diff --git a/mos-platform/c128/CMakeLists.txt b/mos-platform/c128/CMakeLists.txt
index eae189ecf..b0a3d350a 100644
--- a/mos-platform/c128/CMakeLists.txt
+++ b/mos-platform/c128/CMakeLists.txt
@@ -25,7 +25,11 @@ add_platform_object_file(c128-basic-header basic-header.o basic-header.S)
 add_platform_object_file(c128-init-mmu init-mmu.o init-mmu.S)
 target_include_directories(c128-init-mmu PRIVATE .)
 
-add_platform_library(c128-c c128.c kernal.S)
+add_platform_library(c128-c
+  c128.c
+  devnum.s
+  kernal.S
+)
 target_include_directories(c128-c BEFORE PUBLIC .)
 target_include_directories(c128-c BEFORE PUBLIC ../c64)
 
diff --git a/mos-platform/c128/devnum.s b/mos-platform/c128/devnum.s
new file mode 100644
index 000000000..a14a8f98f
--- /dev/null
+++ b/mos-platform/c128/devnum.s
@@ -0,0 +1,17 @@
+; Copyright 2023 LLVM-MOS Project
+; Licensed under the Apache License, Version 2.0 with LLVM Exceptions.
+; See https://github.com/llvm-mos/llvm-mos-sdk/blob/main/LICENSE for license
+; information.
+
+; Originally from cc65. Modified from original version.
+
+;
+; Oliver Schmidt, 2010-02-14
+;
+
+        .include        "c128.inc"
+
+        .globl          devnum
+        .zeropage       devnum
+
+devnum = DEVNUM
diff --git a/mos-platform/cx16/CMakeLists.txt b/mos-platform/cx16/CMakeLists.txt
index 85284a727..da9442147 100644
--- a/mos-platform/cx16/CMakeLists.txt
+++ b/mos-platform/cx16/CMakeLists.txt
@@ -79,6 +79,7 @@ add_platform_library(cx16-c
  cx16_k_screen_set_charset.s
  cx16_k_sprite_set_image.s
  cx16_k_sprite_set_position.s
+ filevars.s
  get_numbanks.s
  get_ostype.s
  get_tv.s
diff --git a/mos-platform/cx16/filevars.s b/mos-platform/cx16/filevars.s
new file mode 100644
index 000000000..ab720b25e
--- /dev/null
+++ b/mos-platform/cx16/filevars.s
@@ -0,0 +1,38 @@
+; Copyright 2024 LLVM-MOS Project
+; Licensed under the Apache License, Version 2.0 with LLVM Exceptions.
+; See https://github.com/llvm-mos/llvm-mos-sdk/blob/main/LICENSE for license
+; information.
+
+; Originally from cc65. Modified from original version.
+
+; 2002-11-15, Ullrich von Bassewitz
+; 2019-11-08, Greg King
+;
+; Variables used for CBM file I/O
+;
+
+        .globl         curunit
+
+        .include        "cx16.inc"
+
+
+.bss
+curunit:
+        .fill    1
+
+
+.section .init,"axR",@progbits
+initcurunit:
+        lda     DEVNUM
+        bne     L0
+        lda     #8              ; Default is SD card
+        sta     DEVNUM
+L0:     sta     curunit
+        rts
+
+
+.section .fini,"axR",@progbits
+updatedevnum:
+        lda     curunit
+        sta     DEVNUM
+        rts
diff --git a/mos-platform/mega65/CMakeLists.txt b/mos-platform/mega65/CMakeLists.txt
index d11ecb0d0..e5c57a0aa 100644
--- a/mos-platform/mega65/CMakeLists.txt
+++ b/mos-platform/mega65/CMakeLists.txt
@@ -20,5 +20,8 @@ merge_libraries(mega65-crt0
 add_platform_object_file(mega65-basic-header basic-header.o basic-header.S)
 add_platform_object_file(mega65-unmap-basic unmap-basic.o unmap-basic.S)
 
