diff --git a/Core/gb.h b/Core/gb.h
index 45deae9..fcae405 100644
--- a/Core/gb.h
+++ b/Core/gb.h
@@ -227,11 +227,13 @@ enum {
     GB_IO_PSWX       = 0x72, // X position of the palette switching window
     GB_IO_PSWY       = 0x73, // Y position of the palette switching window
     GB_IO_PSW        = 0x74, // Key combo to trigger the palette switching window
-    GB_IO_UNKNOWN5   = 0x75, // (8Fh) - Bit 4-6 (Read/Write)
-    GB_IO_PCM12     = 0x76, // Channels 1 and 2 amplitudes
-    GB_IO_PCM34     = 0x77, // Channels 3 and 4 amplitudes
+    GB_IO_PGB        = 0x75, // Bits 0-2 control PHI, A15 and ¬CS, respectively.  Bits 4-6 control the I/O directions of bits 0-2 (0 is R, 1 is W)
+    GB_IO_PCM12      = 0x76, // Channels 1 and 2 amplitudes
+    GB_IO_PCM34      = 0x77, // Channels 3 and 4 amplitudes
 };
 
+static const typeof(GB_IO_PGB) __attribute__((deprecated("Use GB_IO_PGB instead"))) GB_IO_UNKNOWN5 = GB_IO_PGB;
+
 typedef enum {
     GB_LOG_BOLD = 1,
     GB_LOG_DASHED_UNDERLINE = 2,
diff --git a/Core/memory.c b/Core/memory.c
index 7702423..e1b30da 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -735,7 +735,7 @@ static uint8_t read_high_memory(GB_gameboy_t *gb, uint16_t addr)
                 return GB_is_cgb(gb)? gb->io_registers[addr & 0xFF] : 0xFF;
             case GB_IO_PSW:
                 return gb->cgb_mode? gb->io_registers[addr & 0xFF] : 0xFF;
-            case GB_IO_UNKNOWN5:
+            case GB_IO_PGB:
                 return GB_is_cgb(gb)? gb->io_registers[addr & 0xFF] | 0x8F : 0xFF;
             default:
                 if ((addr & 0xFF) >= GB_IO_NR10 && (addr & 0xFF) <= GB_IO_WAV_END) {
@@ -1415,7 +1415,7 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
             case GB_IO_PSWX:
             case GB_IO_PSWY:
             case GB_IO_PSW:
-            case GB_IO_UNKNOWN5:
+            case GB_IO_PGB:
                 gb->io_registers[addr & 0xFF] = value;
                 return;
             case GB_IO_OPRI:
diff --git a/Misc/registers.sym b/Misc/registers.sym
index ef027d0..1da8871 100644
--- a/Misc/registers.sym
+++ b/Misc/registers.sym
@@ -61,7 +61,7 @@
 00:FF72 rPSWX
 00:FF73 rPSWY
 00:FF74 rPSW
-00:FF75 rUNKNOWN5
+00:FF75 rPGB
 00:FF76 rPCM12
 00:FF77 rPCM34
 00:FFFF rIE