In protected mode, SS is a descriptor offset into the GDT. It doesn’t make sense to change SS very often unless there are multiple stacks. Furthermore, it doesn

In protected mode, SS is a descriptor offset into the GDT. It doesn’t make sense to change SS very often unless there are multiple stacks. Furthermore, it doesn’t make sense to overcomplicate an already over-complicated instruction set burdened with vestigal, legacy features. And, loading SS and SP atomtically would probably create a byte pattern that’s too long to encode.