diff --git a/src/test/kc/tmp-zp-problem.c b/src/test/kc/tmp-zp-problem.c
index 88d121a68..8d4e54741 100644
--- a/src/test/kc/tmp-zp-problem.c
+++ b/src/test/kc/tmp-zp-problem.c
@@ -11,8 +11,10 @@ void main() {
     for(;;) {
         for(char i=0;i<10;i++)
             for(char j=0;j<10;j++) {
+                // i and j are put into X/Y. The A=X+Y fragment uses $ff as temporary storage.
                 char k = i+j;
                 if(k>0x80) {
+                    // Problem: This will occasionally trigger due to the IRQ interfering with the value in $ff!
                     VICII->BORDER_COLOR++;
                 }
             }
@@ -25,6 +27,7 @@ __mem char * volatile ptr = (char*)0xff00;
 // The Interrupt Handler
 __interrupt(rom_sys_c64) void irq() {
     *BG_COLOR = WHITE;
+    // ptr has to be moved to ZP for indirection. The fragment uses $fe-f as temporary storage.
     (*ptr)++;
     *BG_COLOR = BLACK;
 }
\ No newline at end of file
diff --git a/src/test/ref/tmp-zp-problem.asm b/src/test/ref/tmp-zp-problem.asm
index b77443db2..3e5f9dfab 100644
--- a/src/test/ref/tmp-zp-problem.asm
+++ b/src/test/ref/tmp-zp-problem.asm
@@ -74,6 +74,7 @@ main: {
     jmp __b1
   __b3:
     // char k = i+j
+    // i and j are put into X/Y. The A=X+Y fragment uses $ff as temporary storage.
     txa
     sty.z $ff
     clc
diff --git a/src/test/ref/tmp-zp-problem.log b/src/test/ref/tmp-zp-problem.log
index 9d656c280..cf804d559 100644
--- a/src/test/ref/tmp-zp-problem.log
+++ b/src/test/ref/tmp-zp-problem.log
@@ -412,6 +412,7 @@ main: {
     // main::@3
   __b3:
     // [13] main::k#0 = main::i#2 + main::j#2 -- vbuaa=vbuxx_plus_vbuyy 
+    // i and j are put into X/Y. The A=X+Y fragment uses $ff as temporary storage.
     txa
     sty.z $ff
     clc
@@ -601,6 +602,7 @@ main: {
   __b3:
     // char k = i+j
     // [13] main::k#0 = main::i#2 + main::j#2 -- vbuaa=vbuxx_plus_vbuyy 
+    // i and j are put into X/Y. The A=X+Y fragment uses $ff as temporary storage.
     txa
     sty.z $ff
     clc