diff --git a/examples/armv4t/gdb/mod.rs b/examples/armv4t/gdb/mod.rs
index 0831566..6c1ed51 100644
--- a/examples/armv4t/gdb/mod.rs
+++ b/examples/armv4t/gdb/mod.rs
@@ -314,6 +314,7 @@ impl target::ext::base::single_register_access::SingleRegisterAccess<()> for Emu
                 );
                 Ok(buf.len())
             }
+            custom_arch::ArmCoreRegIdCustom::Unavailable => Ok(0),
         }
     }
 
@@ -341,6 +342,7 @@ impl target::ext::base::single_register_access::SingleRegisterAccess<()> for Emu
                 Ok(())
             }
             // ignore writes
+            custom_arch::ArmCoreRegIdCustom::Unavailable |
             custom_arch::ArmCoreRegIdCustom::Time => Ok(()),
         }
     }
@@ -460,6 +462,8 @@ mod custom_arch {
         // not sent as part of `struct ArmCoreRegsCustom`, and only accessible via the single
         // register read/write functions
         Time,
+        /// This pseudo-register is valid but never available
+        Unavailable,
     }
 
     impl RegId for ArmCoreRegIdCustom {
@@ -467,6 +471,7 @@ mod custom_arch {
             let reg = match id {
                 26 => Self::Custom,
                 27 => Self::Time,
+                28 => Self::Unavailable,
                 _ => {
                     let (reg, size) = ArmCoreRegId::from_raw_id(id)?;
                     return Some((Self::Core(reg), size));
@@ -530,6 +535,7 @@ mod custom_arch {
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Fps) => "padding",
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Cpsr) => "cpsr",
                         ArmCoreRegIdCustom::Custom => "custom",
+                        ArmCoreRegIdCustom::Unavailable => "Unavailable",
                         _ => "unknown",
                     };
                     let encoding = match r {
@@ -537,6 +543,7 @@ mod custom_arch {
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Sp)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Pc)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Cpsr)
+                        | ArmCoreRegIdCustom::Unavailable
                         | ArmCoreRegIdCustom::Custom => Encoding::Uint,
                         _ => Encoding::Vector,
                     };
@@ -545,6 +552,7 @@ mod custom_arch {
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Sp)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Pc)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Cpsr)
+                        | ArmCoreRegIdCustom::Unavailable
                         | ArmCoreRegIdCustom::Custom => Format::Hex,
                         _ => Format::VectorUInt8,
                     };
@@ -555,6 +563,7 @@ mod custom_arch {
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Sp)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Pc)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Cpsr)
+                        | ArmCoreRegIdCustom::Unavailable
                         | ArmCoreRegIdCustom::Custom => "General Purpose Registers",
                         _ => "Floating Point Registers",
                     };
diff --git a/examples/armv4t/gdb/target_description_xml_override.rs b/examples/armv4t/gdb/target_description_xml_override.rs
index 294b513..2c5348c 100644
--- a/examples/armv4t/gdb/target_description_xml_override.rs
+++ b/examples/armv4t/gdb/target_description_xml_override.rs
@@ -96,5 +96,13 @@ const EXTRA_XML: &str = r#"
         this register via the 'p' and 'P' packets respectively
     -->
     <reg name="time" bitsize="32" type="uint32"/>
+
+    <!--
+        pseudo-register that is always unavailable.
+
+        it is supposed to be reported as 'x'-ed bytes in replies to 'p' packets
+        and shown by the GDB client as "<unavailable>".
+    -->
+    <reg name="unavailable" bitsize="32" type="uint32"/>
 </feature>
 "#;
diff --git a/src/protocol/response_writer.rs b/src/protocol/response_writer.rs
index 3dea22a..1ef43ea 100644
--- a/src/protocol/response_writer.rs
+++ b/src/protocol/response_writer.rs
@@ -212,6 +212,14 @@ impl<'a, C: Connection + 'a> ResponseWriter<'a, C> {
         Ok(())
     }
 
+    /// Marks data of 'size' bytes as unavailable.
+    pub fn write_x(&mut self, size: usize) -> Result<(), Error<C::Error>> {
+        for _ in 0..(size * 2) {
+            self.write(b'x')?;
+        }
+        Ok(())
+    }
+
     /// Write a number as a big-endian hex string using the most compact
     /// representation possible (i.e: trimming leading zeros).
     pub fn write_num<D: BeBytes + PrimInt>(&mut self, digit: D) -> Result<(), Error<C::Error>> {
diff --git a/src/stub/core_impl/single_register_access.rs b/src/stub/core_impl/single_register_access.rs
index fd61c58..cace9d6 100644
--- a/src/stub/core_impl/single_register_access.rs
+++ b/src/stub/core_impl/single_register_access.rs
@@ -33,14 +33,22 @@ impl<T: Target, C: Connection> GdbStubImpl<T, C> {
 
                 let len = ops.read_register(id, reg_id, buf).handle_error()?;
 
-                if let Some(size) = reg_size {
-                    if size.get() != len {
+                if len == 0 {
+                    if let Some(size) = reg_size {
+                        res.write_x(size.get())?;
+                    } else {
                         return Err(Error::TargetMismatch);
                     }
                 } else {
-                    buf = buf.get_mut(..len).ok_or(Error::PacketBufferOverflow)?;
+                    if let Some(size) = reg_size {
+                        if size.get() != len {
+                            return Err(Error::TargetMismatch);
+                        }
+                    } else {
+                        buf = buf.get_mut(..len).ok_or(Error::PacketBufferOverflow)?;
+                    }
+                    res.write_hex_buf(buf)?;
                 }
-                res.write_hex_buf(buf)?;
                 HandlerStatus::Handled
             }
             SingleRegisterAccess::P(p) => {
diff --git a/src/target/ext/base/single_register_access.rs b/src/target/ext/base/single_register_access.rs
index f7a9b16..2887fef 100644
--- a/src/target/ext/base/single_register_access.rs
+++ b/src/target/ext/base/single_register_access.rs
@@ -29,7 +29,8 @@ where
     /// Implementations should write the value of the register using target's
     /// native byte order in the buffer `buf`.
     ///
-    /// Return the number of bytes written into `buf`.
+    /// Return the number of bytes written into `buf` or `0` if the register is
+    /// valid but unavailable.
     ///
     /// If the requested register could not be accessed, an appropriate
     /// non-fatal error should be returned.