X86.FR | Doc TB's R&D Lab

Along with AMD, Cyrix was one of the biggest Intel challengers in the 486 era. While most of the AMD Am486s used the exact same microcode as Intel 486s, Cyrix was the first to release a 100% compatible processor based on a custom design. Being a fabless company, Cx486s were manufactured by IBM, ST Microelectronics and Texas Instruments. All of them sold Cyrix 486s under their own brand.

Adding support for Cyrix-based 486 and 586 was more challenging than expected. As many of you probably remember, 486 motherboards were full of jumpers because of the many different pinouts. I want the UCA to be able to test every CPU out of the box without messing with jumpers, so I had to use many tricks to accommodate the different pinouts. I also wasted a lot of time trying to understand the erratic bugs I had when adding more x86 code to detect Cyrix CPU. The cause was finally obvious, but I had a hard time spotting it: two address lines (A11 & A9) had been inverted in the FPGA code for more than one year!

This stupid typo came on top of another Cyrix-only specification I had to deal with. All the HDL code I wrote for the UCA is focused on achieving 0 wait-states. Unfortunately, when I started to work on support for the Cx486, it crashed almost instantly, even at low frequencies. I rewrote a lot of Verilog to achieve near-perfect timing, matching the original Intel datasheet almost perfectly. But the Cx486 kept crashing. I had to wire everything to my 32-channel logic analyzer to understand why all Cyrix 486s failed to work on the UCA. The answer is shown on this screenshot:

Cyrix added an unexpected (normally chipset-related) mechanism that adds hardware wait-states to every I/O. And not just a couple of them: 32 clock cycles for every I/O by default! The state machine that handles the decoding of CPU cycles inside the FPGA wasn’t able to understand why the CPU doesn’t resume operation after an I/O and assumes a timeout has happened. As soon as I changed the HDL code to handle this case, Cyrix CPUs started to work properly on the UCA. I could have saved myself a lot of effort if I had RTFM more carefully: this behavior is indeed described on the 5×86 CPU BIOS Writer’s Guide, page 12:

Maybe I’ll add a software path later to change this setting with the UCA Analyzer tool. Messing with Cyrix-specific internal registers on the fly is an upcoming feature already planned! As with AMD, I grouped all the non-ES Cyrix CPUs I had in a tray and starts testing.

(1) Let’s start with the Cyrix Cx486S-40, one of the first 486-class CPU released by Cyrix in March 1993. It features 2 KB of write-back L1 cache, quite unusual for the time. The CPUID at reset is 0x450, which does not correspond to any Intel 486 (i486SX are 42x). Power consumption is quite high. Also note that the screenshot is done at 25 MHz for a rated maximum clock of 40 MHz. For an unknown reason that deserves a longer investigation, very early Cyrix 486 like this one cannot run at 33 MHz or more on the UCA with the actual HDL code. Maybe it’s due to the added electrical interference from the Logic Analyzer, or maybe It comes from a regression in the code after I messed with timings, but that Cx486S-40 was able to run at 40 MHz some days ago, so I’m quite confident It will be fixed soon. I was just too lazy to unwire everything to take the screenshot.

(2) Cyrix Cx486DX2-66. A clock doubled 486 with FPU. CPUID after reboot is 0x480 (similar to Intel DX4s) but the CPU does not support the cpuid instruction. Cyrix CPU has two registers named DIR0 and DIR1 for identification. This one contains 0x1B in DIR0, the hex value for a Cx486DX2. DIR1 contains 0x0B. DIR1[7:4] is “CPU Step Identification Number” (here 0x00) and DIR1[3:0] is “CPU Revision Identification” (here 0x0B or 11 in decimal). The actual “Cyrix stepping” is 0.11. This CPU is marked A3CM434M and has been manufactured week 34’1994. It’s an early example. Like the Cx486S, it does not work at more than 25 MHz on the UCA yet (but it will soon).

(5) IBM “Blue Lightning DX2” 486-V666GA. An IBM-branded Cyrix 486DX2-66. They are strictly identical from a microarchitectural point of view but are supposed to come with a stricter QC (Quality Control). This one is a 3.45-3.6V part, and not a 5V CPU like the previous one. It is also much newer (manufactured in March/April 1995). Stepping/Revision is 3.2. No problem running it on the Universal Chip Analyzer at 66 MHz (2 x 33.3 MHz).

