PC program counter mainly records the execution position of single chip microcomputer in ROM. Generally, a single chip microcomputer can access the address of 64K, which means the word width of 16 bits is required.

SP is the pointer to the stack, which is built on RAM, and the weight of internal RAM is 0~255, so only 8 bits are needed.

DPTR is a 16-bit register. When assigning a value to this data register, we divide it into upper 8 bits and lower 8 bits to access the register addressing mode.

Because it is 16 bit, we can use it to access 64K ROM space and 64K external RAM space.