Intel 80386, part 1: Introduction

As with all the processor retrospective series, I’m going to focus on how Windows NT used the Intel 80386 in user mode because the original audience for all of these discussions was user-mode developers trying to get up to speed debugging their programs. The flags register is updated by many instructions. Many instructions have a more compact encoding if the destination register is , , or . This is different from most other processors, where each opcode maps to an instruction template, where all that’s left to fill in are the registers and immediates. ¹ This partial register behavior wasn’t a big deal at the time, but it ended up creating register dependencies that made it much harder to add out-of-order execution to later versions of the processor.

Source: blogs.msdn.microsoft.com