I meant 32k bytes - PDP-11 was byte-addressed, not word-addressed. The 64kB address space was split in half between kernel and userland in so called "high and l

I meant 32k bytes - PDP-11 was byte-addressed, not word-addressed. The 64kB address space was split in half between kernel and userland in so called "high and low moby" scheme (as it required minimal logic latching onto single address line). And for I/D split you needed appropriate CPU model. the top 8kB "I/O page" is reserved as part of the kernel space, not userspace, so it does not impact as much the userspace part.