CPU architectures and referring to them sit in a sort of middle ground. They need to be technically accurate but over time their terminology can change. I thought it would be interesting to look into the name origins of the two most widely used CPU architectures for desktop systems today. And, admittedly this is fresh on my mind from some research I was doing.
Nowadays, most 32-bit software for desktops and laptops is referred to as being built for “x86”. What does this mean, exactly? Well, as expected, we have to go back quite a ways. After the 8086, Intel released the 80186, 80286, and 80386. The common architectures and instructions behind these CPUs came to be known as “80×86 instructions”- understandably. The 486 that followed the 80386 dropped the 80 officially from the name- inspection tools would “imply” it’s existence but Intel never truly called their 486 CPUs “80486”. It’s possible this is how the 80 got dropped. Another theory could be that it was simply dropped for convenience- x86 was enough to identify what was being referenced, after all.
The term survived up to today, even though, starting with the Pentium, the processors themselves never truly bore the “mark” of an x86 processor.
x64 is slightly more interesting in it’s origins. 64-bit computing had existed on other architectures before but x64 now references the “typical” x86-compatible 64-bit operating mode. Intel’s first foray into this field was with the Itanium processor. This is a 64-bit processor and it’s instruction set is called “IA-64” (as in Intel-Architecture 64”). This did not work well as it was not directly compatible with x86 and therefore required software emulation.
it was AMD who extended the existing x86 instruction set to add support for 64-bit through a new operating mode. Much as 32-bit instructions were added to the 80386 and compatibility preserved by adding a new “operating mode” to the CPU, the same was done here; 64-bit operations would be exclusive to the 64-bit Long Protected Mode, where 32-bit was still 32-bit protected mode, and the CPU was still compatible with real mode.
This AMD architecture was called “AMD64” and the underlying architecture that it implemented was “x86-64”.
Intel, as part of a series of settlements, licensed AMD’s new architecture and implemented x86-64. This implementation went through a few names- x86E, EM64T- but Intel eventually settled on Intel64. Intel64 and AMD64 aren’t identical, so software targets a subset- this subset is where we get the name x64.