Hello Spectrum

The game board is waiting. Eight rows, eight columns, sixty-four cells to claim.

This unit gives you a working ZX Spectrum program immediately. You’ll run it, see the board, move the cursor with the keyboard, and make your first changes to the code. Experience first, explanation after.

Run It

Download the code and assemble it:

pasmonext --tapbas inkwar.asm inkwar.tap

Load the TAP file in Fuse (or your preferred emulator). You’ll see this:

A black screen with a white 8×8 grid in the centre. The top-left cell flashes - that’s your cursor.

Press Q to move up. Press A to move down. Press O to move left. Press P to move right.

The cursor moves around the board. You’re controlling a ZX Spectrum program.

The Complete Code

Here’s everything that makes this work:

That’s about 200 lines. Don’t worry about understanding all of it yet - we’ll explore each piece. For now, let’s look at the structure and make some changes.

The Main Loop

Every game follows the same pattern: set up, then loop forever.

The halt instruction waits for the next frame - the Spectrum draws the screen 50 times per second, and halt synchronises our code to that rhythm. Without it, the cursor would move impossibly fast.

Try This: Change the Border

Find this line near the top of init_screen:

            xor     a               ; A = 0 (black)
            out     (KEY_PORT), a   ; Set border colour

The xor a sets A to 0 (black). Change it to:

            ld      a, 1            ; A = 1 (blue)
            out     (KEY_PORT), a   ; Set border colour

Reassemble and run. The border is now blue.

The Spectrum’s border colour is set by sending a value (0-7) to port $FE. The colours are:

Try different values. See what happens with 2, 4, or 6.

Try This: Change the Board Colour

The board cells are white because of this constant:

EMPTY_ATTR  equ     %00111000       ; White paper, black ink (empty cell)

That binary value %00111000 means:

• Bits 5-3 (PAPER): 111 = 7 = white
• Bits 2-0 (INK): 000 = 0 = black

Change it to cyan paper:

EMPTY_ATTR  equ     %00101000       ; Cyan paper, black ink

The 101 in bits 5-3 is 5 = cyan. Reassemble and run - the board is now cyan.

Try This: Change the Cursor

The cursor flashes because of the FLASH bit:

CURSOR_ATTR equ     %10111000       ; White paper, black ink + FLASH

The 1 at the start (bit 7) enables FLASH. Try changing it to BRIGHT instead:

CURSOR_ATTR equ     %01111000       ; White paper, black ink + BRIGHT

Now the cursor doesn’t flash - it’s just brighter than the other cells. Which do you prefer?

The Attribute Byte

You’ve been changing attribute values without fully understanding them. Here’s the format:

Bit:  7      6      5  4  3    2  1  0
      FLASH  BRIGHT PAPER      INK

• FLASH (bit 7): When set, the cell alternates between normal and inverted colours
• BRIGHT (bit 6): When set, colours are brighter
• PAPER (bits 5-3): Background colour (0-7)
• INK (bits 2-0): Foreground colour (0-7)

So %00111000 means: no flash, no bright, paper=7 (white), ink=0 (black).

And %10111000 means: flash ON, no bright, paper=7, ink=0.

Where Attributes Live

The Spectrum screen has 768 attribute cells arranged in a 32×24 grid. Each cell controls the colours of an 8×8 pixel area.

Attributes start at address $5800 (22528 decimal) and run to $5AFF. To find the attribute for row R, column C:

Address = $5800 + (R × 32) + C

Our board starts at row 8, column 12, so the top-left board cell is at:

$5800 + (8 × 32) + 12 = $5800 + 256 + 12 = $590C

That’s what the draw_board routine calculates.

The Keyboard

The Spectrum keyboard is arranged in half-rows. Each half-row has its own port address:

To check if Q is pressed, we read port $FBFE and check bit 0. If the bit is 0 (active low), the key is down.

That’s what read_keyboard does for Q, A, O, and P.

What You’ve Learnt

• Running Z80 code - The org 32768 puts code at a safe address; end start tells the assembler where execution begins
• The main loop - Setup once, then halt-read-update forever at 50Hz
• Border colour - out (254), a sets the border to colour 0-7
• Attribute memory - $5800-$5AFF, one byte per 8×8 cell
• Attribute format - FBPPPIII: flash, bright, paper colour, ink colour
• Keyboard reading - in a, (port) reads a half-row; bits are active low

What’s Next

In Unit 2, we’ll add the ability to claim cells. Press Space, and the cell turns your colour. The game begins.