Write to unused memory-mapped IO address (0xfffffffc)

In some assembly code I am writing I decrement the frame_pointer with 4
after executing a store word instruction in a loop:

sw $2, 0($30)
addi $30, $0, -4 # fp point to next element in a.

When the program starts $30 is initialized to:

7FFFEFEC

The first time the SW instruction is executed it works fine and the frame
pointer is afterwards decremented with 4 and gets the value:

FFFFFFFC

But the next time the SW instruction is executed I get this error:

"Write to unused memory-mapped IO address (0xfffffffc)"

I use SPIM to simulate my code.

Why is it not possible to write to 0xfffffffc??


.

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