-add_platform_library(mega65-c kernal.S)
+add_platform_library(mega65-c
+  filevars.s
+  kernal.S
+)
 target_include_directories(mega65-c BEFORE PUBLIC .)
diff --git a/mos-platform/mega65/filevars.s b/mos-platform/mega65/filevars.s
new file mode 100644
index 000000000..80b17c919
--- /dev/null
+++ b/mos-platform/mega65/filevars.s
@@ -0,0 +1,25 @@
+; Copyright 2024 LLVM-MOS Project
+; Licensed under the Apache License, Version 2.0 with LLVM Exceptions.
+; See https://github.com/llvm-mos/llvm-mos-sdk/blob/main/LICENSE for license
+; information.
+
+; Originally from cc65. Modified from original version.
+
+;
+; Ullrich von Bassewitz, 15.11.2002
+;
+; Variables used for Mega65 file I/O
+;
+
+        .globl         curunit
+
+
+.bss
+curunit:
+        .fill    1
+
+
+.section .init,"axR",@progbits
+initcurunit:
+        lda     #8              ; Default is disk
+        sta     curunit
diff --git a/mos-platform/pet/CMakeLists.txt b/mos-platform/pet/CMakeLists.txt
index 55c4057c7..d33a52fab 100644
--- a/mos-platform/pet/CMakeLists.txt
+++ b/mos-platform/pet/CMakeLists.txt
@@ -23,5 +23,8 @@ install(FILES pet.inc DESTINATION ${ASMINCDIR})
 
 add_platform_object_file(pet-basic-header basic-header.o basic-header.S)
 
-add_platform_library(pet-c kernal.S)
+add_platform_library(pet-c
+  devnum.s
+  kernal.S
+)
 target_include_directories(pet-c BEFORE PUBLIC .)
diff --git a/mos-platform/pet/devnum.s b/mos-platform/pet/devnum.s
new file mode 100644
index 000000000..3839fe5a8
--- /dev/null
+++ b/mos-platform/pet/devnum.s
@@ -0,0 +1,17 @@
+; Copyright 2024 LLVM-MOS Project
+; Licensed under the Apache License, Version 2.0 with LLVM Exceptions.
+; See https://github.com/llvm-mos/llvm-mos-sdk/blob/main/LICENSE for license
+; information.
+
+; Originally from cc65. Modified from original version.
+
+;
+; Oliver Schmidt, 2010-02-14
+;
+
+        .include        "pet.inc"
+
+        .globl       devnum
+        .zeropage    devnum
+
+devnum = DEVNUM
diff --git a/mos-platform/vic20/CMakeLists.txt b/mos-platform/vic20/CMakeLists.txt
index 5dd90aeab..e601ef6dd 100644
--- a/mos-platform/vic20/CMakeLists.txt
+++ b/mos-platform/vic20/CMakeLists.txt
@@ -21,6 +21,9 @@ install(FILES vic20.inc DESTINATION ${ASMINCDIR})
 
 add_platform_object_file(vic20-basic-header basic-header.o basic-header.S)
 
-add_platform_library(vic20-c kernal.S)
+add_platform_library(vic20-c
+  devnum.s
+  kernal.S
+)
 target_include_directories(vic20-c BEFORE PUBLIC .)
 
diff --git a/mos-platform/vic20/devnum.s b/mos-platform/vic20/devnum.s
new file mode 100644
index 000000000..fd174baaa
--- /dev/null
+++ b/mos-platform/vic20/devnum.s
@@ -0,0 +1,17 @@
+; Copyright 2024 LLVM-MOS Project
+; Licensed under the Apache License, Version 2.0 with LLVM Exceptions.
+; See https://github.com/llvm-mos/llvm-mos-sdk/blob/main/LICENSE for license
+; information.
+
+; Originally from cc65. Modified from original version.
+
+;
+; Oliver Schmidt, 2010-02-14
+;
+
+        .include        "vic20.inc"
+
+        .globl       devnum
+        .zeropage    devnum
+
+devnum = DEVNUM