diff --git a/hw/acpi/aml-build.c b/hw/acpi/aml-build.c
index 8d01959..77ce00b 100644
--- a/hw/acpi/aml-build.c
+++ b/hw/acpi/aml-build.c
@@ -636,9 +636,11 @@
 }
 
 /* ACPI 1.0b: 16.2.5.2 Named Objects Encoding: DefField */
-Aml *aml_field(const char *name, AmlFieldFlags flags)
+Aml *aml_field(const char *name, AmlAccessType type, AmlUpdateRule rule)
 {
     Aml *var = aml_bundle(0x81 /* FieldOp */, AML_EXT_PACKAGE);
+    uint8_t flags = rule << 5 | type;
+
     build_append_namestring(var->buf, "%s", name);
     build_append_byte(var->buf, flags);
     return var;
diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index c193ca5..085dc6d 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -746,7 +746,7 @@
 
         aml_append(dev, aml_operation_region("PEOR", aml_system_io,
                                               misc->pvpanic_port, 1));
-        field = aml_field("PEOR", aml_byte_acc);
+        field = aml_field("PEOR", aml_byte_acc, aml_preserve);
         aml_append(field, aml_named_field("PEPT", 8));
         aml_append(dev, field);
 
@@ -783,7 +783,7 @@
         /* declare CPU hotplug MMIO region and PRS field to access it */
         aml_append(sb_scope, aml_operation_region(
             "PRST", aml_system_io, pm->cpu_hp_io_base, pm->cpu_hp_io_len));
-        field = aml_field("PRST", aml_byte_acc);
+        field = aml_field("PRST", aml_byte_acc, aml_preserve);
         aml_append(field, aml_named_field("PRS", 256));
         aml_append(sb_scope, field);
 
@@ -857,7 +857,8 @@
             pm->mem_hp_io_base, pm->mem_hp_io_len)
         );
 
-        field = aml_field(stringify(MEMORY_HOTPLUG_IO_REGION), aml_dword_acc);
+        field = aml_field(stringify(MEMORY_HOTPLUG_IO_REGION), aml_dword_acc,
+                          aml_preserve);
         aml_append(field, /* read only */
             aml_named_field(stringify(MEMORY_SLOT_ADDR_LOW), 32));
         aml_append(field, /* read only */
@@ -870,7 +871,8 @@
             aml_named_field(stringify(MEMORY_SLOT_PROXIMITY), 32));
         aml_append(scope, field);
 
-        field = aml_field(stringify(MEMORY_HOTPLUG_IO_REGION), aml_byte_acc);
+        field = aml_field(stringify(MEMORY_HOTPLUG_IO_REGION), aml_byte_acc,
+                          aml_preserve);
         aml_append(field, aml_reserved_field(160 /* bits, Offset(20) */));
         aml_append(field, /* 1 if enabled, read only */
             aml_named_field(stringify(MEMORY_SLOT_ENABLED), 1));
@@ -879,7 +881,8 @@
             aml_named_field(stringify(MEMORY_SLOT_INSERT_EVENT), 1));
         aml_append(scope, field);
 
-        field = aml_field(stringify(MEMORY_HOTPLUG_IO_REGION), aml_dword_acc);
+        field = aml_field(stringify(MEMORY_HOTPLUG_IO_REGION), aml_dword_acc,
+                          aml_preserve);
         aml_append(field, /* DIMM selector, write only */
             aml_named_field(stringify(MEMORY_SLOT_SLECTOR), 32));
         aml_append(field, /* _OST event code, write only */
diff --git a/include/hw/acpi/aml-build.h b/include/hw/acpi/aml-build.h
index 1705001..15579e6 100644
--- a/include/hw/acpi/aml-build.h
+++ b/include/hw/acpi/aml-build.h
@@ -47,7 +47,13 @@
     aml_dword_acc = 3,
     aml_qword_acc = 4,
     aml_buffer_acc = 5,
-} AmlFieldFlags;
+} AmlAccessType;
+
+typedef enum {
+    aml_preserve = 0,
+    aml_write_as_ones = 1,
+    aml_write_as_zeros = 2,
+} AmlUpdateRule;
 
 typedef enum {
     aml_system_memory = 0x00,
@@ -205,7 +211,7 @@
 Aml *aml_package(uint8_t num_elements);
 Aml *aml_buffer(void);
 Aml *aml_resource_template(void);
-Aml *aml_field(const char *name, AmlFieldFlags flags);
+Aml *aml_field(const char *name, AmlAccessType type, AmlUpdateRule rule);
 Aml *aml_varpackage(uint32_t num_elements);
 
 void