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The 70-R21 was IBM's first true 486 machine. It came out immediately after Intel released the first 486 CPUs in quantities. IBM took advantage of the fact that the up-to-now high-end Model 70 had its CPU on a daughtercard, so they only had to design a new CPU module, which was simplified by the higher integration of the i486 CPU compared to its predecessor.
The 70-R21 therefore looks mostly like all other model 70s, you have to take a close look at the IBM label at the front to see the tiny '486'...
IBM also offered the 486 module as an upgrade for people who already had a Model 70 with a 386 module and refunded the value of the old module - except for the optional i387 coprocessor...
One has to remove the plastic frame holding drives and cards in their place to take a closer look at the CPU module. Compared to the old 386/25 module, it's rather empty - the cache, cache controller and coprocessor are already integrated. The heatsink is my own addition - the 486DX25 used in this machine is a very early chip release and becomes quite hot. It works fine without a heatsink when the cover is closed, but without the cover, the air doesn't flow along the CPU any more and you run into trouble quite quickly...
A closer look onto the CPU module reveals that the 486 CPU module was a real quick-hack - note the amount of corrective wires ;-)