diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
index ee219c8358..4e65286ff5 100644
--- a/docs/specs/acpi_cpu_hotplug.txt
+++ b/docs/specs/acpi_cpu_hotplug.txt
@@ -30,6 +30,18 @@ Register block base address:
 Register block size:
     ACPI_CPU_HOTPLUG_REG_LEN = 12
 
+All accesses to registers described below, imply little-endian byte order.
+
+Reserved resisters behavior:
+   - write accesses are ignored
+   - read accesses return all bits set to 0.
+
+The last stored value in 'CPU selector' must refer to a possible CPU, otherwise
+  - reads from any register return 0
+  - writes to any other register are ignored until valid value is stored into it
+On QEMU start, 'CPU selector' is initialized to a valid value, on reset it
+keeps the current value.
+
 read access:
     offset:
     [0x0-0x3] reserved
@@ -86,9 +98,3 @@ write access:
                  ACPI_DEVICE_OST QMP event from QEMU to external applications
                  with current values of OST event and status registers.
             other values: reserved
-
-Selecting CPU device beyond possible range has no effect on platform:
-   - write accesses to CPU hot-plug registers not documented above are
-     ignored
-   - read accesses to CPU hot-plug registers not documented above return
-     all bits set to 0.