Night Gunner is a combat flight simulation game that uses machine code (invoked via RAND USR 20337) for performance-critical routines, with the BASIC code serving primarily as a loader and initialization framework. The program makes extensive use of LPRINT and COPY statements to transfer screen content to a printer, and employs a rich set of graphics characters including block graphics for rendering the game display. Variable arrays and string manipulation through CHR$, ACS, LN, TAN, and STR$ are used heavily throughout the BASIC stubs, suggesting the BASIC acts as a scaffolding layer over the underlying machine code engine. The SAVE “NG” line at line 25 stores the program to tape, while the main game loop and rendering logic reside in the machine code loaded into memory above the BASIC workspace.
Program Analysis
Program Structure
The program is organized across five numbered lines: REMs at lines 5, 10, 15, and 20 that encode the bulk of the program data as tokenized BASIC stored inside REM statements, a SAVE at line 25, and the machine code entry point at line 30. This is a well-known technique for storing executable machine code and compressed BASIC data in REM lines, which the ZX81 interpreter skips during normal execution but which can be read and used by machine code routines.
- Lines 5, 10, 15, 20 — REM blocks containing machine code, tokenized BASIC fragments, and graphic data
- Line 25 —
SAVE "NG"to tape - Line 30 —
RAND USR 20337transfers control to machine code
Machine Code Entry
Line 30 uses RAND USR 20337 to jump directly into machine code stored at memory address 20337. This is the standard idiom for launching machine code from BASIC on this platform. All game logic, rendering, and input handling are almost certainly implemented in the machine code payload embedded in the REM lines, with the BASIC skeleton providing only initialization and tape I/O.
REM Line Encoding
The four REM lines (5, 10, 15, 20) are not documentation — they contain the actual machine code and tokenized data of the game. The listing shows dense sequences of BASIC keywords, block graphics characters, and numeric tokens interspersed with what appear to be encoded Z80 opcodes. This technique packs machine code bytes into the program file in a form that the BASIC LOAD command can handle transparently, since REM statement bodies are stored verbatim in the program area.
Display and Graphics
The program makes extensive use of block graphic characters (▘, ▝, ▀, ▖, ▌, ▞, ▛, ▄, ▟, █, etc.) embedded within the REM data, suggesting these are used to render the game’s visual display — aircraft, terrain, explosions, and targeting elements. The title screen text visible in the REM data includes the string PRESS RUN TO PLAY and BEST SCORE OF THE DAY, indicating a high-score display on startup.
Printer Usage
Multiple LPRINT and COPY tokens appear throughout the REM data, indicating the game can output scores or screen contents to a printer. The COPY statement on the ZX81 dumps the screen to the ZX Printer, so these calls are likely used for printing the score table or game results.
Key BASIC Idioms Visible in Data
Although the REM lines are primarily machine code, the tokenized BASIC fragments within them reveal several idioms used by the game’s subroutines:
RAND USR— machine code call conventionGOSUB/RETURN— subroutine structure for modular game routinesFAST/SLOW— mode switching for rendering speedUNPLOT/PLOT— low-resolution graphics drawingPAUSE— timing delays within game sequencesINKEY$— keyboard polling for real-time inputLPRINT+COPY— printer output of score dataDIM,FOR/NEXT,IF— standard array and loop controlCLS,SCROLL— screen managementSAVE— in-game save of state or high score
Notable Techniques
The use of VAL "number" in GO TO and GO SUB targets within the data is a memory optimization that stores the destination line number as a string rather than a two-byte integer. The heavy use of ACS, LN, TAN, ATN, SIN, INT, SGN, ABS, and RND tokens throughout the REM data is not mathematical computation — these are single-byte tokens being used as compact data bytes within the machine code payload, a classic space-saving trick.
Mission Sequences
The string -MISSION----1 is visible in the REM data at line 10, suggesting the game has numbered mission stages. Variable names visible in the data (e.g., M, U, Y, Q, R, P, O) suggest a compact variable naming scheme typical of machine-code-integrated BASIC games where variable slots are used as shared memory between the BASIC and machine code layers.