(6) IBM 486 DX4 / 486-4V3100GIC. Well, I can’t remember where this CPU come from, but it doesn’t work. Not a single sign of life on the UCA not on a standard 486 motherboard. The power drawn seems linked to the clock signal applied (so the internal die is not shorted), but when wired to the logic analyzer, not a single pin toggles after reset. Unfortunately, it looks dead. 🙁

(7) It’s ST ST486 DX2-66. While IBM-branded Cx486s are often known for their higher QC (and higher overclocking), ST’s 486 are usually less overclockable. This CPU was manufactured in February 1995 but still uses the Stepping 0.12. A single step newer than the very old Cyrix Cx486DX2-66 but much older than the IBM Blue Lightning DX2. It works as expected at 66 MHz

(8) It’s ST ST486 DX2-80. This part is very close to the IBM Blue Lightning DX2. It uses the same 3.2 Stepping but works at 5V instead of 3.45V. Power consumption is quite high (~4.5W) and it runs hot. CPUID is 0x480. No problem to have it running on the UCA at 80 MHz (2 x 40 MHz).

(9) It’s ST ST486 DX4-100. Very late 3.45V clock-tripled CPU manufactured in 1997. The stepping is 3.6, which corresponds to the latest Cyrix 486 revision ever produced. CPUID is still 0x480 and L2 cache is limited to 8 KB Write-back (instead of 16KB for latest Intel 486 DX4s). It runs fine at 66 MHz (2×33.3 MHz) and 100 MHz (3×33.3 MHz).

(3) Cyrix 5×86-100GP. The 5×86 is a short-lived, stripped-down version of the Cyrix 6×86. It features 16 KB of L1 Write-Back cache and a 5^th generation (Pentium class) microarchitecture. The vast majority of 5×86 processors run at 2x or 3x multipliers. This example is a quite early “1.3” revision. CPUID changes from 0x429 at 2x to 0x42D at 3x. It can work on the UCA at 120 MHz (3×40 MHz) with 3.6V.

(4) Cyrix 5×86-120GP. Some late (and rare) 5x86s are able to work with a 4x multiplier (in addition to the default 3x multiplier). For some unknown reason, the revision/stepping drops to 0.5 even if the CPU was manufactured way after the previous one (in 1996). CPUID at 4x is 0x42C (and stay at 0x42D at 3x). Here is that nice 5×86-120 running at its rated 120 MHz (3×40 MHz) and then overclocked at 133 MHz (4×33.3 MHz) @ 3.6 volts

For fun, I also tried to increase the voltage to 3.7V before restarting the UCA at … 160 MHz (4x 40 Mhz) ! To my surprise, it successfully completed a test pass. I stopped to avoid any damage to the CPU, but that was probably the fastest pass ever run on the UCA.

Awesome!

Next step is to add support for the remaining 486 brands (and solve the frequency regression on early Cx486s). I also have a nice feature upgrade for the 486 adapter planned soon. Stay tuned!

After the initial support for Intel 486s, the Universal Chip Analyzer with the new 486 adapter now supports all 486s from AMD. I’m an avid CPU collector but I only collect Engineering sample (check my collection here!). Of course, some analysis on these ES will be published here soon, but to add support for AMD 486s, I bought some “retail” Am486 and Am5x86. Here they are:Good news : they all work well on the UCA! Here are some notes I took while testing.

Am486DX2-50 (1) – Am486DX2-80NV8T (2) : 8 KB L1 Write-Through Cache. Both CPUID 0x432 with cpuid instruction not supported. Virtually undetectable from Intel DX2 : Same microcode. Same power consumption. Exact same performances. Maybe distinguishable from Intel DX2 with JTAG. Work in progress on this point.

Am486DX4-100NV8T (3) : 8 KB L1 Write-Through Cache. CPUID also 0x432 without cpuid instruction. Real nightmare to distinguish from Intel. B-Step a bit lower power (-5%) Vs AY-Step. INT Perfomance is lower than Intel DX4 (-20%). FP Performance almost identical. Power consumption is also lower (-25%). One interesting thing : In 2x mode, the CPU is exactly as fast as an Intel DX2. However, in 3x mode, it seems significantly slower than a DX4, but only in INT. Performances looks like 2.5x in INT (according to cycle count) and “real” 3x in FP. Strange. That deserves some additional investigation.

