diff --git a/xa/src/xat.c b/xa/src/xat.c
index 681570c..7ec226c 100644
--- a/xa/src/xat.c
+++ b/xa/src/xat.c
@@ -704,8 +704,14 @@ fprintf(stderr, "E_NODEF pass1 xat.c\n");
                } else
                     sy=4+nk;	/* absolute or zero page */
 
-		/* length counter set to maximum length + 1 */
-               bl=Maxbyt+1;
+	       /* length counter set to maximum length + 1 */
+	       if (w65816 || (t[l-1]=='@' || t[l-1] == '!')) {
+		       	/* for 65816 allow addressing modes up to 4 byte overall length */
+               		bl=Maxbyt+1;
+	       } else {
+		       	/* for other modes only check for addressing modes up to 3 byte overall length */
+		 	bl=Maxbyt;
+	       }
                
 		/* find best fit for length of this operand */
                while(--bl)
@@ -1375,9 +1381,19 @@ fprintf(stderr, "Kdsb E_DSB %i\n", j);
                          }
                     }
                }
-                
-               bl=Maxbyt+1;
                
+	       /* set bl to maximum overall length +1 as while() below starts with decrementing it */
+	       if (w65816 || (t[*ll-1]=='@' || t[*ll-1] == '!')) {
+		       	/* for 65816 allow addressing modes up to 4 byte overall length */
+               		bl=Maxbyt+1;
+	       } else {
+		       	/* for other modes only check for addressing modes up to 3 byte overall length */
+		 	bl=Maxbyt;
+	       }
+              
+#ifdef DEBUG_AM
+	      printf("--- trying to find am using: (max+1) bl=%d, sy=%d\n", bl, sy); 
+#endif
                while(--bl)
                {
                     if((am=at[sy][bl-1])>=0)
@@ -1415,8 +1431,8 @@ fprintf(stderr, "Kdsb E_DSB %i\n", j);
                {
 #ifdef DEBUG_AM
 fprintf(stderr,
-"b4: pc= %d, am = %d and vv[0] = %d, optimize = %d, bitmask = %d\n",
-	pc[segment], am, vv[0], fl, (vv[0]&0xffff00));
+"b4: pc= %d, am = %d and vv[0] = %d, optimize = %d, bitmask = %u, er=%d\n",
+	pc[segment], am, vv[0], fl, (vv[0]&0xffff00), er);
 #endif
 
 /* terrible KLUDGE!!!! OH NOES!!!1!
@@ -1455,15 +1471,21 @@ fprintf(stderr,
                else
                {
                     bl=le[am];
+		    if ((am != 11 && am != 16) && (vv[0] > 255 || vv[0] < -256) && bl == 2) {
+			    er = E_OVERFLOW;
+		    } else
+		    if ((am != 11 && am != 16) && (vv[0] > 65535 || vv[0] < -65536) && (bl == 2 || bl == 3)) {
+			    er = E_OVERFLOW;
+		    } else
                     if( ((ct[n][am]&0x400) && memode) || ((ct[n][am]&0x800) && xmode)) {
                          bl++;
-		}
+		    }
                     *ll=bl;
 
                }
 
 #ifdef DEBUG_AM
-fprintf(stderr, "byte length is now %d\n", bl);
+fprintf(stderr, "byte length is now %d, am=%d, er=%d\n", bl, am, er);
 #endif
 
                if(!er)
diff --git a/xa/tests/neg_offset/Makefile b/xa/tests/neg_offset/Makefile
index 86ae8c3..7c4bc33 100644
--- a/xa/tests/neg_offset/Makefile
+++ b/xa/tests/neg_offset/Makefile
@@ -1,12 +1,33 @@
-default: test1 test2
+default: test1 test2 test3 test4 test5 test6 test7 test8 
 
 test1:
 	../../xa -R -LLIB6502 test1.s -o test1.o
-	../hextool -cmp=ok < test1.o
+	../hextool -cmp=ok1 < test1.o
 
 test2:
 	../../xa -R -LLIB6502 test2.s -o test2.o
-	../hextool -cmp=ok < test2.o
+	../hextool -cmp=ok2 < test2.o
+
+test3:
+	../../xa -R -LLIB6502 test3.s -o test3.o
+	../hextool -cmp=ok3 < test3.o
+
+test4:
+	../../xa -R -LLIB6502 test4.s -o test4.o
+	../hextool -cmp=ok4 < test4.o
+
+test5:
+	../../xa -R -LLIB6502 test5.s -o test5.o
+	../hextool -cmp=ok5 < test5.o
+
+test6:
+	../../xa -R -LLIB6502 test6.s -o test6.o || exit 0 && exit 1
+
+test7:
+	../../xa -R -LLIB6502 test7.s -o test7.o || exit 0 && exit 1
+
+test8:
+	../../xa -R -LLIB6502 test8.s -o test8.o || exit 0 && exit 1
 
 clean:
 	rm -f *.o
diff --git a/xa/tests/neg_offset/a.o65 b/xa/tests/neg_offset/a.o65
new file mode 100644
index 0000000..0bea5d1
Binary files /dev/null and b/xa/tests/neg_offset/a.o65 differ
diff --git a/xa/tests/neg_offset/ok1 b/xa/tests/neg_offset/ok1
new file mode 100644
index 0000000..9f821a0
Binary files /dev/null and b/xa/tests/neg_offset/ok1 differ
diff --git a/xa/tests/neg_offset/ok2 b/xa/tests/neg_offset/ok2
new file mode 100644
index 0000000..9f821a0
Binary files /dev/null and b/xa/tests/neg_offset/ok2 differ
diff --git a/xa/tests/neg_offset/ok3 b/xa/tests/neg_offset/ok3
new file mode 100644
index 0000000..3663025
Binary files /dev/null and b/xa/tests/neg_offset/ok3 differ
diff --git a/xa/tests/neg_offset/ok4 b/xa/tests/neg_offset/ok4
new file mode 100644
index 0000000..40a1a4a
Binary files /dev/null and b/xa/tests/neg_offset/ok4 differ
diff --git a/xa/tests/neg_offset/ok5 b/xa/tests/neg_offset/ok5
new file mode 100644
index 0000000..0bea5d1
Binary files /dev/null and b/xa/tests/neg_offset/ok5 differ
diff --git a/xa/tests/neg_offset/test3.s b/xa/tests/neg_offset/test3.s
new file mode 100644
index 0000000..29a90cd
--- /dev/null
+++ b/xa/tests/neg_offset/test3.s
@@ -0,0 +1,6 @@
+
+	.text
+
+	jsr LIB6502-255
+
+
diff --git a/xa/tests/neg_offset/test4.s b/xa/tests/neg_offset/test4.s
new file mode 100644
index 0000000..1358dfb
--- /dev/null
+++ b/xa/tests/neg_offset/test4.s
@@ -0,0 +1,6 @@
+
+	.text
+
+	jsr LIB6502+256
+
+
diff --git a/xa/tests/neg_offset/test5.s b/xa/tests/neg_offset/test5.s
new file mode 100644
index 0000000..41d582a
--- /dev/null
+++ b/xa/tests/neg_offset/test5.s
@@ -0,0 +1,6 @@
+
+	.text
+
+	jsr LIB6502-256
+
+
diff --git a/xa/tests/neg_offset/test6.s b/xa/tests/neg_offset/test6.s
new file mode 100644
index 0000000..aa2b73f
--- /dev/null
+++ b/xa/tests/neg_offset/test6.s
@@ -0,0 +1,6 @@
+
+	.text
+
+	jsr LIB6502-65537
+
+
diff --git a/xa/tests/neg_offset/test7.s b/xa/tests/neg_offset/test7.s
new file mode 100644
index 0000000..b3ea18e
--- /dev/null
+++ b/xa/tests/neg_offset/test7.s
@@ -0,0 +1,6 @@
+
+	.text
+
+	jsr 65536
+
+
diff --git a/xa/tests/neg_offset/test8.s b/xa/tests/neg_offset/test8.s
new file mode 100644
index 0000000..4571b2a
--- /dev/null
+++ b/xa/tests/neg_offset/test8.s
@@ -0,0 +1,6 @@
+
+	.text
+
+	jsr LIB6502+65536
+
+