The registers AX, BX,CX and DX are the general purpose 16-bit registers.
AX is used as 16-bit accumulator. The lower 8-bit is designated as AL and higher 8-bit is designated as AH . AL can be used as an 8-bit accumulator for 8-bit operation.
All data register can be used as either 16 bit or 8 bit. BX is a 16 bit register, but BL indicates the lower 8-bit of BX and BH indicates the higher 8-bit of BX .
The register CX is used default counter in case of string and loop instructions.
The register BX is used as offset storage for forming physical address in case of certain addressing modes.
DX register is a general purpose register which may be used as an implicit operand or destination in case of a few instructions.