The following demonstration programs can all be compiled into machine-code by the Compiler program to be described, and run on the Atom or BBC Computer. They illustrate some of the features of SPL.
The first program performs a bubble sort of the characters in the top half of the screen memory. The sort works by successively comparing pairs of adjacent locations; if two are in the wrong order they are exchanged, and then the smaller one is moved back to its correct position in the locations that have already been sorted.
5 {BUBBLE SORT OF SCREEN}
10 PROC BUBBLE();
15 BEGIN I=0;
20 LOOP:J=I;
25 LOOPA:IF SCREEN[J]>SCREEN[J+1] THEN
28 BEGIN
30 TEMP=SCREEN[J];
31 SCREEN[J]=SCREEN[J+1];
32 SCREEN[J+1)-TEMP;
35 IF J=0 THEN GOTO OK;
40 J=J-1;GOTO LOOPA
42 END;
45 OK:I=I+1+IF I<255 THEN GOTO LOOP
60 END
When the compiled program is executed the characters on the top part of the screen will be sorted into order.
The following SPL program moves a snake across the top half of the screen, and demonstrates the use of the language’s shift operators '>>’ and '<<'. The program is only suitable for the Atom, and uses a routine WAIT to make sure that the screen accesses are noise-free. After being compiled it should be executed by linking to the address corresponding to the label ENTER.
5 {CRAWLING SNAKE}
10 PROC SNAKE();
15 BEGIN
20 PROC CLEAR(K);
25 {CLEAR SCREEN TO CHARACTER K}
28 BEGIN
30 X=0;CLR:SCREEN[X]=K;
35 X=X+1;IF X<>0 THEN GOTO CLR
40 END;
42 PROC WAIT();
43 {WAIT FOR FLYBACK SYNC}
44 BEGIN
45 WA:IF PORT[2]>127 THEN GOTO WA
46 END;
47 ENTER:L=0;CLEAR(64);
48 SCREEN[0]=127;SCREEN[1]=127;
49 SCREEN[2]=127;SCREEN[3]=127;
50 RUN:X=0;
60 LOOP:WAIT();
70 C=(SCREEN[X]&63)<<2;
80 SCREEN[X]=C|192+L;L=C>>6;
90 X=X+1;
95 IF X<>0 THEN GOTO LOOP ELSE GOTO RUN
100 END
The following procedure will print a number as two hexadecimal numbers on the screen. On the Atom this routine is pre-defined.
40 proc wrhex(n); 50 begin 60 if n>=160 then wrch(n>>4+55) 70 else wrch(n>>4+48); 80 n=n&15; 90 if n>=10 then n=n+7; 100 wrch(n+48) 110 end;
When compiled and executed the procedure will print the contents of the accumulator in hexadecimal.
The following SPL program finds all the prime numbers up to 127, and prints them in hexadecimal:
10 PROC MAIN();
20 BEGIN
30 PROC PRIME(N);
35 {PRINT N IF PRIME}
40 BEGIN D=l;
60 TRY:D=D+1;E=N;
65 IF D<N-1 THEN
68 BEGIN
70 TEST:E=E-D;
75 IF E<>0 THEN
80 IF E<128 THEN GOTO TEST
85 ELSE GOTO TRY
88 END
90 ELSE WRHEX(N)
110 END;
115 {MAIN PROGRAM}
120 ENTER:T=1;
125 ALL:PRIME(T);WRCH(32);
130 IF T<128 THEN
140 BEGIN
150 T=T+1;GOTO ALL
170 END
190 END
On the BBC Computer the definition of the 'wrhex' routine
should be inserted at the start of the program between lines 20
and 30. The compiled program, when executed, prints the following
sequence:
01 02 03 05 07 0B 0D 11 13 17 1D 1F 25 29 2B 2F 35 3B 3D 43 47 49 4F 53 59 61 65 67 6B 6D 71 7F
It prints a space for every number tested, so the primes can be seen to become more sparse as they get larger.
The following function finds the greatest common divisor (GCD) of the numbers A and B using Euclid's algorithm:
5 PROC TEST(); BEGIN
10 PROC GCD(); {GCD OF A,B IN B}
20 BEGIN LOOP!IF A<>B THEN
25 BEGIN
30 IF A<B THEN B=B-A
35 ELSE A=A-B END;
40 RETURN B END;
45 ENTER:A=9; B=12; WRHEX(GCD())
50 END
The routine, once compiled, should be entered at ENTER, when the GCD of 9 and 12, i.e. 3, will be displayed. Again, for the BBC Computer version the 'wrhex' routine should be included since it is not pre-defined.
The following test program demonstrates an 8-bit multiply routine written in SPL:
5 PROC TEST();
8 BEGIN
10 PROC MULT();
15 {A*B - RESULT IN C}
20 BEGIN C=O;
25 MULL:IF B>0 THEN
28 BEGIN
30 IF B&1=1 THEN C=C+A;
40 A=A<<1;B=B>>1;GOTO MULL
45 END
50 END;
8 ENTER:A=6;B=19;MULT();WRHEX(C)
60 END
The machine code is executed from the address corresponding to the label ENTER, and should print out '72'; in other words, 6*19=114, or 72 in hexadecimal.
In the following SPL Mastermind program the computer generates a random 4-digit code, which the player must guess. The guess is entered as four decimal digits, and the computer displays the result as two digits: the first digit gives the number of digits correctly guessed in the correct position; the second digit gives the number of correct digits incorrectly placed. When the computer's code is correctly guessed, with a score of '40', the program gives a 'bleep'.
The following sample run shows each of the player's 4-digit guesses followed by the computer's 2-digit reply:
1122 00 3344 10 5566 00 7788 20 9900 10 3780 02 7948 40
The version of the program shown below is for the BBC Computer; on the Atom all the symbols and variables should be in upper case, and lines 40 to 110 can be omitted.
10 {mastermind} 20 proc mastr(); 30 begin array my[3],your[3],temp[3]; 40 proc wrhex(n); 50 begin 60 if n>=160 then wrch(n>>4+55) 70 else wrch(n>>4+48); 80 n=n&15; 90 if n>=10 then n=n+7; 100 wrch(n+48) 110 end;120 proc rnd(); 130 begin rndy:rndxmndx<<2+rndx+7; 340 if rndx&15>9 then goto rndy; 150 return rndx&15 160 end;170 proc input(); 180 begin n=0; 190 readc:j=rdch();wrch(j);j=j-48; 200 if j>9 than goto readc; 210 your[n]=j;n=n+1; 220 if n<4 then goto readc 230 end;240 {main program} 250 enter:n=0; 260 myno:my[n]=rnd();n=n+1; 270 if n<4 then goto myno; 280 try:input();n=0; 290 copy:temp[n]=my[n];n=n+1; 300 if n<4 then goto copy; 310 n=0;score=0; 320 bull:if temp[n]=your[n] then 330 begin temp[n]=10;your[n]=11; 340 score=score+16 350 end; 360 n=n+1;if n<4 than goto bull; 370 n=0; 380 cow:m=0; 390 cowx:if temp[n]=your[m] then 400 begin temp[n]=10;your[m]=11; 410 score=score+1 420 end; 430 mm+1;if m<4 then goto cowx; 440 n=n+1;if n<4 then goto cow; 450 wrch(32);wrhex(score);wrch(10);wrch(13); 460 if score<>64 then goto try; 470 wrch(7) 480 end