How to get the circular buffer pointer to go outside of the prescribed range.

I'm pretty sure on this I have a certain sequence and a circular buffer pointer will go outside of the range limit. I looked at some errat sheets but I didn't see this listed.

A "C" program will call an ASM routine. The C program passes a structure of the form

struct {

long ArrayPtr;

long Arrary[256];

};

Here is the problem ASM code and the above structure is passed in AR1.

1: OR #0002h,mmap(ST2_55)    ; enable AR1 for circular

2: MOV dbl(*AR1+),AC0                 ; This reads the index from the above struture and now AR1 points to Array[0]. For me AR1=0x146DA

3: mov mmap(AR1),BSA01           ; set the cirular base address

4: Mov #512,BK03                          ; length of the circular addressing

5: MOV dbl(*AR1+),AC1                ; Now look at AR1 and it's 0x144DC so it subtracted 512 and added 2 and it's out of range of 0x146DA + 512.

It looks like at line 2 it sets a trigger to tell it to do the 512 subtraction. Then at line 5 when it increment AR1+ it's subtracting 512 and adding 2. AR1 should be 0x146DC but it's 0x144DC.

My expectaion was that it only does the check with a *AR1+.

Also when this happens BSA01=0 and BK03=0 before they are changed at lines 3 and 4 but I'm not sure that makes a difference.