“The 386 was originally supposed to have an on-chip cache, but there wasn’t room and the cache was dropped in the middle of the project.”
The 386 shipped with 285,000 transistor sites and only 181,000 actual transistors, and most sources still print a third number, 275,000. The 386 SL in 1990 packed 855,000 by bolting on a bus controller, memory controller and cache tag RAM, and the 386 core shrank to a quarter of the die. It also introduced System Management Mode, the hidden mode every x86 chip still carries. The chip that set the PC architecture for decades lost its cache to a lack of room.