Am486DX4-120SV8B (4) – Am486DX4-100SV8B (5) : Newer core with SMI and Write-Back L1 Cache (still 8KB). CPUID instruction supported. 2x/3x Mode and WT/WB change CPUID (0x434/0x474 in 2x mode, 0x484/0x494 in 3x mode). Exact same performance than NV8T. Bit higher power consumption (+10%).

Am5x86-P75 (Am486DX5-133V16BGC) (6). 16KB Write-Back L1 Cache ! CPUID instruction supported. 3x/4x Mode and WT/WB change CPUID (0x484/0x494 in 3x mode mode, 0x4E4/0x4F4 in 4x mode). INT Performance identical to 486DX4-100 (looks like INT units are locked in 3x mode). FPU performance are far better (real 4x). Doesn’t support overclocking to 160 MHz.

Am5x86-P75 (Am486DX5-133W16BGC) (7). Same than previous one, but supports overclocking to 160 MHz with 3.6V !

More 486s will soon be supported!

When I started studying FPGAs more than two years ago to build a simple IC tester, I didn’t expect to support anything faster than the Intel 8086. The learning curve for Verilog has been quite harsh since then, but I’m now much more comfortable with complex logic, states machine and timings diagrams. I also learned a lot about how old CPU architectures work in depth and how to interface vintage circuit with modern hardware. Acquiring this knowledge step by step is quite exciting, even if I still have a lot to understand.

By the end of 2018, I was able to successfully interface an Intel 486 DX-33. That was quite a challenge and the HDL code was horrible (and the electronic as well, to be honest), but I knew that it was possible for the modest FPGA I use (a Xilinx Spartan 6 LX9) to support up to 32-bit architecture with quite fast bus frequencies and 0 wait state. Awesome!

* Now Truly Universal

In 2019, I worked on a truly universal hardware platform that can support anything from the original Intel 4004 to much more advanced 32-bits CPUs like Motorola 68040 or Intel 486 (and also MCU, RAM, the Xeon ID platform and almost anything else). The goal was to raise (a bit) the cost of the UCA base board to keep the top adapters as cheap (and easy to design) as possible. The Universal Chip Analyzer now consists of a 3 layers stack.

• • The first (bottom) layer is still the modified Mojo V3 development board with the Spartan 6 LX9 FPGA. I thought for a long time about redesigning the board, but the embedded Micro team did quite a good job and cheap clones Mojo V3 are available. The main changes I made on the board are a much bigger Flash (256 Mb) to accommodate many different FPGA configuration files, a rewritten firmware to support all the new features and some tuning on the FPGA power stage.
  • The second (middle) layer is the heart of the Universal Chip Analyzer. It integrated all the bus transceivers, an internal management bus, and a much more advanced power delivery stage for the CPU under test. Bus transceivers can handle up to a 32-bit bus. The internal management bus can create interconnections between all the components (MCU, FPGA, etc.) spread on the 3 PCB layers. It’s used to bypass the FPGA for some later adapters and to support some nice features out of the box like adapters with embedded displays. The most interesting part is the power stage. The first iAPX86 UCA Shield was powered by USB only. It’s not possible anymore because advanced CPUs sometimes require lower voltages (+3.3V, +3.45V or +3.6V) and much higher current. A DC-DC converter is now integrated, along with a precision current/voltage monitor circuit that is also able to act as a programmable fuse. The voltage converter can be set externally to 5V or 3.3V, but it can also be set by the UCA software to any voltage from 2.2V to 5.5V by 50 mV step! I also added a standard 3-pin fan connector. Mandatory to keep some CPU like 486 DX4s cool! PS : The final PCB color will be black. The green one is cheaper and faster for prototypes…
  • The third (top) layer is a simple passive adapter with just two 50-pin connectors and a Socket (standard or ZIF) to accommodate the CPU under test. The UCA auto-detects the correct FPGA firmware to load according to the adapter plugged (except for later adapters able to support more than one CPU family). A jumper is often present to set a fixed voltage (usually 5V) or to select a software-defined voltage.

This hardware platform was designed for many upcoming adapters and should not evolve any further. It allows a much faster development of adapters, both in hardware and in FPGA HDL code. Many of them are already in development – some almost finished – and they will be released throughout 2020. Some very cool adapters are planned and yes, even for non-CPU!

