What is an 8-bit register on a desktop cpu?

Question

Reading through the following instruction table manual I see that integer multiplication is often much faster for 8-bit registers.

In the context of a normal desktop cpu, what does 8-bit register mean? That the value stored within ie a 32-bit register simply happens to be within an 8-bit range? There aren't actual 8-bit registers, right?

Answer 1

There are actual 8-bit registers, and they are just a part of the full register. For example, on Intel cpus:

AL  8-bit
AH  8-bit
AX  16-bit
EAX 32-bit
RAX 64-bit

In pictures:

63                             32      24      16       8       0
|-------|-------|-------|-------|-------|-------|-------|-------|
                                                        |<- AL->|
                                                |<- AH->|
                                                |<----- AX ---->|
                                |<------------ EAX ------------>|
|<---------------------------- RAX ---------------------------->|