Namtir Raiders

Namtir Raiders is a space shooter game that loads and executes machine code via RAND USR 16514, with the BASIC loader storing the game data in a large string array dimensioned to 1790 bytes using DIM A$(1790). The program uses POKE 16477 to adjust system variables and checks memory configuration with PEEK 16386 and PEEK 16387 before proceeding, branching to NEW if the expected values aren’t found. The REM statement in line 1 contains the bulk of the machine code payload, encoded as tokenized BASIC—a common technique for embedding binary data in listings. A smaller DIM A$(20) and SAVE command in line 45 suggest a two-stage loading process, with the initial short array used before the full game array is allocated. The title screen data is stored in REM line 100, containing inverse-video block graphics that form a decorative splash screen including a high score display area.

Program Analysis

Program Structure

The loader is organized into a small number of BASIC lines that perform a carefully sequenced initialization before handing control to machine code:

1. Line 1 — A REM statement containing the machine code payload, encoded as tokenized BASIC keywords and characters. This is the largest block in the program.
2. Line 2 — An empty REM, acting as a separator or padding.
3. Line 10 — POKE 16418,0 resets the error register (ERR NR system variable) to clear any prior error state.
4. Line 20 — PRINT A$ outputs the string array, which at this point is uninitialized; likely used to verify or display something before clearing.
5. Line 30 — CLEAR reclaims memory.
6. Line 40 — DIM A$(20) allocates a small 20-byte string array, used as staging before the full allocation.
7. Line 45 — SAVE "NAMTIR RAIDERS" saves the loader to tape.
8. Line 50 — CLEAR frees memory again.
9. Line 55 — Memory check: IF PEEK 16386<>252 OR PEEK 16387<>127 THEN NEW verifies that 16K of RAM is present (RAMTOP at address &7FFC = 32764), aborting with NEW if not.
10. Line 60 — DIM A$(1790) allocates the full 1790-byte array to hold game data or machine code workspace.
11. Line 70 — POKE 16477,26 sets the system variable PROG (or a related variable) to adjust the interpreter’s view of program memory.
12. Line 80 — RAND USR 16514 transfers execution to machine code at address 16514, which is within the REM statement of line 1.
13. Line 100 — A large REM containing the title screen, rendered using inverse-video block graphic characters, and ending with a “HIGH SCORE -” prompt.
14. Line 101 — An empty REM terminator.

Machine Code Embedding Technique

The primary technique used here is storing the machine code inside the REM statement at line 1. Because a REM line is not executed by the BASIC interpreter, arbitrary bytes can be stored there without causing syntax errors. The address 16514 corresponds to a location inside that REM’s data area (the ZX81 program area starts at 16509, and the REM content begins 5 bytes into line 1 after the line number, length, and REM token). RAND USR 16514 jumps directly into this embedded Z80 machine code.

Memory Configuration Check

Line 55 inspects system variables at addresses 16386–16387, which hold the RAMTOP value. The expected values 252 and 127 correspond to the high and low bytes of address 32764 (&7FFC), the standard RAMTOP for a 16K RAM machine. If this condition is not met—indicating insufficient or unexpected memory—the program executes NEW to halt safely rather than corrupting memory.

Two-Stage Array Allocation

The loader performs two distinct array allocations: a small DIM A$(20) at line 40, followed after a CLEAR by the full DIM A$(1790) at line 60. This pattern is consistent with a save-then-reload workflow: the small array is allocated before the SAVE so that the tape image includes a minimal footprint, while the large array is only needed at runtime to provide the machine code with a safe workspace or data buffer.

System Variable Manipulation

POKE 16418,0 writes to ERR NR (the error number system variable), clearing any pending error condition before the program proceeds. POKE 16477,26 targets the system variable at that address; adjusting this before the USR call likely repositions the interpreter’s internal pointer to ensure the machine code executes in the correct memory context.

Title Screen in REM

Line 100 contains an elaborate title screen encoded entirely within a REM statement using block graphic characters (▀, ▛, ▜, ▌, ▐, ▗, ▘, ▟, etc.) and inverse-video letters spelling out game title elements. Since this data is never directly printed by BASIC—the machine code handles display—the REM serves as a convenient read-only data area accessible via PEEK or direct memory read from Z80 code. The line ends with the text HIGH SCORE - , suggesting the machine code reads from this REM to construct the HUD display.

Notable Idioms and Techniques

• USR entry into REM: The canonical ZX81 technique for embedding and executing machine code without a separate loader file.
• Memory guard with NEW: Using THEN NEW rather than a GOSUB or error message is a compact way to abort on hardware mismatch.
• SAVE mid-loader: Placing SAVE at line 45, after the initial small DIM but before the large one, produces a tape image that is self-contained and minimal.
• RAND USR vs. USR: Using RAND USR discards the return value silently, avoiding a “0 OK” report that PRINT USR would produce.

Potential Anomalies

• Line 20: PRINT A$ executes before A$ is dimensioned (it is dimensioned at line 40). On a fresh run this would print an empty or zero-filled string, or trigger an error if A$ doesn’t exist—suggesting this line is vestigial or only meaningful in a reload scenario where A$ was previously set.
• The SAVE at line 45 would pause execution awaiting a tape; in normal use the operator would save here and then allow the program to continue running, which is an unusual but workable design.

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   2 REM 
  10 POKE 16418,0
  20 PRINT A$
  30 CLEAR
  40 DIM A$(20)
  45 SAVE "NAMTIR RAIDER[S]"
  50 CLEAR
  55 IF PEEK 16386<>252 OR PEEK 16387<>127 THEN NEW
  60 DIM A$(1790)
  70 POKE 16477,26
  80 RAND USR 16514
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 101 REM