PLOT ROUTINE

by Lawrence Simons

One major drawback in Mallard BASIC is the absence of a PLOT command - a command that allows you to switch a pixel on or off at a specific place on the screen. But now there is thanks to the sterling work of Mr Simons of London. What he has produced is a bit of machine code that works the same as the PLOT command in a Sinclair Spectrum (if anyone admits to remembering such things).

The idea is that once you have typed in the machine code routine (the really complicated looking bit from line 50000) you can run this using a GOSUB routine (see line 10). You have three variables to set. For the x axis (how far across the screen) you use x%, and for the y axis you set y%. As the screen is 720 pixels wide by 256 pixels high. x% has to be between 0 and 719 and y% has to be between 0 and 255. The program will ignore anything beyond these limits.

The final variable is f% which decides whether to switch the pixel on or off. If f%=1 you switch on; 0 switches off. Any other number and it will reverse the situation - if it finds a pixel on it switches it off and vice versa.

GOSUB to the machine code once and then every time thereafter you put CALL PLOT (x%,y%,f%) it will switch the pixel at x% across and y% down on or off.

Now, switching individual pixels on and off by itself may not seem too impressive - but if you use it with a formula (as in the examples) you can produce impressive graphics in boring old Mallard. Perhaps not the fastest way of producing circles or ellipses but it saves you having to take a crash course in GSX and it is more fun than learning LOGO.

No doubt we will see some ingenious uses appearing in listings over the next few months.

10 GOSUB 50000: x0=360:y0=12: f%=1                                                     0961
20 PRINT CHR$(27)+"0";CHRS(27)+"E";CHR$(27)+"f";                                       0BD5
30 FOR a=0 TO 6.2832 STEP 0.005                                                        0974
40 x%=CINT(x0+350*COS(a)):y%=CINT(y0+120*SIN(a))                                       0FD5
50 CALL PLOT(x%,y%,f%):NEXT                                                            08D9
60 y%=y0:FOR x%=x0-350 TO x0+350:CALL plot(x%,y%,f%):NEXT                              1169
70 x%=x0:FOR y%=y0-120 TO y0+120:CALL plot(x%,y%,f%):NEXT                              1136
80 PRINT CHR$(27)+"e";CHR$(27)+"1";CHR$(27)+"Y"+CHR$(59)+CHR$(32):END                  11B4
50000 MEMORY &HBFFF:plot=&HC000:A=PLOT:E=0                                             0F5F
50010 RESTORE 50120                                                                    05C2
50020 FOR f=0 TO 14                                                                    0537
50030 s=0                                                                              024B
50040 FOR G=0 TO 7                                                                     052C
50050 READ X$:X=VAL("&H"+X$)                                                           0789
50060 S=S+X:POKE A,X:A=A+1:NEXT                                                        0B3C
50070 READ X$:IF S<>VAL("&H"+X$) THEN E=E+1:PRINT "ERROR IN LINE ";10*(F+5015)         19F7
50080 NEXT                                                                             02D2
50090 IF E=0 THEN RETURN                                                               0922
50100 PRINT E; "ERROR";:IF E=1 THEN PRINT ELSE PRINT "S"                               12FD
50110 STOP                                                                             02C6

⇓

50120 DATA 0A,32,7C,C0,4E,23,46,21,250                                                 0887
50130 DATA CF,02,A7,ED,42,D8,ED,43,4AF                                                 09B1
50140 DATA 78,C0,EB,5E,23,56,7A,A7,41B                                                 0994
50150 DATA C0,93,3D,5F,ED,53,7A,C0,469                                                 094F
50160 DATA F3,ED,73,7D,C0,31,9D,C0,51E                                                 095C
50170 DATA 01,36,C0,CD,5A,FC,E9,00,403                                                 0953
50180 DATA ED,7B,7D,C0,FB,C9,2A,7A,50D                                                 09F3
50190 DATA C0,29,11,00,B6,19,5E,23,24A                                                 0823
50200 DATA 56,7B,E6,F8,CB,27,CB,12,47E                                                 08FC
50210 DATA 47,7B,E6,07,B0,5F,2A,78,360                                                 08CD
50220 DATA C0,45,7D,E6,F8,6F,EB,19,4D3                                                 0955
50230 DATA 78,E6,07,3C,47,AF,37,1F,2ED                                                 09C0
50240 DATA 10,FD,47,3A,7C,C0,A7,20,391                                                 08E7
50250 DATA 05,78,2F,A6,77,C9,3D,78,347                                                 0926
50260 DATA 20,03,B6,77,C9,AE,77,C9,407                                                 091B

[BASIC listing with corrected line 50070] and [assembler listing]