The theoretical reduction in power consumption between 0.6 µm and 0.4 µm is 33%, so that makes perfect sense. This Engineering sample is probably an early U5S manufactured with the original 1993 0.6 µm process. It is unknown at this point if retail (non-prototype) UMC Green CPU have ever been built with that process. Another analysis with a retail U5S produced earlier than August 1994 (Week 33’94) would allow us to be sure…

Identifying “blank” 486s with the UCA #1

The Universal Chip Analyzer is useful to test and spot counterfeits CPUs, but also to help identify CPUs without markings. The lack of printings on a CPU can be caused by a poor ink quality that gradually faded out over years, by abrasion with other ICs (common when you saved a nice CPU from a “scrap lot”) or because it’s an early engineering sample (prototype). Here are two examples.

Let’s start with the first one.

It’s supposed to be an early engineering sample coming from ST Microelectronics. Hand-writing on top are “X2, Y4 #7”, probably related to the coordinates of this particular die on the wafer (X=2 , Y=4) and the wafer number (#7), and also “PLL”, which probably mean it was designed to test the integrated Phase-Lock Loop (clock multiplier). The back of the CPU shows that the PLL was configured for “3XCLK”. So it’s a DX4 class CPU. But it could also be a 5×86 ES. I have tested it on the UCA at 3.45V.

All 486s from ST are just rebranded Cyrix 486s and this one makes no exception. It identifies itself as a Cyrix Cx486DX4 (M0.7). The most interesting point is the stepping. I have one It’s ST ST486 DX4-100 (you can see it here) and another Cyrix Cx486DX-100GP4. Both come with stepping 3.6. This sample uses stepping 4.0. I do not have any Cyrix 486s (or IBM or It’s ST) with such a late stepping. I am not even sure this stepping finally reached the commercial status. This sample works perfectly fine at 120 MHz (3×40 MHz) with 3.45V while my Cyrix DX4-100 requires 3.6V to work at 120 MHz and the retail It’s ST doesn’t work at all when overclocked at 120 MHz.

At this point, I have no proof that this sample comes from TI and not directly from Cyrix. Anyway, it could be an engineering sample for a hypothetical Cx486DX4-120, that was finally canceled to avoid hurting 5×86 sales. Interesting.

Here is the other one.

The package marking (25253) tell us it’s an early AMD 486s assembled by Kyocera, but there is nothing more written on top or back of the chip. Package number is almost often used for 3.3V parts (while 5V parts from the same era come on the 25220 package). Time to plug it on the UC!

Early AMD 486s use the Intel 486 microcode, so they’re virtually indistinguishable by software. I’m testing a very nice way to distinguish them but that’s another story (I’m waiting for a new PCB and I’ll tell you more if it works as expected). The CPUID have 8 KB of L1 write-through cache and the CPU doesn’t support write-back, so it’s a (N)V8T revision and not a later SV8B. It doesn’t support 3x multiplier, so it’s a DX2 and not a DX4. Testing various frequencies shows it can work fine at 40 MHz. This unmarked CPU is probably an Am486DX2-80 NV8T, or maybe an Am486DX2-66 NV8T with a good overclocking capability. Nothings suggest it’s an engineering sample. Markings have probably faded over time (or have been removed due to mechanical action).

[UCA CPU Analysis] Intel 486 DX2-66 SYE36 ES

I’m starting a new section: IC Analysis! The goal is to study odd or rare CPUs with the Universal Chip Analyzer. As an avid CPU collector, I have many of them. Since I started collecting back in the early 00s, I have only been interested in Engineering Samples. These are basically prototypes of retail CPUs. Knowing their specification is often very interesting for historical purposes (ie: to retrace the timeline of the development).

Let’s start with this 486 DX2-66 Engineering Sample:

This processor is uncommon in many respects, even for an engineering sample. It comes with the standard Intel i486 DX2 logo but other writings are printed instead of being laser-engraved. The part number on the first line (“A80486DX2-66”) is the retail one, while Intel often used the code number (“P24” or “A80P24”) on early prototypes of the 486DX2.