* Now with i486 support (and soon much more)

I started the development of this base platform with the fastest CPU with a 32-bit bus in mind: the clock-tripled 486 DX4. Because who can do more can do less. If the platform – and especially the FPGA – can support the DX4, it can also support every CPUs down to the 4004. That was a giant step versus the Intel 8086, the fastest CPU supported on the first iAPX86 shield. The 486DX4 is much more advanced, way faster and much more complex to interface. Tricks like forcing a stripped-down bus to 16 bit or adding wait states were forbidden.

Right now, the UCA supports any Intel 486 SX, DX, DX2 and DX4 CPU, from early engineering sample to QFP-on-adapter to late DX4s with write-back cache. The UCA is also able to set different bus frequencies, from 16 MHz to 40 MHz, with CPU frequencies up to 120 MHz (Intel 486 DX4-100 overlocked with a 40 MHz FSB).

At this time, you probably wonder: what about the only 486-class CPU with a 50 MHz bus, the 486 DX-50? Well, it works fine at 40 MHz, but after many weeks, I’m now sure the UCA platform will not be able to support a “true” 50 MHz FSB. That’s just too close to the limitation of the bus transceivers I use. That doesn’t mean the UCA will never support a 486 DX-50 running at 50 MHz (or even higher), that only mean it will not support it without wait states. Back in the day, I don’t think there was a single 486 DX-50 based computer without wait states. When released, this specific CPU was well-known for its instabilities and manufacturers had to add (many) wait states to make it work properly.

Now it’s time to talk about non-Intel 486s. The hardest part was to build an adapter able to support all the different 486 manufacturers out of the box (from AMD to Texas Instrument to Cyrix) without any dip switches despite the different pinout they use. I’m confident I now found the perfect hardware tricks and, even if they’re not working right now due to lack of software implementation, the 486 Adapter for the UCA will support (very) soon all 486s. Yes, that includes AMD & Cyrix 5×86, and some outsiders like UMC 486s and the elusive Texas Instrument 486 SXL2-66!

* Now Truly an Analyzer

I always wanted the UCA to be a true Analyzer (and not just a tester), able to dig into the microarchitecture by allowing the user to mess with internal registers and CPU pins. The path is quite complex because the data has to be transferred between several layer. The FPGA is directly connected to the CPU under test and loaded with custom HDL code (the “FPGA Firmware”). A microcontroller IP is inferred inside the FPGA and internally connected to various control signals, to the main RAM used by the CPU, and also to the external microcontroller of the base board. Some C code, written with the Xilinx SDK, is executed by this internal MCU to handle the communication between the FPGA parts and the external MCU. I call it the “iMPU Firmware”. More C code is needed on the external MCU (for the ATMega32u4, called “UCA Firmware”) to pass the data to the USB connector. Finally, a Windows 10 software program written in C# to communicate with the UCA.

Here is the actual UCA Analyzer tool, running with an Intel 486 DX4-100 overclocked to 120 MHz.

On the main tab: the testing status (INT or FP Test in progress, Pass, etc.), various version information on the internal firmware of the UCA, the actual CPU under test detected with actual CPU & FSB frequency, multiplier, voltage, current, power and process. You can also select a different frequency and reset the CPU.

On the specific 486 tab, the actual CPUID of the CPU (acquired from the CPUID instruction or at reset if CPUID instruction is not supported), various information on supported/enabled features and a work-in-progress “control” section to assert some pins of the CPU. For example, we have an Intel 486 DX4-100 in Write-Back mode with CPUID 0x490. If you set the cache to Write-Through mode, the CPUID for this CPU changes to 0x483. Many new features are planned here.

Another work-in-progress tab is for a benchmark feature. The values displayed are random for now, but everything is already implemented to support INT & FP benchmark. The goal is very different than traditional benchmark. Here, we will focus on microarchitecture benchmark only. There are absolutely no bottlenecks usually found on a “real-world” motherboard like chipset or EDO/FPM RAM. The UCA is able to feed the CPU with 0 wait states, so all data is read/written immediately, as if they were in cache.  The true power of the internal ALU/FPU, at their maximum process capabilities, can be revealed. More about the benchmark mode later.

