"In protected mode, the IA-32 architecture provides a normal physical address space of 4 GBytes (2 32 bytes). This is the address space that the processor can address on its address bus. This address space is flat (unsegmented), with addresses ranging continuously from 0 to FFFFFFFFH. This physical address space can be mapped to read- write memory, read-only memory, and memory mapped I/O. The memory mapping facilities described in this chapter can be used to divide this physical memory up into segments and/or pages."
It correlates to my experience of developing in protected mode in QEMU. Once entered protected mode, I can access to any address above 0x10000 without being wrapped around. When I was writing my first kernel (https://github.com/tuhdo/os-study) in real-mode, indeed A20 must be enabled.
