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.
If 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!