The second line shows the date when the die (the piece of silicon where the CPU has been engraved) was assembled inside the ceramic packaging: week 22 of 1992, so between May 25th and May 31st, 1992. The date when the die itself has been produced is marked on the back: week 17 of 1992 (between April 20 & 26, 1992). Intel officially introduced the first clock-doubled 486DX2 at 50 MHz on March 3rd, 1992. The 486DX2 at 66 MHz was launched five months later, on August 10th, 1992. This sample has been produced before the initial production of the 486DX2-66.

Another very rare feature of this CPU is the Intel’s product spec number used. From the 70s until today, Intel has used a 5-digit alphanumeric code (named “S-Spec”) to identify all their retail products. An S-Spec always starts with the letter “S” (ie: SX366 is a 80386 DX-33 and SR147 is a Core i7 4770K). The only exception is for prototypes (engineering or qualification samples), where the code begins with a “Q”. The presence of that “Q-Spec” (also named QDF) on a CPU is the most effective way to distinguish a pre-production sample from its standard production counterpart. On this obvious engineering sample (also marked “ES” on front), the QDF starts with S: “SYE36”. For a very short period (1991/1992), Intel produced some 386/486 Engineering samples with a spec code starting with “SXE”, “SYE” and “SZE”. The reason is still unknown, but this sample is one of them.

It’s now time to test this SYE36 sample with the UCA

And It works fine! This early sample does not support the CPUID instruction, but the value at reset is 0x433. The first commercial stepping is A2 with a CPUID set at 0x432. Only a DX2-50 has been released with this stepping, which didn’t seem able to run properly at 66 MHz. This sample uses the B1 stepping, like the first retail 486 DX2-66 (SX645) released. Power consumption measured on FPU benchmark mode is quite high (4.3 W) but still within specs (4.5 W). Later DX2-66s need less energy.

Despite its unusual markings, it seems this sample was a qualification sample rather than a “true” engineering sample. It was probably sent to Intel’s customers for validation some weeks before the official launch. Other than that, it’s strictly identical to a SX645 486 DX2-66.

The UCA 486 Adapter now supports UMC 486s!

Last but not least, UMC’s 486s can now be tested on the Universal Chip Analyzer. All 486s ever manufactured are now supported! UMC is a Taiwanese semiconductor company still active today, albeit much smaller than its well-known competitors (TSMC, GlobalFoundries, …). In the mid-90s, UMC produced some rare 486s compatible CPU named “UMC Green CPU”. They were in-house design and not Intel-licensed like AMD 486s. Almost immediately after the initial announcement in 1993, Intel sued UMC and its distributors over patent infringements (including the infamous ‘338 patent, read more here). In response, UMC filled an anti-trust suit against Intel, but finally choose to give up and cease production of x86 compatible CPUs. Here are the retail UMC 486s I have:

UMC Green CPUThe most common one was the U5SX (sometimes marked U5S) clocked at 25, 33 or 40 MHz and without FPU. Some UMC Green CPUs labeled “U5SD” were also released. Contrary to what you might think, they don’t include a FPU – verified with the UCA – but are just supposed to use the “DX pinout”. The meaning of this mention is unclear because both the i486SX and the i486DX shares the same layout for non-FPU related pins. Some guys have claimed that the difference is a relocation of the NMI pin, but I have not noticed that. UMC also released extremely rare U5D (with FPU) and U486DX2 (clock doubled with FPU), but only a couple samples are known today worldwide.

U5S SUPER-40 Tested on the UCAThe Universal Chip Analyzer is able to test all UMC 486s, even if I still have an issue at 33 MHz and 40 MHz to grab every I/O. Code fetch works fine, but the output on I/O ports are sometimes dropped. It’s probably easy to solve, but I really need a copy of the UMC 486 manual / datasheet. Unfortunately, it seems nobody has one in the CPU collector’s community. If you can help, you’re more than welcome!

Looking at threads about the UMC 486 on cpu-world, vogons and vcfed, I saw that some people were wondering if the CPU marked “SUPER” were different from the “non-SUPER” ones. So, I ran some INT benchmark with the UCA.

As you can see, all UMC CPUs (U5S-SUPER, U5SX and U5SD) offer the exact same results when clocked at the same frequency. You also probably noticed the awesome relative performance of the UMC 486 versus the Intel 486s. Clocked at 33 MHz, the UMC 486 is as fast as an Intel 486DX2-66 and, when clocked at 40 MHz, it’s almost as fast as an Intel DX4-75. That’s not a bug. The UCA doesn’t have any wait-state on memory subsystem and actually uses a mix of heavy Integer divide and multiplication instructions for the benchmark. This result comes from the ultra-fast ALU designed by UMC, especially on divides. While Intel 486s requires 40 cycles to perform a INT divide, UMC 486s only need 7 cycles. That’s more than 5 times faster! However, in real-world applications, the UMC 486-SUPER at 40 MHz was on par with an Intel i486SX2-66. Still excellent!

