diff --git a/asminc/cpu.mac b/asminc/cpu.mac
index be35265b7..c66641078 100644
--- a/asminc/cpu.mac
+++ b/asminc/cpu.mac
@@ -4,31 +4,30 @@ CPU_ISET_NONE      = $0001
 CPU_ISET_6502      = $0002
 CPU_ISET_6502X     = $0004
 CPU_ISET_6502DTV   = $0008
-
 CPU_ISET_65SC02    = $0010
 CPU_ISET_65C02     = $0020
 CPU_ISET_65816     = $0040
 CPU_ISET_SWEET16   = $0080
-
 CPU_ISET_HUC6280   = $0100
 CPU_ISET_M740      = $0200
 CPU_ISET_4510      = $0400
 CPU_ISET_45GS02    = $0800
 
 ; CPU capabilities
+; make sure to only combine the instruction sets that are 100% compatible
 CPU_NONE           = CPU_ISET_NONE
 CPU_6502           = CPU_ISET_6502
-CPU_6502X          = CPU_ISET_6502|CPU_ISET_6502X
-CPU_6502DTV        = CPU_ISET_6502|CPU_ISET_6502DTV
-CPU_65SC02         = CPU_ISET_6502|CPU_ISET_65SC02
-CPU_65C02          = CPU_ISET_6502|CPU_ISET_65SC02|CPU_ISET_65C02
-CPU_65816          = CPU_ISET_6502|CPU_ISET_65SC02|CPU_ISET_65816
+CPU_6502X          = CPU_ISET_6502X   | CPU_ISET_6502
+CPU_6502DTV        = CPU_ISET_6502DTV | CPU_ISET_6502
+CPU_65SC02         = CPU_ISET_65SC02  | CPU_ISET_6502
+CPU_65C02          = CPU_ISET_65C02   | CPU_ISET_6502 | CPU_ISET_65SC02
+CPU_65816          = CPU_ISET_65816   | CPU_ISET_6502 | CPU_ISET_65SC02
 CPU_SWEET16        = CPU_ISET_SWEET16
 ; FIXME: CPU_ISET_65SC02 does not apply to the following, because the zp-indirect
 ; addressing was replaced with zp-indirect,z-indexed in 652SCE02
 ; NOTE: HUC6280 removes "wai" ($cb) and "stp" ($db) from the 65C02 instruction set
-CPU_HUC6280        = CPU_ISET_6502|CPU_ISET_65C02|CPU_ISET_HUC6280
+CPU_HUC6280        = CPU_ISET_HUC6280 | CPU_ISET_6502 | CPU_ISET_65C02
 ; NOTE: 4510 replaces "wai" ($cb) and "stp" ($db) of the 65C02 instruction set
-CPU_4510           = CPU_ISET_6502|CPU_ISET_65C02|CPU_ISET_4510
-CPU_45GS02         = CPU_ISET_6502|CPU_ISET_65C02|CPU_ISET_4510|CPU_ISET_45GS02
-CPU_M740           = CPU_ISET_6502|CPU_ISET_M740
+CPU_4510           = CPU_ISET_4510    | CPU_ISET_6502 | CPU_ISET_65C02
+CPU_45GS02         = CPU_ISET_45GS02  | CPU_ISET_6502 | CPU_ISET_65C02 | CPU_ISET_4510
+CPU_M740           = CPU_ISET_M740    | CPU_ISET_6502
diff --git a/libsrc/c65/kernal.s b/libsrc/c65/kernal.s
index ea07f9ce6..fac8be514 100644
--- a/libsrc/c65/kernal.s
+++ b/libsrc/c65/kernal.s
@@ -42,7 +42,6 @@
         .export         STOP
         .export         GETIN
         .export         CLALL
-        .export         UDTIM
         .export         SCREEN
         .export         IOBASE
         .export         PLOT
diff --git a/libsrc/mega65/kernal.s b/libsrc/mega65/kernal.s
index 45f4c4d02..198e8fbb4 100644
--- a/libsrc/mega65/kernal.s
+++ b/libsrc/mega65/kernal.s
@@ -42,7 +42,6 @@
         .export         STOP
         .export         GETIN
         .export         CLALL
-        .export         UDTIM
         .export         SCREEN
         .export         IOBASE
         .export         PLOT