Register Sub-Registers (aliases)

Each 64-bit register exposes smaller 32/16/8-bit views.

; RAX = 0x1122334455667788
;   EAX = 0x55667788   AX = 0x7788   AL = 0x88   AH = 0x77

Gotcha: writing to a 32-bit reg (e.g. mov eax, ...)

zeroes the upper 32 bits. Writing to 16/8-bit does not.