Game 7

Drive It with the Stick

Open a loop that reads the joystick on every pass, decode the direction, and move the rover — erase the old block, draw the new one. PEEK(56320), an AND to pick the bit, two POKEs. The first time the machine answers you in real time.

Now the rover comes alive. Open a loop that, on every pass, reads the joystick and moves the rover to match. Hold the stick and it rolls; let go and it stops. This is the first time in the volume the machine answers you the instant you act.

10 POKE 53281,0
20 PRINT CHR$(147)
30 R=12:C=20
40 POKE 1024+R*40+C,160:POKE 55296+R*40+C,7
50 J=PEEK(56320)
60 NR=R:NC=C
70 IF (J AND 1)=0 THEN NR=R-1
80 IF (J AND 2)=0 THEN NR=R+1
90 IF (J AND 4)=0 THEN NC=C-1
100 IF (J AND 8)=0 THEN NC=C+1
110 IF NR=R AND NC=C THEN 50
120 POKE 1024+R*40+C,32
130 R=NR:C=NC
140 POKE 1024+R*40+C,160:POKE 55296+R*40+C,7
150 GOTO 50
A C64 screen, black background, the rover block driven up and to the right of centre.
The rover, driven: pushed right and up with the stick, it travelled there a cell at a time. Hold a direction and it keeps going; release and it stops where it is.

The loop is lines 50–150, and it does three things each pass. Read the stick: line 50, J = PEEK(56320), reads control port 2, where the joystick lives. Decode the direction: the bits are active-low — holding a direction clears its bit — so you test with AND. Lines 70–100 read up (J AND 1), down (2), left (4) and right (8); a result of 0 means that direction is held, so the new row NR or column NC shifts by one. Move: if the position changed, line 120 erases the old cell (POKE a space), then lines 130–140 update R/C and draw the rover at its new spot. That’s erase-move-draw — clear where it was, draw where it is.

Then GOTO 50 runs it all again, dozens of times a second. The rover follows the stick because the loop never stops asking what the stick is doing. The bit-testing looks fiddly, but it’s the C64’s real-time input in three lines — the same PEEK(56320) every action game on the machine leans on.

Try this

  • Change the feel. The rover moves one cell per pass, as fast as the loop runs. Add a tiny delay — FOR D=1 TO 20:NEXT — inside the loop to slow it to a steady crawl.
  • One axis only. Comment out the left/right tests (lines 90–100) and the rover only goes up and down. Proof each direction is its own independent bit.

What’s next

The rover drives anywhere — including straight off the screen. In Unit 3 walls give the world edges, and a look-ahead PEEK stops the rover dead when it meets one.