Anomalies and Observations
- The trailing
00000000000sequences at the ends of lines 5, 10, and 20 are padding bytes within the REM bodies, likely null bytes (character code 0) used as terminators or alignment fillers in the machine code data. - The copyright notice
(C)DIGITAL INTEGRATION 1982is embedded at the start of line 5’s REM body, confirming the commercial origin of the title. - Line 25 saves only the BASIC program; the machine code embedded in the REMs is saved as part of the program file since REM data is included in the
SAVEoperation.
Content
Image Gallery
Source Code
5 REM (C)DIGITAL INTEGRATION 1982##Y█LN 7#LN GOSUB PILN CHR$ RNDE£RND▘▞ \,, FOR ▘= 5[M]INKEY$ GOSUB [K]E£RND▘[T]▝\,, FOR ▘+ 5LEN INKEY$ GOSUB [K]E▐#6#RND FOR 7LN [-]#TAN E£RND▘# \,,##▘) 5TINKEY$ GOSUB [K]##▘( \,,##▘) 5#INKEY$ GOSUB [K]##▘( \,,##▘) 5#INKEY$ GOSUB [K]E£RND▘+▝\,,##▘* 5#INKEY$ GOSUB [K]##▘\~~ \,,##▘* 5▄INKEY$ GOSUB [K]##▘\~~ \,,##▘* 5[,]INKEY$ GOSUB [K]TAN ▌▜▌█▌█▌▄▟█▌█▐█▙▖▄▌▙▘█▌█▌▙▐█▌▄▐██▌█▙█▙█▙█▙▄▟█▙█▟██▙█▀▀██▜▛██▜█▜█▛█▛██▀▀██▀▜▐▀██▐▌██▗▜▐█▌▞▌██▝███▝▟▝▘██▞▚██▐▙▐█▌█▖██▝▀██▐▞ [P][R][E][S][S]██[R][U][N]██[T][O]██[P][L][A][Y][B][E][S][T]█[S][C][O][R][E]█[O][F]█[T][H][E]█[D][A][Y] FASTVAL Y[-]#7Y[O]#7Y[-]#▘6 [R] GOSUB PIY[.]#\,,FF▞ COPY:=$4 CLEAR( RAND Y█#7#7#▘6 [R] GOSUB PI#LN GOSUB PIAT LPRINT U9RNDACS #"" LPRINT TAN LN [3]RNDE£RND▘# \,,▞0)5 LN <=INKEY$ 7( IF ▞▞LN <=INKEY$ ;( IF ▞.LN <=INKEY$ F( IF ▞£LN <=INKEY$ ;( IF ▞.LN <=INKEY$ 7( IF ▞$LN <=INKEY$ [R] GOSUB #( SAVE ▞/LN <=INKEY$ F( IF ▞▌LN <=INKEY$ [R] GOSUB #( SAVE ▞"LN <=INKEY$ 7( IF FY[-]#7Y[O]#7Y[-]#▘6 [R] GOSUB PIY[.]#Y:MQRNDY▝MRRNDLN UNPLOT # LN [3]RNDE£RND▘# \,,)5 ▞/LN <=INKEY$ F( IF ▞▒LN <=INKEY$ ;( IF ▞;LN <=INKEY$ 7( IF ▞\~~LN <=INKEY$ ;( IF ▞.LN <=INKEY$ F( IF ▞\~~LN <=INKEY$ [R] GOSUB #( SAVE ▞:LN <=INKEY$ 7( IF Y[O]#LN ABS #LN GOSUB PI #;PI FASTVAL E£RND▘ RUN Y[.]