dec /34a issue (more info)

I did test those locations (420-430) and I can read/write the "normal" values as well as the complemented values VIA the front panel. It's also not "any data written by the processor is complemented" because the trap catcher program does write the correct data to locations 0-770. I'm certain I'm typing in the memory address test correctly. I have no other -11 handy. Is it possible there's a typo in the program I got? It follows: 000004 000226 bus error trap 000006 000000 000010 000012 reserved instruction trap 000012 000000 000200 012706 mov #410,sp set the stack 000202 000410 000204 012700 mov #420,r0 set target memory address 000206 000420 000210 010001 mov r0, r1 copy memory address 000212 010111 loop1: mov r1, (r1) write memory 000214 020111 cmp r1, (r1) check write 000216 001401 beq .+2 OK 000220 000000 halt initial check error 000222 005121 com (r1)+ complement the data 000224 000772 br loop1 loop till bus error 000226 012737 mov #6,@#4 reload trap vector 000230 000006 000232 000004 000234 010001 mov r0,r1 copy memory address 000236 005111 loop2: com (r1) complement memory 000240 020111 cmp r1,(r1) recheck memory 000242 001401 beq .+2 OK 000244 000000 halt memory addressing error 000246 005721 tst (r1)+ increment memory address 000250 000773 br loop2 When I enter and run the above program, it halts at 246. R1 contains 422. In going back after this halt, the program at 200 is unchanged and just as entered. However, location 4 contains a 6, not a 226. Puzzled.... J