Mobile CPU Upgrades
Siemens-Nixdorf PCD-4ND is a 486-based laptop from 1993-1995. It was quite popular in Germany and you could have seen a few of them still in use even ten years ago (mostly connected to expensive industrial devices using a serial port).
I like how easy it was to upgrade the CPU in this particular laptop. It took about 20 seconds to remove a small plastic cover and replace the CPU board. There were probably four options during the lifecycle of the machine – 25-MHz 486SX, 50-MHz 486DX2 and 75-MHz/100-MHz 486DX4 (Intel called it ‘Intel DX4’).
Note the WDC graphics chip below the CPU board slot. It shared the bus with the CPU as it was connected using VLB (= VESA Local Bus) instead of the older ISA/AT bus. This allowed to use fast 32-bit transfers on a frequency equal to the CPU external clock (25/33-MHz) without a sophisticated bus controller (EISA, PCI). This chip was used in many laptops of this era and it was surprisingly powerful. It allowed up to 1024×768 with 256 colors and 640×480 with 65k colors and it could accelerate bit-block transfers as well as graphics primitives.