Document SGB command $19 - PAL_PRI (#465)

Co-authored-by: Antonio Vivace <[email protected]>
Co-authored-by: Eldred Habert <[email protected]>

Author: 3 people

src/SGB_Command_Palettes.md

@@ -82,3 +82,19 @@ The palette data is sent by VRAM-Transfer (4 KBytes).
 
 Each Palette consists of four 16-bit color definitions (8 bytes). Note:
 The data is stored at 3000h-3FFFh in SNES memory.
+
+## SGB Command $19 — PAL_PRI
+
+If the player overrides the active palette set (a pre-defined or the custom one), it stays in effect until the smiley face is selected again, or the player presses the X button on their SNES controller.
+
+However, if `PAL_PRI` is enabled, then changing the palette set (via any of the above commands except `PAL_TRN`) will switch back to the game's newly-modified palette set, if it wasn't already active.
+
+_Donkey Kong_ (1994) is one known game that appears to use this.
+
+```
+ Byte  Content
+ 0     Command*8+Length (fixed length=1)
+ 1     Palette priority when a palette packet is sent
+         Bit 0 - Priority (0=User, 1=Software)
+ 2-F   Not used (zero)
+```

src/SGB_Command_Summary.md

@@ -29,3 +29,5 @@ Code | Name                                      | Explanation
  $16 | [ATTR_SET](<#SGB Command $16 — ATTR_SET>) | Set Data to ATF
  $17 | [MASK_EN](<#SGB Command $17 — MASK_EN>)   | Game Boy Window Mask
  $18 | [OBJ_TRN](<#SGB Command $18 — OBJ_TRN>)   | Super NES OBJ Mode
+ $19 | [PAL_PRI](<#SGB Command $19 — PAL_PRI>)   | System palette priority
+ 

src/SGB_Command_Undocumented.md

@@ -4,16 +4,10 @@ The following information has been extracted from disassembling a SGB1v2
 firmware; it should be verified on other SGB revisions.
 
 The SGB firmware explicitly ignores all commands with ID >= $1E. This
-leaves undocumented commands $19 to $1D inclusive.
+leaves undocumented commands $1A to $1D inclusive.
 
 ## Stubbed commands
 
 Commands $1A to $1F (inclusive)'s handlers are stubs (only contain an
 `RTS`). This is interesting, since the command-processing function
 explicitly ignores commands $1E and $1F.
-
-## SGB command $19
-
-The game _Donkey Kong_ (1994) appears to send this command, and it appears
-to set a flag in the SGB's memory. It's not known yet what it does,
-though.