diff --git a/hw/i386/x86.c b/hw/i386/x86.c
index 29167de97d..c61f4ebfa6 100644
--- a/hw/i386/x86.c
+++ b/hw/i386/x86.c
@@ -1128,12 +1128,25 @@ void x86_load_linux(X86MachineState *x86ms,
     nb_option_roms++;
 }
 
+void x86_isa_bios_init(MemoryRegion *isa_bios, MemoryRegion *isa_memory,
+                       MemoryRegion *bios, bool read_only)
+{
+    uint64_t bios_size = memory_region_size(bios);
+    uint64_t isa_bios_size = MIN(bios_size, 128 * KiB);
+
+    memory_region_init_alias(isa_bios, NULL, "isa-bios", bios,
+                             bios_size - isa_bios_size, isa_bios_size);
+    memory_region_add_subregion_overlap(isa_memory, 1 * MiB - isa_bios_size,
+                                        isa_bios, 1);
+    memory_region_set_readonly(isa_bios, read_only);
+}
+
 void x86_bios_rom_init(X86MachineState *x86ms, const char *default_firmware,
                        MemoryRegion *rom_memory, bool isapc_ram_fw)
 {
     const char *bios_name;
     char *filename;
-    int bios_size, isa_bios_size;
+    int bios_size;
     ssize_t ret;
 
     /* BIOS load */
@@ -1171,14 +1184,8 @@ void x86_bios_rom_init(X86MachineState *x86ms, const char *default_firmware,
     g_free(filename);
 
     /* map the last 128KB of the BIOS in ISA space */
-    isa_bios_size = MIN(bios_size, 128 * KiB);
-    memory_region_init_alias(&x86ms->isa_bios, NULL, "isa-bios", &x86ms->bios,
-                             bios_size - isa_bios_size, isa_bios_size);
-    memory_region_add_subregion_overlap(rom_memory,
-                                        0x100000 - isa_bios_size,
-                                        &x86ms->isa_bios,
-                                        1);
-    memory_region_set_readonly(&x86ms->isa_bios, !isapc_ram_fw);
+    x86_isa_bios_init(&x86ms->isa_bios, rom_memory, &x86ms->bios,
+                      !isapc_ram_fw);
 
     /* map all the bios at the top of memory */
     memory_region_add_subregion(rom_memory,
diff --git a/include/hw/i386/x86.h b/include/hw/i386/x86.h
index 55c6809ae0..d7b7d3f3ce 100644
--- a/include/hw/i386/x86.h
+++ b/include/hw/i386/x86.h
@@ -129,6 +129,8 @@ void x86_cpu_unplug_request_cb(HotplugHandler *hotplug_dev,
 void x86_cpu_unplug_cb(HotplugHandler *hotplug_dev,
                        DeviceState *dev, Error **errp);
 
+void x86_isa_bios_init(MemoryRegion *isa_bios, MemoryRegion *isa_memory,
+                       MemoryRegion *bios, bool read_only);
 void x86_bios_rom_init(X86MachineState *x86ms, const char *default_firmware,
                        MemoryRegion *rom_memory, bool isapc_ram_fw);