pet32: don't use problematic zeropage locations

codeCore/src/prog8/code/target/zp/PETZeropage.kt

@@ -32,7 +32,10 @@ class PETZeropage(options: CompilationOptions) : Zeropage(options) {
             }
             ZeropageType.FLOATSAFE,
             ZeropageType.BASICSAFE -> {
-                free.addAll(0xb1u..0xbau)       // TODO more?
+                // Only scratch register area is safe for transient use.
+                // $b5 and $ba are clobbered by PET BASIC/Kernal calls like CHROUT ($ffd2).
+                // User variables requiring persistent storage must NOT be placed here.
+                free.addAll(listOf(0xb1u, 0xb2u, 0xb3u, 0xb4u, 0xb6u, 0xb7u, 0xb8u, 0xb9u))
             }
             ZeropageType.DONTUSE -> {
                 free.clear()  // don't use zeropage at all
@@ -83,4 +86,4 @@ class PETZeropage(options: CompilationOptions) : Zeropage(options) {
         allocatedVariables["cx16.r12r13sl"] = VarAllocation((base+6*4).toUInt(), DataType.LONG, 4)
         allocatedVariables["cx16.r14r15sl"] = VarAllocation((base+7*4).toUInt(), DataType.LONG, 4)
     }
-}
+}