The UCA now supports Ti486s (featuring the interesting Ti486SXL)

In the 90s, Texas Instruments (TI) manufactured some 486-class CPUs under its own brand. TI was one of the third-parties who produced Cyrix processors (Cyrix was a fabless company). Consequently, the vast majority of TI-branded CPUs were just rebranded Cyrix 486s in PGA132 (386 pinout) or PGA168 (486 pinout). But Texas Instruments also designed its own 486 micro-architecture: the short-lived TI486SXL.

Good news: the Universal Chip Analyzer with the 486 adapter now supports all TI 486s!

Let’s start with the fastest one, the TI486DX4-G100-GA.

TI486DX4-G100-GAThe “Colorful” Ti486DX4 is a rebranded Cyrix 486DX4-100: clock-tripled with 8KB L1 WB cache. Compared to IBM 486DX4 or ST 486DX4, both also rebranded Cyrix, the Ti486DX4 includes a tiny difference. IBM and ST CPUs are indistinguishable from a Cyrix CPU. Texas Instrument insisted to add a way to distinguish their CPU. This was implemented by setting a bit (DIR1[7]) in one of the Cyrix-specific registers. If the bit is “1”, the CPU is Ti-branded. If the bit is clear, it’s a Cyrix, IBM or ST 486. Other than that, nothing changed.

TI also released some 486 DX2, like this TI486DX2-G80-GA.

TI486DX2-G80-GAAgain, it’s just a rebranded clock-doubled Cyrix 486DX2 with a specific identification bit set. The DX2-80 seems more common than the DX2-66. The Universal Chip Analyzer is perfectly able to test it at 2×40 MHz.

But the most interesting Texas Instruments 486s are the ones based on their own micro-architecture like this TI486SXL2-G66-GA.

TI486SXL2-G66-GACodenamed “Potomac”, this clock-doubled CPU with 8KB of L1 cache doesn’t integrate a FPU. Ti was probably thinking of developing a FPU later. The mechanism to enable the clock-doubled PLL is different than all other CPU, who uses a specific pin to toggle between 3x and 2x (DX4s) or just boot at 2x by default (DX2s).  The Ti 486SXL2 powers up in the non clock-doubled mode. To enable the 2x PLL, you have to write a specific bit in a proprietary register called CCR0 (with the same mechanism involving reading/writing to port 0x22h/0x23h as on Cyrix 486s). Setting bit 6 on CCR0 switchs on the clock-doubling PLL instantly (within 20 µs). I was able to overclock this CPU to 80 MHz at 2×40 MHz (still at 3.45V).

Texas Instruments also released non-clockdoubled “Potomac” CPUs like this Ti486SXL-40.

Ti486SXL-40As you can see on the UCA Analyzer screenshot, it seems there is no way to distinguish them from SXL2 from a hardware point of view. Of course, it’s possible to check if the CCR0 bit 6 is set to 1 to know if clock-doubled mode is enabled, but the SXL2 can also work as a SXL in 1x mode.

While messing with CCR0 and the x86 code that run on the CPU, I found something very interesting: my SXL-40 also supports the clock-doubling mode! I had to run several benchmarks to be sure, but yes: that Ti486 SXL-40 can work as a SXL2 with the exact same power consumption and the same performances.

Ti486SXL - Benchmarks

I used the preliminary Benchmark Mode on the UCA. These Integer scores are not calibrated so they’re only valid to compare these CPUs on a relative scale. We can see that the Ti SXL microarchitecture is ~30% slower than the Intel 486 one at equal clock frequency,  but the SXL-40 is just 18% slower than a Intel 486DX-33. The Ti SXL2-66 is much faster then the DX-33, but the i486DX2-66 is far beyond.

