diff --git a/example/gdrom_test.cpp b/example/gdrom_test.cpp
index 353066b..21efcd1 100644
--- a/example/gdrom_test.cpp
+++ b/example/gdrom_test.cpp
@@ -1,6 +1,7 @@
 #include "gdrom.hpp"
 #include "gdrom_bits.hpp"
 #include "memorymap.hpp"
+#include "systembus.hpp"
 
 #include "sh7091/serial.hpp"
 
@@ -234,7 +235,7 @@ void cd_read()
 void main()
 {
   // gdrom unlock undocumented register
-  *((volatile unsigned long *)0xa05f74e4) = 0x1fffff;
+  g1_if.GDUNLOCK = 0x1fffff;
 
   // Without this read from system_boot_rom, the read value of
   // gdrom_if.status is always 0xff
diff --git a/regs/systembus.csv b/regs/systembus.csv
index 09244e7..4ed3745 100644
--- a/regs/systembus.csv
+++ b/regs/systembus.csv
@@ -83,6 +83,7 @@
 "G1_IF","b4","4","G1CRDYC","W","G1IORDY signal control"
 "G1_IF","b8","4","GDAPRO","W","GD-DMA address range"
 ,,,,,
+"G1_IF","e4","4","GDUNLOCK","W","(undocumented unlock register)"
 "G1_IF","f4","4","GDSTARD","R","GD-DMA address count (on Root Bus)"
 "G1_IF","f8","4","GDLEND","R","GD-DMA transfer counter"
 ,,,,,
diff --git a/regs/systembus.ods b/regs/systembus.ods
index 90f79c2..a486842 100644
Binary files a/regs/systembus.ods and b/regs/systembus.ods differ
diff --git a/systembus.hpp b/systembus.hpp
index 9190727..0eb9117 100644
--- a/systembus.hpp
+++ b/systembus.hpp
@@ -142,7 +142,9 @@ struct g1_if_reg {
   reg32 G1SYSM;              /* System mode */
   reg32 G1CRDYC;             /* G1IORDY signal control */
   reg32 GDAPRO;              /* GD-DMA address range */
-  reg8  _pad5[56];
+  reg8  _pad5[40];
+  reg32 GDUNLOCK;            /* (undocumented unlock register) */
+  reg8  _pad6[12];
   reg32 GDSTARD;             /* GD-DMA address count (on Root Bus) */
   reg32 GDLEND;              /* GD-DMA transfer counter */
 };
@@ -163,6 +165,7 @@ static_assert((offsetof (struct g1_if_reg, G1GDWC)) == 0xa4);
 static_assert((offsetof (struct g1_if_reg, G1SYSM)) == 0xb0);
 static_assert((offsetof (struct g1_if_reg, G1CRDYC)) == 0xb4);
 static_assert((offsetof (struct g1_if_reg, GDAPRO)) == 0xb8);
+static_assert((offsetof (struct g1_if_reg, GDUNLOCK)) == 0xe4);
 static_assert((offsetof (struct g1_if_reg, GDSTARD)) == 0xf4);
 static_assert((offsetof (struct g1_if_reg, GDLEND)) == 0xf8);