--- a/src/cs/system/main/gcc/exceptions.S	Fri Jul 20 17:33:14 2018 +0000
+++ b/src/cs/system/main/gcc/exceptions.S	Fri Jul 20 18:21:46 2018 +0000
@@ -1,14 +1,8 @@
 /*
  * This module contains ARM exception handlers which used to be
- * in chipsetsw/system/Main/int.s in TI's Leonardo code.
+ * in chipsetsw/system/Main/int.s in TI's original version.
  */
 
-    .section "except_stack","aw",%nobits
-    .balign  4
-    .space   512
-    .globl   _Except_Stack_SP
-_Except_Stack_SP:
-
 	.text
 	.code	32
 
@@ -84,3 +78,48 @@
 
         mov     r0,r11          @ put index into 1st argument
         b       dar_exception
+
+@ the second part
+
+/*
+ * For the SP-in-RAM validity check, we use the following simplification:
+ * it doesn't really matter what the actual IRAM and XRAM sizes are on
+ * any given target, as the address decoder hooked up to the ARM7TDMI core
+ * always decodes the full 8 MiB address range for each, causing the
+ * actual memories to be aliased multiple times in those two ranges.
+ * Furthermore, the XRAM address range falls right after the IRAM one,
+ * thus we can get away with only a single range check.
+ */
+
+#define	RAM_LOW		0x00800000
+#define	RAM_HIGH	0x01800000
+
+#define	XDUMP_STACK_SIZE	20
+
+	.globl	exception
+exception:
+        ldr     r12,=xdump_buffer
+        ldr     r11,[r12,#4*13]   @ get svc mode r13
+        add     r12,r12,#4*18     @ base of stack buffer
+
+	@ check if svc r13(sp) is within internal/external RAM.
+	@ It *could* be invalid.
+        cmp     r11,#RAM_LOW
+        blt     nostack
+        mov     r0,#RAM_HIGH
+        sub     r0,r0,#XDUMP_STACK_SIZE
+        cmp     r11,r0
+        bge     nostack
+
+stack_range:              
+        ldmfd   r11!,{r0-r9}    @ copy ten stack words..
+        stmia   r12!,{r0-r9}
+        ldmfd   r11!,{r0-r9}    @ copy ten stack words..
+        stmia   r12!,{r0-r9}
+
+nostack:
+	@ we're finished saving all state.
+	@ Now execute C code for more flexibility.
+	@ set up a stack for this C call
+	ldr	sp,=_Stack_segment_end
+	b	dar_reset