Claiming Cells

The board is ready. Now it’s time to claim it.

This unit adds the core gameplay: press Space to claim the cell under your cursor. The cell turns your colour. Then it’s the other player’s turn. Two players, one keyboard, taking turns until the board is full.

Run It

Assemble and run:

pasmonext --tapbas inkwar.asm inkwar.tap

The board looks the same as before - but now press Space.

The cell under your cursor turns red (bright red, actually). A short rising tone plays. Move the cursor and press Space again - now it turns blue. You’re Player 2.

Keep pressing Space. Red, blue, red, blue. The board fills up with colour.

Hand the keyboard to a friend. Now you’re playing Ink War.

Try This: Change Player Colours

Edit P1_ATTR and P2_ATTR to use different colours:

P1_ATTR     equ     %01100000       ; Yellow paper + BRIGHT
P2_ATTR     equ     %01011000       ; Cyan paper + BRIGHT

Remember the colour values:

• 0 = black, 1 = blue, 2 = red, 3 = magenta
• 4 = green, 5 = cyan, 6 = yellow, 7 = white

The PAPER bits are 5-3, so shift your colour left by 3. Yellow (6) becomes 110 in bits 5-3, plus BRIGHT (bit 6) = %01110000… wait, that’s white. Let me recalculate.

Actually: %01100000 = BRIGHT + paper 4 (green). For yellow: %01110000 = BRIGHT + paper 6. Try different combinations and see what looks good.

Try This: Change the Sound

Make the claim sound descending instead of ascending:

sound_claim:
            ld      hl, 100         ; Starting pitch (higher)
            ld      b, 20

.loop:
            ; ... same tone generation ...

            ; Decrease pitch (add to delay)
            ld      de, 20
            add     hl, de          ; Changed from sbc to add

            djnz    .loop
            ret

Or make it longer, shorter, different starting pitch. The beeper is primitive but expressive.

What You’ve Learnt

• Game state arrays - defs 64, 0 reserves 64 bytes initialised to zero
• Turn-based logic - Track current player, switch after valid moves
• Input validation - Check cell state before allowing claims
• XOR toggling - xor 3 switches between 1 and 2
• Beeper sound - Toggle bit 4 of port $FE at varying speeds for pitch

What’s Next

In Unit 3, you’ll customise the game’s appearance - different colours, different cursor style, making it your own. You’ll learn exactly how attribute bytes work by changing them.

What Changed

We added three things:

1. Board state - An array tracking who owns each cell
2. Claiming - Press Space to take ownership
3. Turn switching - Players alternate automatically

Player Colours

Each player gets a distinct colour:

Player 1 is red (%01010000), Player 2 is blue (%01001000). Both use BRIGHT for vivid colours that stand out from the white empty cells.

The cursor attributes (P1_CURSOR, P2_CURSOR) combine the player colour with FLASH, so when you move onto your own cell, it still flashes.

Game State

We need to remember who owns each cell:

The board_state array has 64 bytes - one per cell. Each byte is:

• 0 = empty
• 1 = Player 1 (red)
• 2 = Player 2 (blue)

The current_player variable tracks whose turn it is, starting with Player 1.

Initialisation

At startup, we clear the board and set Player 1 to go first:

The djnz loop writes zero to all 64 cells. Simple and fast.

Trying to Claim

When you press Space, we check if the claim is valid:

If the cell isn’t empty (or a / ret nz), we ignore the press. No stealing opponent cells - not yet, anyway.

The player switch uses a clever trick: xor 3 toggles between 1 and 2. Binary 01 XOR 11 = 10, and 10 XOR 11 = 01.

The Claim Itself

Two things happen:

1. We store the player number in board_state
2. We change the attribute colour on screen

The array index is row * 8 + col. We calculate it using shifts: add a, a three times multiplies by 8.

Sound Feedback

A claim should feel satisfying:

This generates a rising tone by decreasing the delay between speaker toggles. The Spectrum’s beeper is just a single bit - toggle it fast for high pitch, slow for low pitch.

The speaker is bit 4 of port $FE (the same port as border colour). We alternate between $10 (speaker on) and $00 (speaker off).

The Complete Code