How does A20 relate to Pmode

Hello, Im new to asm and programming in general. I been reading up on a20 gates and so fourth. Would like to make my own Os. I dont get how A20 relates to pmode programming however. Setting the bit in cr0 to 1 I thought enables pmode and a20 is just accessing 64k more memory with the 20th gate. but it cant access more then that. Thuoght the a20 gate is for real mode programming with extra memory. Why does all the programming code or most of it show that it is needed to access protected mode memory. I thought switching to protected mode made it where you can access all the memory. and a20 is just High memory or something. Well Thanks and Maybe you all can clear this up for me. :D byebye