On linux it depends on a sysctl parameter, and on 4kB platforms some runtimes used the fact that it was set to "one page" as a method to have very short pointer

On linux it depends on a sysctl parameter, and on 4kB platforms some runtimes used the fact that it was set to "one page" as a method to have very short pointers starting at address 4097 for things very often references (and which might need guard page, like NULL value). Made for some borked code with Clozure Common Lisp when people started running 64kB pagesize on PPC64, and that meant that even with minimal zero page it still was too big to map the "low memory" where CCL stuck it's NULL definition.