50 SYS 700
100 *=$0A18  ; $1E52 FOR 128
110 ; SET UP INIT FLIGHT DATA
120  LDX #0
130 ILP LDA IVAL,X
140  STA ALT,X
150  INX
160  CPX #ISZ
170  BNE ILP
180 ; BIG LOOP - RECALCULATE, DISPLAY
190 BLP LDA #$01
200  STA TOG
210 ; WAIT UNTIL RASTER FLYBACK
220 RPL LDA $D012
230  CMP RST
240  STA RST
250  BCS RPL
260  LDA $D011
270  BMI RPL
280 ; IF NO FUEL, THRUST = 0
290 ZLP SED
300  LDA #$30
310  LDX FUE
320  BMI NOF1
330 ; READ KEYBOARD, CHECK 1 TO 9
340  JSR $FFE4
350  CMP #$31
360  BCC NOKY
370  CMP #$3A
380  BCS NOKY
390 ; SET NEW THRUST VALUES
400 NOF1 STA TH2
410  STA $04A8
420  AND #$0F
430  STA TH1
440  LDY #0
450  SEC
460  SBC #5
470  BPL THX
480  LDY #$99
490 THX STA ACC+1
500  STY ACC
510 ; CALCULATE ACCEL, VELOC, ALT
520 NOKY LDX #5
530 FLY CLC
540  LDA ALT,X
550  ADC ALT+2,X
560  STA ALT,X
570  DEX
580  LDA ALT,X
590  ADC ALT+2,X
600  STA ALT,X
610  LDA #0
620  LDY ALT+2,X
630  BPL POZZ
640  LDA #$99
650 POZZ DEX
660  ADC ALT,X
670  STA ALT,X
680  DEX
690  BPL FLY
700 ; FUEL = FUEL - THRUST
710  SEC
720  LDA FUE+2
730  SBC TH1
740  STA FUE+2
750  LDA FUE+1
760  SBC #0
770  STA FUE+1
780  LDA FUE
790  SBC #0
800  BPL GFU
810 ; SET SPECIAL VALUE FOR NO FUEL
820  LDA #$00
830  STA FUE+2
840  STA FUE+1
850  LDA #$F0
860 GFU STA FUE
870  DEC TOG
880  BPL NOKY
890 ; EXIT IF LANDED/TOO HIGH
900  LDA ALT
910  BMI EXIT
920  LDY ALT+1
930 ; SETUP ALTITUDE DISPLAY
940  JSR DIG4
950  LDX #3
960 DGL1 LDA DGS,X
970  STA $430,X
980  DEX
990  BPL DGL1
1000 ; SPECIAL CODE FOR "+/-" VELOCITY
1010  LDY VEL+1
1020  LDA #$2B
1030  LDX VEL
1040  BPL PVL
1050  SEC
1060  LDA #0
1070  SBC VEL+1
1080  TAY
1090  LDA #0
1100  SBC VEL
1110  TAX
1120  LDA #$2D
1130 PVL STA $0458
1140  TXA
1150 ; SETUP VELOCITY DISPLAY
1160  JSR DIG4
1170  LDX #2
1180 DGL2 LDA DGS+1,X
1190  STA $459,X
1200  DEX
1210  BPL DGL2
1220  LDA FUE
1230  LDY FUE+1
1240 ; SETUP FUEL DISPLAY
1250  JSR DIG4
1260  LDX #2
1270 DGL3 LDA DGS+1,X
1280  STA $481,X
1290  DEX
1300  BPL DGL3
1310 ; LOOP UNLESS STOP KEY PRESSED
1320  CLD
1330  JSR $FFE1
1340  BEQ EXIT
1350  JMP BLP
1360 ; EXIT BACK TO BASIC
1370 EXIT CLD
1380  RTS
2000 ; THIS SUBROUTINE SPLITS Y/A
2010 ;  INTO 4 DECIMAL DIGITS.
2020 ;  NOTE THE RE-ENTRANT CODE.
2030 DIG4 LDX #3
2040  PHA
2050  TYA
2060  JSR DIG2
2070  PLA
2080 DIG2 PHA
2090  AND #$0F
2100  JSR DIG1
2110  PLA
2120  LSR A
2130  LSR A
2140  LSR A
2150  LSR A
2160 DIG1 ORA #$30
2170  STA DGS,X
2180  DEX
2190  RTS
3000 ; HERE ARE THE INITIAL FLIGHT
3010 ; PARAMETERS:
3020 ; ALTITUDE & VELOCY (3 BYTES EACH)
3030 IVAL .BYTE $45,0,0, $99,$98,0
3040 ; ACCEL & FUEL (3 BYTES EACH)
3050  .BYTE $99,$98,0, 8,0,0
3060 ; THRUST AS BINARY AND ASCII
3070  .BYTE 3,$33
4000 ; WORKING VARIABLES, NOT LOADED
4010 ;    WITH THE PROGRAM.
4020 ALT *=*+3   ; ALTITUDE
4030 VEL *=*+3   ; VELOCITY
4040 ACC *=*+3   ; ACCELERATION
4050 FUE *=*+3   ; FUEL
4060 TH1 *=*+1   ; THRUST BINARY
4070 TH2 *=*+1   ; THRUST ASCII
4080 ISZ =*-ALT  ; SIZE OF INIT TABLE
4090 RST *=*+1   ; RASTER TRACKER
4100 TOG *=*+1   ; TWO-TIMING COUNT
4110 DGS *=*+4   ; DECIMAL DIGITS