"private address space to each running program; each virtual address space contains all of a program's instructions and data, which can be referenced by the program via virtual addresses"
Ah, excellent!
"I think I wanted to use internal frag there"
Mhm that makes sense. I was more confused by the usage of "thus" than "internal fragmentation".
Using "thus" after a sentence fragment (in bold below) which nearly defines "internal fragmentation" felt odd to me.
restricted to placing an address space in a fixed-sized slot and thus internal fragmentation can arise
Perhaps something along the lines of:
restricted to placing an address space in a fixed-sized slot. If the address space's size does not exactly match the slot's fixed size, there will be what's known as internal fragmentation.
Perhaps something along the lines of: