1. For 32-bit linux, for physical memory, when the kernel is designed, the actual physical memory that can be directly accessed is only 1GB at most (reason: the number of fixed page tables in the kernel is limited)
1. For 32-bit linux, for physical memory, when the kernel is designed, the actual physical memory that can be directly accessed is only 1GB at most (reason: the number of fixed page tables in the kernel is limited)