The “Power” tab is one of the most significant improvements. On the top block, you can precisely monitor the voltage and current needed by the CPU, but also configure the internal ADCs (averaging & conversion time) and set the type and value of the protection (usually overcurrent). You can also reset the Alert Flag when shutdown due to overcurrent happens. On the bottom block, you can set the actual voltage for the CPU under test, from 2.2V to 5.5V by 50 mV increments. Useful to test undervoltage or overclocking. Ie: the 486 DX4-100 I used for the screenshot doesn’t work at 120 MHz at 3.3V but everything is fine at 3.45V.

The original Mojo v3 comes with a 60+ MB FPGA bitfile (firmware) uploader written in Java. I adapted it to handle many different bitfile “slots”, but it was time to rewrite it from scratch. I integrated the FPGA firmware uploader tool to the UCA analyzer, with the same features. You can still check all installed bitfile, slot by slot, and upload/delete them as needed. Now with a tool less than 1 MB and without Java.

The “Debug” tab logs all sent/received communication for debugging purpose. I also added a feature on the last “About” tab to easily upload the base UCA firmware (for the external microcontroller).

* Soon truly on sale everywhere?

Many CPU collectors or retro-enthusiasts already expressed their interest for the universal shield described above. A beta-testing phase will begin soon, starting with the 486 Adapter then with many others. I’m still wondering if it’s better to sell the adapters one by one, as soon as they’re ready, or to wait until at least 3 or 4 are available. The latter option can help keep costs as low as possible by panelizing the adapters. The final “retail” price for the UCA (base FPGA board + Universal Shield, without top adapter) is expected to be around $150. Adapters cost will be around $50 or less, depending of the Socket included. Finding ZIF PGA sockets at decent price is the most challenging part, especially for CPUs like the 286s. All those who have already bought an UCA with the iAPX 86 Shield can reuse the FPGA board with a firmware update, so they can expect a big rebate.

Progress is going to be fast, so stay tuned for more exciting information and in-depth CPU analysis!

Since the end of last year, I faced a lot of change in my personal life: new job, new house, new car, etc. Except my girlfriend, almost everything else changed in the last 3 months. Damn exciting, but I had very little time to spend on side projects.

It’s now time to resume the work on the Universal Chip Analyzer! My next goal was to add support for the very successful MOS 6502, an 8-bit microprocessor released in mid-70s that was at the heart of many designs well into the 80s.  The original 6501 (circa 1975) was pin-compatible with the M6800 while being much less expensive, but Motorola sued MOS almost instantly and the 6501 has been withdrawn very quickly. The 6502 shares the same internal micro-architecture with the original 6501 but uses a different pin arrangement.  It’s a very simple CPU, with a 8-bit accumulator, two index registers and a 16-bit address bus (able to interface with a maximum of 64 KB of memory). The 6502, just like many other early CPU, doesn’t have a specific way to handle I/O: you must define an arbitrary memory-mapped I/O range and configure your hardware accordingly.

Several variants of the MOS 6502 has been launched through the decade, some of the them with reduced addressable memory (6503 to 6507) and some others with additional features (6508 to 6510, …). Many early computers and consoles are based on a 6502: Apple IIe, Commodore 64, Atari 2600, Nintendo NES, Oric Atmos, etc.

Adding support for the 6502 wasn’t as easy as I expected. The main issue came from the timings. Usually, I drive all the logic inside the FPGA with the the clock signal generated locally and sent to the CPU. That doesn’t work with this particular CPU because it integrates an internal two-phase clock generator used to sync external IC. So, I finally had to feed these two clocks back in the FPGA and sync the internal logic with them. Some delays were needed to match the original timings (FPGAs hate delays) but everything now seems to work properly.

Here is the UCA testing a NMOS Rockwell R6502P at 1 MHz and a much more modern CMOS Rockwell R65C02 at 4 MHz (overclocked from 3 MHz):

Testing frequency can be set at 1.0 / 2.0 / 3.0 or 4.0 MHz to match the most popular 65xx

I’m waiting to get some samples to support other 6502-based CPU like 6510 or 28-pin 6503 -> 6507. A Rockwell 65C112 has already been seen working properly. The passive adapter isn’t final now, because I still need to work on my next goal: the Motorola 6800! This CPU Is the last (but not least) big one I need to support with the UCA. Stay tuned!