Intro

Because the Game.com always has the OS loaded in memory, it offers a bunch of functions to simplify various tasks; These are referred to as System Calls, and they function a lot like common C-style functions.

Function arguments are setup by setting registers 8-15 And registers 0-4 are set when a function has any returning values.

So if we take FBFillColor for example. Filling the framebuffer with black is as simple as:

mov r8, #3 ; color 3 is black.
call FBFillColor 

If you're not using one of the existing templates, and would rather implement system calls manually, Here's how you would implement them:

 System_Call     equ     20f1h

 ; replace SC with System Call hex value (#1Bh for FBFillColor)
 pushw   rr6
 mov     r7,#SCh 
 br      System_Call

Known System Calls

0x1B - FBFillColor
FUNCTION: Fills the framebuffer with a specified color
INPUT:
    r8 = Color to fill with
OUTPUT:	N/A
IMPLEMENTATION:
--------------------------------------------------
FBFillColor:
        pushw   rr6
        mov     r7,#1Bh
        br      System_Call
--------------------------------------------------

0x1D - FBDrawGraphic
FUNCTION: Draws specified graphics to the framebuffer
INPUT:
    r8 = Horizontal framebuffer displacement
    r9 = Vertical framebuffer displacement
    r10 = Horizontal graphic offset displacement
    r11 = Vertical graphic offset displacement
    r12 = Horizontal graphic size
    r13 = Vertical graphic size
    r14 = Bank #
    r15 = Drawing behavior (Boolean flags)
        - 0x00  = Draw Absolute / Override
        - 0x01  = Draw with white as Transparency
        - 0x02  = Draw to Page B
        - 0x04  = Draw to Current Page
        - 0x08  = Draw in XOR mode
        - 0x40  = Draw Vertical flipped
        - 0x80  = Draw Horizontal flipped
OUTPUT:	N/A
IMPLEMENTATION:
--------------------------------------------------
FBDrawGraphic:
        pushw   rr6
        mov     r7,#1Dh
        br      System_Call
--------------------------------------------------

0x26 - FBDrawLineV
FUNCTION: Draws a completely vertical line on-screen (draws from top to bottom)
INPUT:
    r8 = Starting horizontal coordinate
    r9 = Starting vertical coordinate
    r10 = Line length
    r11 = Color to draw line with
    r12 = Line width
    r13 = ??? (Look into what exactly this is)
        - 0 = Line gets drawn
        - 1 = Line does not get drawn
OUTPUT:	N/A
IMPLEMENTATION:
--------------------------------------------------
FBDrawLineV:
        pushw   rr6
        mov     r7,#26h
        br      System_Call
--------------------------------------------------

0x27 - FBDrawLineH
FUNCTION: Draws a completely horizontal line on-screen (draws from left to right)
INPUT:
    r8 = Starting horizontal coordinate
    r9 = Starting vertical coordinate
    r10 = Line length
    r11 = Color to draw line with
    r12 = Line width
    r13 = ??? (Look into what exactly this is)
        - 0 = Line gets drawn
        - 1 = Line does not get drawn
OUTPUT:	N/A
IMPLEMENTATION:
--------------------------------------------------
FBDrawLineH:
        pushw   rr6
        mov     r7,#27h
        br      System_Call
--------------------------------------------------

0x2E - FBFillColorRect
FUNCTION: Draws a block of a filled color on-screen
INPUT:
    r10 = Horizontal block size
    r11 = Vertical block size
    r12 = Color to fill block with
OUTPUT:	N/A
--------------------------------------------------
FBFillColorRect:
        pushw   rr6
        mov     r7,#2Eh
        br      System_Call
--------------------------------------------------

0x2F - IOInputScan
FUNCTION: Gets the player's input
INPUT: N/A
OUTPUT:
    r0 = Player's inputted keys (document how this works better)
        - 0x83 = Up
        - 0x84 = Down
        - 0x85 = Left
        - 0x86 = Right
        - 0x8B = A
        - 0x8C = B
        - 0x8D = C
        - 0x8E = D
--------------------------------------------------
IOInputScan:
        pushw   rr6
        mov     r7,#2Fh
        br      System_Call
--------------------------------------------------

0x54 - FBPageB
FUNCTION: Switches to framebuffer B
INPUT: N/A
OUTPUT:	N/A
--------------------------------------------------
FBSwapPage:
        pushw   rr6
        mov     r7,#54h
        br      System_Call
--------------------------------------------------

0x55 - FBPageA
FUNCTION: Switches to framebuffer A
INPUT: N/A
OUTPUT:	N/A
--------------------------------------------------
FBSwapPage:
        pushw   rr6
        mov     r7,#55h
        br      System_Call
--------------------------------------------------

0x57 - FBSwapPage
FUNCTION: Switches the current, on-screen framebuffer page to render (A or B)
INPUT: N/A
OUTPUT:	N/A
--------------------------------------------------
FBSwapPage:
        pushw   rr6
        mov     r7,#57h
        br      System_Call
--------------------------------------------------