$[5]$ Regarding an ARM Cortex$-$M processor core, which of the following statement is wrong?

a$.$ Typically, it uses on$-$chip flash as the instruction memory and on$-$chip SRAM as the data memory.

b$.$ The ALU cannot directly access a memory value since its input and output operands are limited to registers or immediate numbers.

c$.$ Its memory address has $32$ bits; thus, the total addressable memory space is $4MB.$

d$.$ Each instruction has a length of either $16$ or $32$ bits.