diff --git a/src/avrdude.1 b/src/avrdude.1
index 0340c4a0..b12d56f8 100644
--- a/src/avrdude.1
+++ b/src/avrdude.1
@@ -770,6 +770,10 @@ flash-type memories the -U option can only write 0-bits but not 1-bits.
 Hence, usersig needs to be erased before a file can be uploaded to this
 memory region, e.g., using -T "erase usersig" -U
 usersig:w:parameters.hex:i
+.It io
+Volatile register memory; it cannot be accessed by external programming
+methods only by bootloaders, which has limited use unless the bootloader
+jumps to the application directly, i.e., without a WDT reset
 .El
 .Pp
 ATxmega devices have the following memory types in addition to eeprom, flash, signature and lock:
@@ -795,6 +799,10 @@ Production signature (calibration) area
 .It usersig
 Additional flash memory page that can be used for firmware settings; this
 memory is not erased during a chip erase
+.It io
+Volatile register memory;
+.Nm
+can read this memory but not write to it using external programming
 .El
 .Pp
 Modern 8-bit AVR devices have the following memory types in addition to eeprom, flash, signature and lock:
@@ -837,6 +845,13 @@ Extra page of EEPROM memory that can be used for firmware settings; this
 memory is not erased during a chip erase
 .It sib
 Special system information block memory with information about AVR family, chip revision etc.
+.It io
+Volatile register memory;
+.Nm
+can program this memory but this is of limited utility because anything
+written to the io memory will be undefined or lost after reset; writing to
+individual registers in the terminal can still be used, e.g., to test I/O
+ports
 .El
 .Pp
 The
diff --git a/src/doc/avrdude.texi b/src/doc/avrdude.texi
index b53bbfec..a3803ae7 100644
--- a/src/doc/avrdude.texi
+++ b/src/doc/avrdude.texi
@@ -814,6 +814,10 @@ memories the @code{-U} option can only write 0-bits but not 1-bits.
 Hence, usersig needs to be erased before a file can be uploaded to this
 memory region, e.g., using @code{-T "erase usersig" -U
 usersig:w:parameters.hex:i}
+@item io
+Volatile register memory; it cannot be accessed by external programming
+methods only by bootloaders, which has limited use unless the bootloader
+jumps to the application directly, i.e., without a WDT reset
 @end table
 
 ATxmega devices have the following memory types in addition to
@@ -838,6 +842,9 @@ Production signature (calibration) area
 @item usersig
 Additional flash memory page that can be used for firmware settings; this
 memory is not erased during a chip erase
+@item io
+Volatile register memory; AVRDUDE can read this memory but not write to it
+using external programming
 @end table
 
 Modern 8-bit AVR devices have the following memory types in addition to
@@ -880,7 +887,11 @@ Extra page of EEPROM memory that can be used for firmware settings; this
 memory is not erased during a chip erase
 @item sib
 Special system information block memory with information about AVR family, chip revision etc.
-
+@item io
+Volatile register memory; AVRDUDE can program this memory but this is of
+limited utility because anything written to the io memory will be undefined or
+lost after reset; writing to individual registers in the terminal can
+still be used, e.g., to test I/O ports
 @end table
 
 The @var{op} field specifies what operation to perform: