Why is memory area $1020 unwriteable on M68HC12 with staa?

Question

I have the following asm code:

    org $1000             ;Table Origin is at $1000
fcb $02,$04,$06,$08   ; values of table from $1001 - $1004
fcb $0a,$0c,$0e,$10   ; values of table from $1005 - $1009


        org $400          ; Program Start
        lds #$4000        ; Set Stack Pointer at value (#) $4000
        ldy #$1000        ; Set Index Y at $1000
        ldaa #$04         ; Load Accumulator a with value $04
loop    staa $20,y        ; Store value of Accumulator a at y = $1000 {DOESNT WORK} 
        staa $21,y        ; Store value of Accumulator a at y = $1021 {DOESNT WORK}
        iny               ; increment y (at this point $1020 and $1021 are still ff)
        iny               ; increment y (at this point $1020 and $1021 are still ff)
        deca              ; decrement a
        bne loop          ; loop until a = 0
hold    bra hold          ; end of program
        end

The table is set properly but the staa writes nothing at 1020 on. Why?

Answer 1

I believe there is ROM there

10.3 ROM Array

The ROM is made of a 28K byte array mapped from $1000 to $7FFF and of a 32 K byte array mapped from $8000 to $FFFF at reset. The MAPROM bit in the MISC register allows the swapping of the two arrays. ROMON28 and ROMON32 enable or disable the ROM module

From the M68HC12 datasheet