\,,#▘5 \,,#\,,#\,,\,,#\,,#\,,#▘# [R] GOSUB PIY[-]#7#7#7Y[+]#7Y[-]#7#7#AT LPRINT TAN FASTVAL E£RND7▞-:4#7$4 CLS7( PLOT AT LPRINT TAN FASTVAL E▄# GOSUB #▟#▞ \,, GOSUB #▙#▞ [R] GOSUB PI▘# \,,6▄#LN ##Y# AT LPRINT TAN FASTE▄#6#RNDE£RND7LN [-]# LPRINT TAN FASTSTR$ E▐#6#RNDE£RND): ;LN [-]#SGN LPRINT TAN FASTSTR$ E£RND)0 ;U[R]#LEN [0]#SGN LPRINT TAN STR$ VAL FAST[J]M[O]#M[P]#M[Q]#5## GOSUB ##RND▞(ACS #CA# FAST5[Q]#▚B LPRINT M[Q]#7# FAST5[P]#[:]B LPRINT M[P]#7# FAST5[O]#[:]B LPRINT M[O]#7/▀777ACS 7ACS >(LN U[O]# NEW?4OY█ LPRINT #7U[P]#ACS ZACS ZACS ZACS Z4MY█#7U[P]# NEW?4KY█#7U[Q]#ACS ZACS ZACS ZACS Z4IY█#7/GLEN [0] LPRINT #7U[P]#ACS ZACS ZACS ZACS ZLEN [0]#7U[P]# NEW?LEN [0]#7U[Q]#ACS ZACS ZACS ZACS ZLEN [0]#7U[Q]# NEW?LEN [0]#AT SGN TAN #B▀▜#▘[>]▟ [-]RND #4 8( >▌ #▝ C▘ # M - ▒ ▖ ▝ ▘ ## 3 "\,,::\~~£." COPY3> COPY3> COPY$+ COPY3+)4+████[3]▘ #(9 ▀▒▞▀ :▖▛ ▀ UNPLOT #,[S]###TAN 00000000000
10 REM ## FASTLN 9#LN PLOT #LN TO # LPRINT TAN FASTURRNDLN ▖#UQRNDLN 2#Y[-]#7Y[O]#7Y[-]#VAL ▘6 [R] GOSUB PIAT Y[.]# LPRINT TAN VAL STR$ E£RND7#- ▞▌ACS 7ACS >( IF #▞ \,, GOSUB #SGN AT TAN STR$ #- ;SGN TAN FAST# RETURN 4\~~5RRNDOLN INT ##[K]# RETURN▘4\~~5RRNDPLN SIN ##[K]# RETURN▝4\~~5QRNDOLN [Z]##[K]# RETURN▀4\~~5QRNDPLN [Q]##[K]# RETURN▖4)5QRNDPLN [Q]#5RRNDPLN SIN ##[K]# RETURN▌4)5QRNDOLN [Z]#5RRNDOLN INT ##[K]# RETURN▞4)5QRNDOLN [Z]#5RRNDPLN SIN ##[K]# RETURN▛4:5QRNDPLN [Q]#5RRNDOLN INT # LPRINT ▌ATN NOT #TAN ACS #COS Q LN NOT #TAN #CHR$ 2 SAVE Q1/ POKE #CHR$ ▝ LIST Q▝/ GOTO #CHR$ + SAVE Q=/ SLOWUORNDACS Z FAST5[E]#[I]▞▌ACS ?( UNPLOT # NEW?LEN ▀# GOSUB #ACS ? NEW▛# LPRINT TAN FASTVAL GOSUB #9RND5 COPY COPY[R] GOSUB PIASN L#5 LOAD TO [R] GOSUB PI4?5RRNDAT PPLN O#LN SIN ##M#5 LOAD LOAD [R] GOSUB PI4?5RRNDAT OOLN RND#LN INT ##M#5 LOAD RUN [R] GOSUB PI4?5QRNDAT PPLN ##LN [Q]##M#5 RUN TO [R] GOSUB PI4?5QRNDAT OOLN ##LN [Z]##M#5 SCROLLINT [R] GOSUB PI4,5QRNDAT OOLN ##LN [Z]#5RRNDOOLN RND#LN INT ##M#5 LOAD NOT [R] GOSUB PI4,5QRNDAT PPLN ##LN [Q]#5RRNDPPLN O#LN SIN ##M#5 LOAD SCROLL[R] GOSUB PI4,5QRNDAT PPLN ##LN [Q]#5RRNDOOLN RND#LN INT ##M#5 SCROLL TO [B] GOSUB PI4,5QRNDAT OOLN ##LN [Z]#5RRNDPPLN O#LN SIN ##M#5 FOR <>[B] GOSUB PIASN $#5 FOR GOSUB [B] GOSUB PIASN 8#5 FOR PRINT [B] GOSUB PIASN V#5 NEXT <>[B] GOSUB PIASN ##5 STOPLN [B] GOSUB PIASN ##5 FOR STR$ [B] GOSUB PIASN ["]#5 FOR FAST[B] GOSUB PIASN [H]#5 STOP<>[B] GOSUB PIASN INT #AT LPRINT TAN # RETURN▛COS RETURN COS RETURN▌COS OTAN # RETURN▖COS RETURN▘COS RETURN▞COS PTAN # RETURN▞COS RETURN▝COS RETURN▌COS OTAN # RETURN▖COS RETURN▀COS RETURN▛COS PTAN VAL ▞COS :$$4 CLEAR( RAND AT TAN VAL ▞ COPY:M$4 CLEAR( RAND AT TAN VAL ▞▞[J]LN 7#Y█LN [.]#LN ##Y█LN 7#[J]LN [.]#LN ##( NEWAT TAN FASTVAL STR$ E£RND▘[C] \,,LN £#E£RND▘[H] \,,LN £#E£RND▘[M] \,,LN £#E£RND▘[U] \,,LN £#E£RND▘TAB \,,▞£LN :#E£RND▘[R] \,,▞▒#7( UNPLOT ▘4 \,,▞▒#F( UNPLOT E£RND▘#▘\,,#7#7#▘5 \,,#F#F#E£RND▘#▝\,,#7#SGN AT LPRINT TAN ▞:)4 #7#;( IF TAN 5[<]#▞▖# RETURN█4▀Q[-]7( PLOT Y>E£RND)#▘; FOR 5▗#▘▘ STR$ FASTVAL GOSUB [K]AT LPRINT SGN ▀ RETURN>ATN [T]# RETURN>CODE [;]#.X4 DIM Y:▘> STR$ FASTVAL GOSUB [K]AT LPRINT SGN RETURN▒ATN ##LN [;]#.X4 GOTO Y)▘) <7STR$ FASTVAL GOSUB [K]AT LPRINT SGN ". RETURN)ATN [;]#< RETURN)CODE [T]#X4 SCROLLTAN [(]O▘[<][-][M][I][S][S][I][O][N][-][-][-][-][1]█ FASTVAL ##▘4 [R] GOSUB PI▞▝:[X]#LN ##:[+]#LN ##( PAUSE :█#AT LPRINT TAN VAL ▞▖LN ##( CLSAT TAN VAL ▞▌LN [;]#( CLSAT TAN LN ▄#( PAUSE :█#AT LPRINT TAN VAL ▞▖LN ▄#( CLSAT TAN VAL ▞▌LN [S]#( CLSAT TAN Y▞M>=ZMRRND##Y█XXMT##MYPU#MPTAN
15 REM ## FASTVAL STR$ 5[▒]#ACS #C\,,E[~~]#6QRNDLN ##5[▒]#ACS #C\,,E[£]#6QRNDLN ##5[▒]#ACS #C\,,E[:]#6QRNDLN ##5[▒]#ACS #C\,,E[(]#6QRNDLN ##LN GOSUB PI5[▒]#ACS #CYE[~~]#6QRND GOSUB #[G]##LN PEEK #EQRND6[~~]## GOSUB #[G]# FAST5▗#ACS # LPRINT 40#CHR$ < PAUSE [;]## RETURN▘4)G#M[+]#M[-]##M[*]#5▗#ACS EXP 5[▒]#ACS #CYE[£]#6QRND GOSUB #[I]##LN PEEK #EQRND6[£]## GOSUB #[I]# FAST5▗#ACS # LPRINT 40#CHR$ < PAUSE THEN## RETURN▘4)G#M[/]#M[;]##M[,]#5▗#ACS CHR$ 5[▒]#ACS #CYE[:]#6QRND GOSUB #[K]##LN PEEK #EQRND6[:]## GOSUB #[K]# FAST5▗#ACS # LPRINT 40#CHR$ < PAUSE 7## RETURN▘4)G#M[.]