View source for HOW TO PLAY

[[File:howtoplay_nam.png|thumb|A "scripted" how-to-play screen.]]

The HOW_TO_PLAY ($C00474) [[system ROM]] call builds and animates an "how to play" screen from a command list. Only a few games use this call.

The graphics are set up using the [[fix layer]] with hardcoded tile numbers (todo: get tilemaps).

=Command list=

The command list must be loaded at $10F300 (how many bytes reserved ?).

Each command is a longword (4 bytes):

* The rightmost byte is the command parameter
* The next byte is the command type (0~3)
* If the command type isn't recognized, the longword is processed as being a pointer to a [[MESS OUT]] data buffer
* The end command is $00000000

<pre>
00000000 00000000 000000TT PPPPPPPP
</pre>

==Type 0: Loop control==

* If the parameter byte is less than $FF: Set a loop counter to that value
* If the parameter byte is equal to $FF: Decrement the loop counter and loop back to the previous setup command until it reaches 0

==Type 1: Joystick state==

Sets the color of each joystick direction arrow.

* 1 is light blue (pressed)
* 0 is dark blue (released)

{{8BitRegister
|Up-Right
|1
|Right
|1
|Down-Right
|1
|Down
|1
|Down-Left
|1
|Left
|1
|Up-Left
|1
|Up
|1
}}

If followed by a null command ($0.l) the next 16-bit word specifies a number of frames to wait before executing the next command.

==Type 2: Buttons state==

Sets the state of each button.

* 1 is pressed
* 0 is released

{{8BitRegister
| -
|4
|D
|1
|C
|1
|B
|1
|A
|1
}}

If followed by a null command ($0.l) the next 16-bit word specifies a number of frames to wait before executing the next command.

==Any other type: Text update==

The longword value is a pointer to a [[MESS_OUT]] data buffer.

This command is '''always''' followed by a 16-bit word value which specifies a number of frames to wait before executing the next command.

=Example=

[[NAM-1975]] sets up the following command list:

<pre>
0000 0200    All buttons released
0000 01FF    All arrows highlighted
0010 F3E2    MESS_OUT write "NAM 1975" "CONTROL HERO AND CURSOR"
0004         Wait 4 frames
0000 000C    Init loop counter to 12
0000 0100    All arrows cleared       <--------------,
0000 0000    Wait...                                 |
0008         ...8 frames                             |
0000 01FF    All arrows highlighted                  |
0000 0000    Wait...                                 |
0008         ...8 frames                             |
0000 00FF    Loop back (blink all arrows 12 times) --'
0000 0201    A button is pressed
0000 0100    All arrows cleared
0010 F418    MESS_OUT write "MACHINE GUN"
0004         Wait 4 frames
0000 000C    Init loop counter to 12
0000 0200    All buttons released     <--------------,
0000 0000    Wait...                                 |
0008         ...8 frames                             |
0000 0201    A button is pressed                     |
0000 0000    Wait...                                 |
0008         ...8 frames                             |
0000 00FF    Loop back (blink A button 12 times) ----'
...
</pre>

=List of games known to use HOW_TO_PLAY=

* [[NAM-1975]]
* [[Baseball Stars Professional]]
* [[Mahjong Kyo Retsuden]]
* ...

[[Category:Code]]
[[Category:BIOS calls]]