Add description of SGB detection using initial register values (#309)

* Add description of SGB detection using initial register values
* Link to power-up register values in SGB detection

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

Author: martendo

src/SGB_Command_Multiplayer.md

@@ -4,7 +4,7 @@
 
 Used to request multiplayer mode (that is, input from more than one joypad).
 Because this function provides feedback from the SGB/SNES to the Game
-Boy program, it is also used to detect SGB hardware.
+Boy program, it can also be used to detect SGB hardware.
 
 ```
  Byte  Content

src/SGB_Unlocking.md

@@ -15,34 +15,41 @@ SGB functions.
 
 ## Detecting SGB hardware
 
-The recommended detection method is to send a MLT_REQ command which
-enables two (or four) joypads. A normal handheld Game Boy will ignore
-this command, a SGB will now return incrementing joypad IDs each time
-when deselecting keyboard lines (see MLT_REQ description for details).
-Now read-out joypad state/IDs several times, and if the ID-numbers are
-changing, then it is a SGB (a normal Game Boy would typically always
-return 0Fh as ID). Finally, when not intending to use more than one
-joypad, send another MLT_REQ command in order to re-disable the
-multi-controller mode. Detection works regardless of whether and how
-many joypads are physically connected to the SNES. However, detection
-works only when having unlocked SGB functions in the cartridge header,
-as described above.
-
-## Separating between SGB and SGB2
-
-It is also possible to separate between SGB and SGB2 models by examining
-the inital value of the accumulator (register A) directly after startup.
-
-Value | Console
-------|---------
- $01  | SGB or original Game Boy (DMG)
- $FF  | SGB2 or Game Boy Pocket
- $11  | CGB or GBA
-
-Because values 01h and FFh are shared for both handhelds and SGBs, it is
-still required to use the above MLT_REQ detection procedure. As far as
-I know the SGB2 doesn't have any extra features which'd require
-separate SGB2 detection except for curiosity purposes, for example, the
-game "Tetris DX" chooses to display an alternate SGB border on SGB2s.
+SGB hardware can be detected by examining the initial value of the C
+register directly after startup: a value of $14 indicates SGB or SGB2
+hardware. It is also possible to separate between SGB and SGB2 by
+examining the initial value of the A register directly after startup.
+Note that the DMG and MGB share initial A register values with the SGB
+and SGB2 respectively.
+
+Console | A Register | C Register
+--------|------------|------------
+DMG     | $01        | $13
+SGB     | $01        | $14
+MGB     | $FF        | $13
+SGB2    | $FF        | $14
+CGB     | $11        | $00
+AGB     | $11        | $00
+
+For initial register values on all systems, see the table of all [CPU
+registers after power-up](<#CPU registers>).
+
+The SGB2 doesn't have any extra features which'd require separate SGB2
+detection except for curiosity purposes, for example, the game "Tetris
+DX" chooses to display an alternate SGB border on SGB2s.
 
 Only the SGB2 contains a link port.
+
+SGB hardware has traditionally been detected by sending [`MLT_REQ` commands](<#SGB Command 11h - MLT_REQ>), but this
+method is more complicated and slower than checking the value of the A
+and C registers after startup. The `MLT_REQ` command enables two (or four)
+joypads; a normal handheld Game Boy will ignore this command, but an SGB
+will return incrementing joypad IDs each time when deselecting keypad
+lines ([see `MLT_REQ` description](<#Reading Multiple Controllers (Joypads)>)). The joypad state/IDs can
+then be read out several times, and if the IDs are changing, then it is
+an SGB (a normal Game Boy would typically always return $0F as the ID).
+Finally, when not intending to use more than one joypad, send another
+`MLT_REQ` command in order to disable the multi-controller mode.
+Detection works regardless of how many joypads are physically connected
+to the SNES. However, unlike the C register method, this detection works only when
+SGB functions [are unlocked from the cartridge header](<#Cartridge Header>).