Over in the Facebook CoCo group, Carlos Comacho shared a link to a cool online emulator for the BBC Micro. The link contains a BASIC program listing that drew a very cool galaxy graphic.

Here is the page:

https://bbcmic.ro/?t=9db8C

Here is the BBC Micro program:

10 MODE1
20 VDU5
30 FORG=RND(-64)TO400
40 t=RND(99):q=RND(99)
50 u=RND(1280):v=RND(1024)
60 A=RND(1)*3
70 R=90/(1+RND(200))
80 Q=1+R*(.5+RND(1)/2)
90 a=1+3*RND(1)^2
100 M=1
110 IFRND(9)<4Q=R:t=0:q=0:A=0:M=PI/3:a=1
120 C=(1+3*RND(1)^2)*R*R
130 FORi=0TOC
140 S=-LNRND(1)
150 T=i*M
160 U=S*R*SINT
170 V=S*Q*COST
180 T=S*A
190 X=U*COST+V*SINT
200 Y=V*COST-U*SINT
210 D=(X*X+Y*Y)/(R*R+Q*Q)
220 Z=99*((2.7^-D)+.1)
230 Z=Z*(RND(1)-.5)^3
240 y=Y*COSt+Z*SINt
250 Z=Z*COSt-Y*SINt
260 x=u+X*COSq+y*SINq
270 y=v-X*SINq+y*COSq
280 P=POINT(x,y)+a
290 IFP>3P=3
300 GCOL0,P
310 PLOT69,x,y
320 NEXT,
330 REM

I decided to try converting it to the CoCo. This led me to looking up some BBC Micro Commands to understand what they did. I found a few significant differences:

• All variables need to be uppercase to work on the CoCo.
• PI is not a constant on the CoCo, so it needs to be defined.
• Line 10 – MODE1 sets a 320×256 4 color screen. On the CoCo 1/2, the 4-color screen has a resolution of 128×191.
• Line 20 – VDU5 – Not used on the CoCo. It outputs a screen character of 5 for some reason.
• Line 30 – RND(-64) would seed the random number generator, but on the BBC Micro it returns the -64. On the CoCo, RND(-64) looks like it does the same as RND(0) on the CoCo, returning a value from 0.0 to .9999. The FOR/NEXT will just have to do -64 for the start number.
• Line 80, 90, 120, 140 and 230 – Their RND(1) returns a number from 0.0 to .99999. This will be changed to RND(0) on the CoCo.
• Line 110 and 290 – The CoCo needs a THEN between IF X>4 THEN DO SOMETHING.
• Line 160,170, 190, 200, 240, 250, 260 and 270 – SIN and COS need parens on the CoCo.
• Line 280 – POINT looks like PPOINT on the CoCo.
• Line 290 – If the program was written for a 4-color “MODE1” display, I assume their colors are 0-3. On the CoCo, they are 1-4, so that is a minor change.
• Line 300 – GCOL0,P sets a color, so probably COLOR P.
• Line 310 – PLOT69,X,Y becomes PSET(X,Y)
• Line 320 – Their BASIC has an interesting shortcut for NEXT. After a FOR A you need a NEXT A, but you can also just say “NEXT” and it uses the most recent FOR. If you have nested loops, like FOR A :FOR B … you can end with “NEXT:NEXT” or “NEXT B,A”. It looks like their BASIC allows leaving out the variables, but using the comma. I tried this on CoCo, and it didn’t work. That would have been a fun discovery. That just means “NEXT,” becomes “NEXT:NEXT”.

When I did a quick port, I saw it passing negative values to PPOINT and PSET, so I had to add an extra check that skips plotting anything that would be off the screen (X <0 or >255, and Y<0 or >191).

My attempt at a CoCo 1/2 version looks like this:

0 'BBC MICRO - 320X256	4 COLOUR DISPLAY
1 POKE 65395,0
5 PI=3.141592653589793238
10 'MODE1
11 PMODE 4,1:PCLS:SCREEN 1,1:PMODE 3,1
20 'VDU5
30 'FORG=RND(-64)TO400
31 FORG=-64TO400
40 T=RND(99):Q=RND(99)
50 U=RND(1280):V=RND(1024)
60 A=RND(1)*3
70 R=90/(1+RND(200))
80 'Q=1+R*(.5+RND(1)/2)
81 Q=1+R*(.5+RND(0)/2)
90 'A=1+3*RND(1)^2
91 A=1+3*RND(0)^2
100 M=1
110 'IFRND(9)<4Q=R:T=0:Q=0:A=0:M=PI/3:A=1
111 IFRND(9)<4THENQ=R:T=0:Q=0:A=0:M=PI/3:A=1
120 'C=(1+3*RND(1)^2)*R*R
121 C=(1+3*RND(0)^2)*R*R
130 FORI=0TOC
140 'S=-LNRND(1)
141 S=-(LOG(RND(0)) / 0.4342944819)
150 T=I*M
160 'U=S*R*SINT
161 U=S*R*SIN(T)
170 'V=S*Q*COST
171 V=S*Q*COS(T)
180 T=S*A
190 'X=U*COST+V*SINT
191 X=U*COS(T)+V*SIN(T)
200 'Y=V*COST-U*SINT
201 Y=V*COS(T)-U*SIN(T)
210 D=(X*X+Y*Y)/(R*R+Q*Q)
220 Z=99*((2.7^-D)+.1)
230 'Z=Z*(RND(1)-.5)^3
231 Z=Z*(RND(0)-.5)^3
240 'Y=Y*COST+Z*SINT
241 Y=Y*COS(T)+Z*SIN(T)
250 'Z=Z*COST-Y*SINT
251 Z=Z*COS(T)-Y*SIN(T)
260 'X=U+X*COSQ+Y*SINQ
261 X=U+X*COS(Q)+Y*SIN(Q)
270 'Y=V-X*SINQ+Y*COSQ
271 Y=V-X*SIN(Q)+Y*COS(Q)
275 IF X<0 OR X>255 THEN 321
276 IF Y<0 OR Y>191 THEN 321
280 'P=POINT(X,Y)+A
281 P=PPOINT(X,Y)+A
290 'IFP>3P=3
291 IFP>3THENP=3
300 'GCOL0,P
301 COLORP
310 'PLOT69,X,Y
311 PSET(X,Y)
320 'NEXT,
321 NEXT:NEXT
330 REM
340 GOTO 340

Unfortunately, these changes were not enough.

Can you help?