#M[0]##M[1]#5▗#ACS THEN5[▒]#ACS #CYE[(]#6QRND GOSUB #[M]##LN PEEK #EQRND6[(]## GOSUB #[M]# FAST5▗#ACS # LPRINT 40#CHR$ < PAUSE ### RETURN▘4)G#M[2]#M[3]##M[4]#5▗#ACS NEWSGN AT LPRINT TAN URRNDLN ▖#UQRNDLN 2#Y█#7#7#▘6 [R] GOSUB PI#TAN FASTVAL GOSUB #9RND 5 FOR <>LN ATN #5 FOR GOSUB LN ATN #5 FOR PRINT LN ATN #5 NEXT <>LN ATN #5 STOPLN LN ATN #5 FOR STR$ LN ATN #5 FOR FASTLN ATN #5 STOP<>LN ATN #5 CLS CLEARLN ATN #AT LPRINT TAN [R] GOSUB PI""U[C]#[R]COS 5▗#ACS #""CHR$ ▝M[C]#ACS LEN LN NEW#TAN FASTVAL E£RND▘[Q]▝\,,Y[A]#7Y[M]#7#7Y[O]#U[C]#[R]C£ACS JACS JW#Y[▒]7#( UNPLOT U[C]#ACS JACS JW#Y0[(]#Y█7#( UNPLOT AT LPRINT TAN FASTVAL STR$ E[S]# GOSUB #[U]#- GOSUB #[V]#[R] GOSUB PI PAUSE PI#E[S]#LN [F]#/▞E[S]#[R] GOSUB #▘6 [R] GOSUB PI;..6[S]##M[U]# GOSUB #[X]#[R] GOSUB PI PAUSE ##LN [N]#5▗#ACS ▚/VE[S]#▘# [R] GOSUB PI FAST GOSUB #[X]#[R] GOSUB PI LPRINT IF [-]#Y▞# GOSUB #[U]#- ....[R] GOSUB #Y▚#/)E£RND)#▘;# RETURN[+]C▌5▗#ACS INPUT SGN AT LPRINT TAN [R]Y█# GOSUB ##TAN FASTVAL E£RND▘=▀\,,6[S]#▘# [R] GOSUB PI6[V]#Y,M[U]#E£RND▘#▘\,,6[X]#5▗#ACS ▚AT LPRINT TAN FASTVAL E£RND▘T▘\,,#▘4 \,,#77#\,,#AT LPRINT TAN FASTVAL E£RND▘ RUN \,,#▘3 \,,#▘▖ \,,#▘X \,,#▘▞ \,,#▘X \,,#▘▖ \,,#▘3 \,,#AT LPRINT TAN FASTVAL E£RND▘[=] \,,#▘2 \,,#▘▞ \,,#▘U \,,#▘\~~ \,,#▘# \,,#▘£ \,,#▘# \,,#▘\~~ \,,#▘U \,,#▘▞ \,,#▘2 \,,#AT LPRINT TAN \,,#▘▞ \,,#▘2 \,,#AT LPRINT TAN 00
20 REM ##VAL FAST)[>]#▘▀ GOSUB [K]U[>]##U[<]#[G]CEU[=]#CHR$ + PAUSE TO #U[=]#LN ▖# FASTU[>]#ACS #4▞LN 2#Y█# LPRINT U[<]# RETURN4C▞LN 2#Y█#5[>]#PLN 6#5[<]#OLN C#5[=]#O#5▗#ACS [Y]CHR$ + IF PRINT #5▗#ACS RETURNU[C]#[R]C▛U▟#ACS #CAACS PLOT /6U[=]#LN ▖# FASTU[>]#ACS #4▞LN 2#Y▚# LPRINT U[<]# RETURN4C▞LN 2#Y▞#5[>]#SGN ▘▀ GOSUB [K]AT TAN ACS #COS Q COPYTAN #CHR$ 4 SAVE Q4TAN FASTSTR$ U[,,]#[R]TAB ▘#5[▒]#U["]# RETURN\~~C▞ RETURN"40/"U[~~]# RETURN$C" RETURN?C▛U[~~]# RETURN:4▒ACS [:][J]M[~~]#/#U[$]# RETURN\~~C▞ RETURN"40/"U[£]# RETURN$C" RETURN?C▛U[£]# RETURN:4▒ACS [-][J]M[£]#/#U[?]