Comment by ordu
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.
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.