Finally, it seems the SXL and SXL2 – at least on B0 stepping – are the exact same CPU. The B0 stepping is probably the only one that can work in 2x mode. “Potomac” A0 doesn’t support SMI and probably doesn’t have the integrated PLL. I also tried to activate the 2x mode and keep the 40 MHz FSB on the Ti486SXL-40 and … it works, actually doubling the performance (similar to a hypothetical “486SXL2-80”).

Spotting Counterfeit Am486 with the UCA

While I was adding support for AMD CPU on the Universal Chip Analyzer, I spotted what looked-like a strange chip at first sight. I was then working on the L1 cache size detection, to distinguish between CPUs with 8 KB and others with 16 KB. In their BIOS Development Guide, AMD wrote a specific code that checks the status of a tag bit in a test register (TR4). After implementing this test path in the x86 code run by the CPU on the UCA, I needed a CPU with 16 KB L1 cache to try on 486 (5x86s were OK). I found this uncommon Am486 :

This is a nice Am486 DX4-100V16BGI. This part number decodes as follows:  A clock tripled (“DX4”) CPU rated at 100 MHz (“100”) and 3.3V (“V”), with a 16 KB (“16”) Write-Back (“B”) L1 cache in a 168-pin PGA package (“G”) and qualified at Industrial temperature range (“I”). This last point is uncommon because the vast majority of Am486 are “Commercial” grade (0°C to 85°C) and not “Industrial” (-40°C to +100°C). That’s probably why I bought this CPU years ago.

But the AMD code was not working: the size of the cache detected was 8 KB instead of 16 KB. I began to have doubts about the genuineness of this CPU. I started to play with the UCA. No way to enable Write-Back: the CPU stays in Write-Through Mode and the CPUID does not change accordingly as on “SV8B” AMD 486s. This CPU does not support Write-Back. I suspected a remarked early “NV8T” DX4-100, but that was not the case: they come with a CPUID 0x484 and this CPU was 0x482 in 3x Mode and 0x432 in 2x Mode.

I was able to find a very early Am486DX2-80 V8T (notice the lack of “N”) manufactured in 1994 with the first A-Stepping. The UCA detects a CPUID set at 0x432, which match with my fake DX4 (in 2x Mode). Early Am486DX4-100 V8T also exists with a CPUID 0x482 in 3x Mode. Some of them seem to have been later remarked to Am486 DX4-100V16BGI.

On closer inspection, several points should have caught my attention about this CPU. No way to be certain of what it really was without the UCA, but the fact that it was a fake could have been known sooner.

    1. Package code is wrong

The AMD package code is written in bottom left of all AMD CPUs from this era. The first AMD Am486s like the Am486DX-33/40 or very early Am486SX2/DX2s use the “24361” package. Later 486DX2 “V8T” and “NV8T” CPUS come in the “25220” or “25253” package. Enhanced “SV8B” DX4s (with SMI and Write-Back) are assembled with the “25398” package. Then we have package “25498” for newer CPUs like the Am486DE2. Later models (SV16B and 5×86) use the “25544” package”. This later one was expected for a genuine Am486DX4-100V16BGI, but the fake CPU comes with an old “25253” (N)V8T package.

Package code is “25253”, similar than old (N)V8T Am486
    1. Markings without hatching

As you can see in the picture below, AMD markings on CPUs from this era use a typical hatching pattern. This pattern is not present at all on the fake CPU.

    1. Marking error

But the most obvious error is a big mistake on printing. Here you can see the word “COMPATIBLE” is actually spelled “COMPATTBLE”, with a double “T”.

There is no doubt at this point that this CPU is a counterfeit Am486DX4. The only question remaining is when was it remarked by fakers? Counterfeits CPUs – especially 486s – were common in the 90s to boost frequency, but here, the original CPU was already an Am486DX4-100 (albeit a very early one with 8 KB L1 Write-Though Cache, instead of the expected 16 KB L1 Write-Back Cache). More recently, in the mid-2010s, old CPUs from the 90s were also faked to target CPU collectors all over the world.

Looking at eBay listings right now (2020-04-23), I found 4 vendors selling Am486 DX4-100V16BGI for a (very) high price. Two of them – including one who only sells multiple 30 pcs lots – are obviously the same fake as the sample analyzed here. The other two look different but still highly suspicious, with a Windows Logo not on par with the unusual Windows printing from AMD for the first one, and a very odd font for the second one (seems also marked “COMPATIBLF”)

Collectors beware of these CPUs!