# RETURN\~~C▞ RETURN"40/"U[:]# RETURN$C" RETURN?C▛U[:]# RETURN:4▒ACS [2][J]M[:]#/IU[)]# RETURN\~~C▞ RETURN"40/"U[(]# RETURN$C" RETURN?C▛U[(]# RETURN:4▒ACS [A][J]M[(]#/▛5▗#ACS [I]/#U[,,]# RETURN 40E£RND)S▘;Y█#77#LN GOSUB PIY[*]LN TO #5[,,]#ACS EXP /#ACS #4£Y[*]LN PLOT #5[,,]#ACS CHR$ /RNDACS #4£Y[*]LN A#5[,,]#ACS THEN/KY█LN TO #LN PLOT #LN A#LN GOSUB PI5▗#ACS [I]5[,,]#Q 5[▒]#ACS #ATN S#Y#M▟#U[▒]# RETURN▘4▖[J]M[▒]#SGN LPRINT TAN FASTVAL STR$ U[D]##5▗#ACS #CODE 8#5▗#ACS #C▀LN J#ACS #C)5[+]#LN ▜#5▗#ACS #C▖ACS [Y]ACS [:]ACS #C)5[/]#LN ▜#5▗#ACS #C▖ACS [Y]ACS [-]ACS #C)5[.]#LN ▜#5▗#ACS #C▖ACS [Y]ACS [2]ACS #C)5[2]#LN ▜#5▗#ACS #C▖ACS [Y]ACS [A]VAL ▞ COPYY▞X4 CLEAR( RAND AT ([$]SGN AT LPRINT TAN FASTVAL LN [N]#▞$LN 8#LN ##( SAVE ▞▖Y?M[~~]#Y\~~M["]#[J]M[,,]#LN J#LN ##( SAVE AT LPRINT TAN FASTVAL Y?M[+]#M[-]#Y▝M[*]#▞(5[+]#LN ▜#LN ##( PRINT LN ##AT LPRINT TAN LN NOT ## GOSUB #[G]#LN ## GOSUB #[~~]#LN NOT ## GOSUB #[I]#LN ###LEN ▀# GOSUB #[£]#LN NOT ## GOSUB #[K]#LN ###LEN ▝# GOSUB #[:]#LN NOT ## GOSUB #[M]#LN ###CHR$ ▝# GOSUB #[:]#TAN #▙##ACS BLEN ▀#TAN 5 6▐#5 6▄#6[A]#5[▒]#ACS RETURNY#M▙#Y)M[D]#Y▀M[R]#LN [N]#LN ;PIY3M[▒]#U▙# RETURN+C$CHR$ ▛M▙#U[D]#CHR$ ▘M[D]#Y#M▟#E[A]#76[A]#6#RND5[<]#LN [-]#LN ##Y#M[C]#[J]M▗#M[,,]#Y█LN 7#LN GOSUB PILN ##LN ##LN ▜#LN NEW#LN 2#LN ▗#U▗# RETURN C▌LN ##/\~~▞ COPYU▙#X4 CLEAR( SAVE U[▒]# RETURN C[£]5▗#ACS #435▟#PLN :# GOSUB #9RND5[U]<=[R] GOSUB PI4▘TAN 5 RETURN NEXT [R] GOSUB PI4[X]###LN ##U[R]#XC"M[R]#Y#M▟##ACS #E▐# GOSUB #▄##[G] IF ##[R] GOSUB # PAUSE ##E▄#6▐####Y█LN 7#LN ##LN ##LN ▜#LN /#TAN ▒ ▖ ▝ ▘ ▞ 42 ;=▀=▌▝\~~" ;;£??)▖▖▒ ▒▀▝▝▒▀▒ [9]#,##"#TAN 00000000000
25 SAVE "N[G]"
30 RAND USR 20337
Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.