I started to learn assembler, and this does not looks logical to me.
Why can't I use multiple higher bytes in a register?
I understand the historical reason of rax->eax->ax, so let's focus on new 64-bit registers. For example, I can use r8 and r8d, but why not r8dl and r8dh? The same goes with r8w and r8b.
My initial thinking was that I can use 8 r8b registers at the same time (like I can do with al and ah at the same time). But I can't. And using r8b makes the complete r8 register "busy".
Which raises the question - why? Why would you need to use only a part of a register if you can't use other parts at the same time? Why not just keep only r8 and forget about the lower parts?