This is actually an interesting story,
Back in the days when we had 8 bit processors (8 bit processors means registers in processors will be able to hold 8 bit data), registers were called like,
So, when we moved to 16 bit processors, we named them as,
AX, BX.. (AX can be broken into AH and AL
Similarly, when we moved to 32 bit processors we named registers as,
EAX, EBX, ECX, EDX.
Do you think in 64 bit processors we will have another name for registers?
Yes, You are absolutely right, we have different names for registers. They are called,
RAX, RBX, RCX, RDX etc.
(PS: We have 8 more registers on x64 processors named R8, R9, R10… till R15)
Some more information from Julian,
Not all 8-bit processors used AL, BL etc. For example, the Motorola 6800 used A, B, X, SP, PC, and CCR for flags. The 6801 extended this by allowing A & B to be combined as D. The Zilog Z80 used A, F, B, C, D, E, H, L, IX & IY, SP & PC. B could be combined with C to form BC, D with E (DE)and H with L (HL). A was combined with the flag register F. IX, IY were 16 bit registers.