diff --git a/src/arch/riscv/prefix/libprefix.S b/src/arch/riscv/prefix/libprefix.S
index b011616da..834a48655 100644
--- a/src/arch/riscv/prefix/libprefix.S
+++ b/src/arch/riscv/prefix/libprefix.S
@@ -1088,6 +1088,7 @@ install:
 
 	/* Copy and register system device tree */
 	phys_to_virt a0, s1
+	mv	a1, s4
 	call	fdtmem_register
 
 	/* Return to a virtual address in the relocated copy */
diff --git a/src/core/fdtmem.c b/src/core/fdtmem.c
index 9e8ab2406..18ae5ce96 100644
--- a/src/core/fdtmem.c
+++ b/src/core/fdtmem.c
@@ -66,6 +66,9 @@ struct fdtmem_region {
 /** Region is usable as RAM */
 #define FDTMEM_RAM 0x0001
 
+/** Size of accessible physical address space (or zero for no limit) */
+static size_t fdtmem_limit;
+
 /**
  * Update memory region descriptor
  *
@@ -360,13 +363,17 @@ physaddr_t fdtmem_relocate ( struct fdt_header *hdr, size_t limit ) {
  * Copy and register system device tree
  *
  * @v hdr		FDT header
+ * @v limit		Size of accessible physical address space (or zero)
  * @ret rc		Return status code
  */
-int fdtmem_register ( struct fdt_header *hdr ) {
+int fdtmem_register ( struct fdt_header *hdr, size_t limit ) {
 	struct fdt_header *copy;
 	struct fdt fdt;
 	int rc;
 
+	/* Record size of accessible physical address space */
+	fdtmem_limit = limit;
+
 	/* Parse FDT to obtain length */
 	if ( ( rc = fdt_parse ( &fdt, hdr, -1UL ) ) != 0 ) {
 		DBGC ( colour, "FDTMEM could not parse FDT: %s\n",
diff --git a/src/include/ipxe/fdtmem.h b/src/include/ipxe/fdtmem.h
index a5777f6a2..1bf0015b4 100644
--- a/src/include/ipxe/fdtmem.h
+++ b/src/include/ipxe/fdtmem.h
@@ -13,6 +13,6 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/fdt.h>
 
 extern physaddr_t fdtmem_relocate ( struct fdt_header *hdr, size_t limit );
-extern int fdtmem_register ( struct fdt_header *hdr );
+extern int fdtmem_register ( struct fdt_header *hdr, size_t limit );
 
 #endif /* _IPXE_FDTMEM_H */