diff --git a/mach/vc4/libem/csa.s b/mach/vc4/libem/csa.s
new file mode 100644
index 000000000..1628199a0
--- /dev/null
+++ b/mach/vc4/libem/csa.s
@@ -0,0 +1,33 @@
+#
+/*
+ * VideoCore IV support library for the ACK
+ * © 2013 David Given
+ * This file is redistributable under the terms of the 3-clause BSD license.
+ * See the file 'Copying' in the root of the distribution for the full text.
+ */
+
+#include "videocore.h"
+
+.define	.csa
+.sect .data
+.csa:
+	! on entry:
+	!   r0 = un-fixed-up descriptor
+	!   r1 = value
+	add r0, gp
+
+	ld r2, 4 (r0)            ! check lower bound
+	cmp r1, r2
+	mov.lo r1, r2            ! r1 = min(r1, r2)
+
+	sub r1, r2               ! adjust value to be 0-based
+
+	ld r2, 8 (r0)            ! check upper bound
+	cmp r1, r2
+	mov.hi r1, r2            ! r1 = max(r1, r2)
+
+    add r1, #3
+    ld r1, (r0, r1)          ! load destination address
+    add r1, gp
+    b r1                     ! ...and go
+
diff --git a/mach/vc4/libem/dummy.s b/mach/vc4/libem/dummy.s
deleted file mode 100644
index fdbcc4c38..000000000
--- a/mach/vc4/libem/dummy.s
+++ /dev/null
@@ -1,14 +0,0 @@
-#
-/*
- * VideoCore IV support library for the ACK
- * © 2013 David Given
- * This file is redistributable under the terms of the 3-clause BSD license.
- * See the file 'Copying' in the root of the distribution for the full text.
- */
-
-#include "videocore.h"
-
-.define	__dummy
-.sect .data
-__dummy:
-
diff --git a/mach/vc4/libem/videocore.h b/mach/vc4/libem/videocore.h
index 3e27a7e7b..8ccb981ee 100644
--- a/mach/vc4/libem/videocore.h
+++ b/mach/vc4/libem/videocore.h
@@ -13,3 +13,5 @@
 .sect .data
 .sect .bss
 
+#define gp r15
+