Data $2^{22}$ to $(2^{22}+2^{25})-1$

$6.$ Why can the instruction memory and the data memory share the same address bus in Cortex$-$M$3,$ Cortex$-$M$4$ and Cortex$-$M$7?$ In other words, why can we put the instruction memory and the data memory in the same memory address space $($i$.$e$.$ Unified address space within Harvard architecture$)?$