diff --git a/mandel-core.s b/mandel-core.s
index ddcd7fc..1f7234b 100644
--- a/mandel-core.s
+++ b/mandel-core.s
@@ -599,12 +599,14 @@ bank_switch_table:
             output = dest
 
             ; top 2 bits are the table bank selector
+            ; note we keep arg2 in X after this lookup as we need it later!
             ldx arg2                ; 3 cyc
             lda bank_switch_table,x ; 4 cyc
             sta PORTB               ; 4 cyc
 
             ; bottom 14 bits except the LSB are the per-bank table index
             ; add $4000 for the bank pointer
+            ; note the low byte of ptr is assumed to remain at 0! do not clobber it!
             txa          ; 2 cyc
             and #$3f     ; 2 cyc
             ora #$40     ; 2 cyc