; equates are non arithmetic string substitutions
; numeric literals can be of the following formats:
; hex 0x0000 or $0000
; dec 1234
; oct 0o777 or 0q777
; bin 0b01011111
# comments can be started with ; or #

vram        EQU     0x0800
rand        EQU     0x06
vbase       EQU     0x30
pixels      EQU     0x32 
xyPos       EQU     0x34
xyVel       EQU     0x36

xBounds     EQU     0x9F
yBounds     EQU     0x77


_startAddress_      EQU     0x0200      ; entry point for the code, if this is missing defaults to 0x0200

_callTable_         EQU     0x007E      ; call addresses are automatically stored here by the assembler, it grows downwards
                                        ; *NOTE* gt1 spec only allows for one zero page segment, .vasm files use this for the call table
                                        ; do *NOT* make this address higher than 0x00BE, it will conflict with future ROM loading mechanisms
                                        ; do *NOT* define constants, (DB or DW), between 0x30 -> 0x44 and 0xc0 -> 0xFF, these addresses are
                                        ; used by the loader and the vCPU stack, you can create variables in these areas as long as you don't
                                        ; corrupt your nested CALL return addresses on the stack
                                        
_singleStepWatch_   EQU     xyPos       ; the single step debugger watches this variable location to decide when to step,
                                        ; choose a variable that is updated often


            LDWI    vram     
            STW     vbase       ; vram base address
            STW     pixels      ; pixel address
            LDWI    0x0101
            STW     xyPos       ; XY position
            LDWI    0x0101     
            STW     xyVel       ; XY velocity

; labels are parsed to generate the correct offsets for branches        
xbounds     LD      xyPos       ; x position bounds checking
            BEQ     xflip
            SUBI    xBounds
            BLT     ybounds
        
xflip       LD      xyVel
            XORI    0xFE        ; flip x velocity
            ST      xyVel
        
ybounds     LD      xyPos+1     ; y position bounds checking
            BEQ     yflip
            SUBI    yBounds
            BLT     velocity
        
yflip       LD      xyVel+1
            XORI    0xFE        ; flip y velocity
            ST      xyVel+1
        
velocity    LD      xyPos
            ADDW    xyVel
            ST      xyPos
            LD      xyPos+1
            ADDW    xyVel+1
            ST      xyPos+1
            
            LDW     vbase       ; generate vram address
            ADDW    xyPos
            STW     pixels
            LD      rand        ; grab an interesting colour
            POKE    pixels      ; plot new pixel
            
            BRA     xbounds