Brickbuster

Brickbuster is a Breakout-style bat-and-ball game where the player uses keyboard controls to deflect a ball and clear rows of bricks. The core game logic runs as machine code embedded in REM statements at lines 1 and 2, with BASIC handling setup, scoring, and the high-score table. The machine code routine is called via RAND USR at lines 100 and 180, with POKE statements at lines 150, 160, and 50 used to pass parameters such as speed, row position, and skill level into the machine code. A display variable is established at line 5 by reading the system variable at address 16396/16397 to locate the program’s own data area for score extraction, which is then assembled into a string and converted with VAL at line 230.

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Program Analysis

Program Structure

The program is organized into several distinct phases controlled by BASIC, while the actual game physics and rendering run entirely in machine code:

1. Initialization (lines 1–5): Two REM statements hold machine code. Line 5 calculates the address D of the program’s data area using system variables at 16396/16397.
2. Title / Speed Selection (lines 15–50): Clears the screen, prompts for a speed value 1–9, validates it, and POKEs it into address 16773.
3. Game Loop (lines 100–200): Calls machine code to initialize the board (RAND USR 16802), then iterates five waves (FOR F=5 TO 25 STEP 5), POKEing row and lives parameters before each serve and calling the game engine via RAND USR 16514.
4. End of Game / High Score (lines 210–300): Reads four bytes from the data area to reconstruct the score string, compares against the best score, and optionally records a new champion name.
5. High Score Entry (lines 400–450): Subroutine that accepts a player name via INPUT and stores it in Z$.
6. Save / Restart (lines 500–510): SAVEs the program then RUNs it.

Machine Code Usage

The two leading REM statements are the most technically significant parts of the program. Line 1 contains the primary machine code payload (the game board initializer and physics engine), while line 2 holds a secondary routine. The BASIC entry points are:

• RAND USR 16802 (line 100): Calls the board setup routine, which appears to draw the brick field and reset game state.
• RAND USR 16514 (line 180): Calls the main game loop — ball movement, collision detection, bat control, and scoring — returning to BASIC only when a life is lost.

Parameters are passed to the machine code purely through memory POKEs before the USR calls. Address 16773 holds the speed value; 16556 holds a lives/attempt counter (always set to 5); 16518 holds the current row multiplier F (5, 10, 15, 20, 25), which the machine code presumably uses to determine brick layout or scoring weight.

Score Extraction Technique

Because the score is maintained inside the machine code routine, retrieving it requires reading raw memory. Line 5 computes D as the base address of the program’s variable area:

LET D=PEEK 16396+256*PEEK 16397

Lines 220–230 then read four consecutive bytes at offsets +2 through +5 from D, construct a string S$ of their CHR$ equivalents, and recover the numeric score with VAL S$. This implies the machine code stores the score as four ASCII digit characters at a known offset within the BASIC program area — a compact and clever interface between machine code and BASIC.

Key BASIC Idioms

• Speed input validation: Lines 30–40 read INKEY$, convert it to a code, subtract 28 (the offset from character code to digit value on this system), and loop until the result is in range 1–9. Line 31 guards against a spurious 118 (newline) code that could corrupt the input.
• Inverse-video flash on prompt: Line 280 first PRINTs the “PRESS P TO PLAY AGAIN” message using inverse-video character escapes, then immediately overwrites it with a normal-video version at the same position — a trick to produce a brief visual flash without a PAUSE or timing loop.
• FOR loop as wave controller: The five-wave structure (FOR F=5 TO 25 STEP 5) is elegant: F doubles as the loop counter and the parameter POKEd into the machine code, avoiding a separate variable.
• Score display prefix: Line 110 PRINTs AT 0,0;"▖0000", placing a block graphic and four placeholder zeros at the top-left, which the machine code presumably overwrites in-place as the score updates.

Notable Anomalies

• Line 20 contains a typo: "ENTER SPEED 1 T0 9" uses the digit zero (0) instead of the letter O in “TO”. This is a display-only cosmetic issue.
• Line 1000 is a REM comment reading BAORD RUTEEN STARTS 16802 (“BOARD ROUTINE STARTS 16802”), confirming the developer used this as an inline note. The misspellings are original.
• The SAVE filename in line 500 uses an inverse-video R at the end of “BRICKBUSTE”, stored as an escape sequence in the listing.

Memory Map Summary

Address	Role
16514	Entry point: main game engine (USR call)
16556	Lives counter (POKEd to 5 each wave)
16773	Speed parameter (1–9, POKEd from user input)
16802	Entry point: board initializer (USR call)
16396/16397	System variable: start of BASIC variables area
D+2 … D+5	Score stored as 4 ASCII bytes by machine code