diff --git a/include/PR/gbi_extension.h b/include/PR/gbi_extension.h
index 352385b36..9c80856de 100644
--- a/include/PR/gbi_extension.h
+++ b/include/PR/gbi_extension.h
@@ -1,9 +1,52 @@
 #pragma once
 
+//
+// GFX COMMANDS FREE SLOTS
+//
+// The following op codes are free to use for G_ commands.
+// For RSP commands, use incrementing numbers starting from 00.
+// For RDP commands, use decrementing numbers starting from ff.
+// Please update the following table when implementing a new command.
+//
+// RSP ->                     09 0a 0b 0c 0d 0e 0f
+// 10          14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
+// 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
+// 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f
+// 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f
+// 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f
+// 60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f
+// 70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f
+// 80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f
+// 90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f
+// a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af
+// b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf
+// c0 c1 c2 c3 c4 c5 c6 c7                        
+// d0                                       <- RDP
+//
+//
+
 ///////////////////////
 // G_SETGEOMETRYMODE //
 ///////////////////////
 
+//
+// GEOMETRY MODES FREE FLAGS
+//
+// The following flags are free to use for geometry modes.
+// Please remove the corresponding flag from the list below when implementing a new geometry mode.
+// Note: geometry mode flags have only 24 bits, the 8 upper bits cannot be used.
+//
+// 0x00000002
+// 0x00000008
+// 0x00000010
+// 0x00000020
+// 0x00001000
+// 0x00002000
+// 0x00008000
+// 0x00080000 // G_TEXTURE_GEN_LINEAR, but not implemented
+// 0x00100000 // G_LOD, but not implemented
+// 0x00800000 // G_CLIPPING, but not implemented
+
 #define G_LIGHT_MAP_EXT       0x00000800
 #define G_LIGHTING_ENGINE_EXT 0x00004000
 #define G_PACKED_NORMALS_EXT  0x00000080
@@ -97,10 +140,14 @@
 #define gDPSetTextureClippingDjui(pkt, x1, y1, x2, y2)         gSetClippingDjui(pkt, G_TEXCLIP_DJUI, x1, y1, x2, y2)
 #define gDPSetTextureOverrideDjui(pkt, texture, w, h, bitSize) gSetOverrideDjui(pkt, G_TEXOVERRIDE_DJUI, texture, w, h, bitSize)
 
-////////////////////
-//  RDP commands  //
-////////////////////
+/////////////////
+// G_SETENVRGB //
+/////////////////
+
 #define G_SETENVRGB		0xd1
+
+#define	gDPSetEnvRGB(pkt, r, g, b)					\
+            DPRGBColor(pkt, G_SETENVRGB, r,g,b,255)
 #define	gsDPSetEnvRGB(r, g, b)					\
             sDPRGBColor(G_SETENVRGB, r,g,b,255)
 
@@ -129,9 +176,9 @@
     ((2 * ((part) + 1)) + 1 + offset)                                \
 }}
 
-////////////////////
-//// G_MOVEWORD ////
-////////////////////
+////////////////
+// G_MOVEWORD //
+////////////////
 
 #define G_MW_FX                  0x00 /* replaces G_MW_MATRIX which is no longer supported */
 #define G_MWO_FRESNEL            0x0C