// Move a sprite around the screen using interrupts
LDISR Button // Set interrupt handler

CALL UpdateScreen	// Clear background and draw player
HLT

Button: LDA Up // Interrupt handler

BRZ NoBtnUp // Check if up button was pressed
LDA pos
SUB #8
STA pos
LDA #0
STA Up

NoBtnUp:LDA Down // Check if Down button was pressed
BRZ NoBtnDown
LDA pos
ADD #8
STA pos
LDA #0
STA Down

NoBtnDown: LDA Left // Check if Left button was pressed
BRZ NoBtnLeft
LDA pos
SUB #1
STA pos
LDA #0
STA Left

NoBtnLeft: LDA Right // Check if right button was pressed
BRZ NoBtnRight
LDA pos
ADD #1
STA pos
LDA #0
STA Right

NoBtnRight: CALL UpdateScreen // Update screen
RET

UpdateScreen: CALL ClearScreen // Display game
LDA player
STA ~pos
RET

ClearScreen: LDA #63 // Clear game background
STA i
ScreenLoop: LDA Background
STA ~i
LDA i
SUB #1
STA i
BRP ScreenLoop
RET

ORG 64 // Start putting labels at address 64 (mapped to up button)

Up:DAT // Buttons
Down:DAT
Left:DAT
Right:DAT

i: DAT              // Used for looping through each pixel
pos: DAT            // Player position 
background: DAT 50  // Background sprite
player: DAT 41      // Player sprite