>> Additionally, the higher 8 bits of rax, rbx, rcx and rdx can be referred to as ah, bh, ch and dh.
Did you mean “ax, bx, cx, dx”?
>> Additionally, the higher 8 bits of rax, rbx, rcx and rdx can be referred to as ah, bh, ch and dh.
Did you mean “ax, bx, cx, dx”?
ax, bx, cx, dx are 16 bit registers referring to the lower 16 bits of rax, rbx, rcx, and rdx respectively. Bits 0..8 can be referred as al/bl/cl/dl, bits 8..16 as ah/bh/ch/dh.
It's ambiguous, but I believe the comment you are replying to suggests that the sentence should read:
>> Additionally, the higher 8 bits of ax, bx, cx and dx can be referred to as ah, bh, ch and dh.
Those would be the lower 8 bits, no?
In a 64 bit register, e.g., RAX, AL refers to the lowest 8 bits [0-7] and AH refers to the next 8 bits [8-15].
Together, AX refers to bits [0-15]. EAX refers to [0-31].
It's counterintuitive (or at least, inconsistent) that we have a name for bits [8-15] but not for [16-31] or [32-63]. My fuzzy understanding is that this came about from legacy decisions.
This page has a helpful visualization at the top: https://www.cs.uaf.edu/2017/fall/cs301/lecture/09_11_registe...
It's neither. al is the lower 8 bits of ax (ah the higher 8 bits). ax is the lower 16 bits of eax. eax the lower 32 bits of rax.
Here's the AMD manual: https://docs.amd.com/v/u/en-US/40332-PUB_4.08
ax=ah:al eax=?:ax rax=?:eax