The Linux kernel, for example, uses the following definitions for page-table entries in different levels of amd64's four-level page tables: typedef struct

The Linux kernel, for example, uses the following definitions for page-table entries in different levels of amd64's four-level page tables: typedef struct { unsigned long pte; } pte_t; typedef struct { unsigned long pmd; } pmd_t; typedef struct { unsigned long pud; } pud_t; typedef struct { unsigned long pgd; } pgd_t; That way, even though they're all represented as 64-bit values, the compiler will catch you if you try to mix-and-match different types without doing appropriate work. Passing small structs in registers means that Linux can do that, and pass around `pte_t`s as parameters, without having to worry about paying a runtime cost for their typechecking.