I was programming in Turbo Pascal at the time, which was still 16-bit. But when I upgraded my 286 to a Cyrix 486, on a 386 motherboard[1], I could utilize the full 32-bit registers by prefixing assembly instructions with 0x66 using db[1].

This was a huge boost for a lot of my 3D rendering code, despite the prefix not being free compared to pure 32-bit mode.

[1]: https://en.wikipedia.org/wiki/Cyrix_Cx486DLC

[2]: http://www.c-jump.com/CIS77/ASM/DataTypes/T77_0030_allocatio...