Gyrating cubes in BBC BASIC
Thursday, 12th June 2008
Work has been keeping me busy recently, but I've tried to set aside a small amount of time each evening to reclaim some sanity and do a little work on BBC BASIC. Not much progress has been made, but there has been some at least.
On the left is the program running on an 83+ SE at 15MHz, on the right on the regular 83+ at 6MHz. If you really wanted to do 3D in BBC BASIC you could probably get away with writing some of the more expensive operations — such as transforming/projecting vertices in batches — in assembly, but that would sort of go against the whole point of trying to write a program to test the speed of BASIC. 
Here's the rather naïve code:
10 *REFRESH OFF 20 DIM p%(15) 30 fps%=0 40 lfps%=0 50 fpst%=TIME+100 60 REPEAT 70 rX=TIME/300 80 rY=TIME/400 90 SrX=SIN(rX) 100 CrX=COS(rX) 110 SrY=SIN(rY) 120 CrY=COS(rY) 130 pt%=0 140 FOR x=-1TO1STEP2 150 FOR y=-1TO1STEP2 160 FOR z=-1TO1STEP2 170 tX=y*CrX-x*SrX 180 tY=-x*CrX*SrY-y*SrX*SrY-z*CrY 190 tZ=3-x*CrX*CrY-y*SrX*CrY+z*SrY 200 p%(pt%)=tX*40/tZ+48 210 pt%=pt%+1 220 p%(pt%)=tY*40/tZ+32 230 pt%=pt%+1 240 NEXT 250 NEXT 260 NEXT 270 CLG 280 PRINTTAB(10,0)lfps%" FPS" 290 MOVE p%(0),p%(1) 300 DRAW p%(4),p%(5) 310 DRAW p%(12),p%(13) 320 DRAW p%(8),p%(9) 330 DRAW p%(0),p%(1) 340 DRAW p%(2),p%(3) 350 DRAW p%(6),p%(7) 360 DRAW p%(14),p%(15) 370 DRAW p%(10),p%(11) 380 DRAW p%(2),p%(3) 390 MOVE p%(4),p%(5) 400 DRAW p%(6),p%(7) 410 MOVE p%(12),p%(13) 420 DRAW p%(14),p%(15) 430 MOVE p%(8),p%(9) 440 DRAW p%(10),p%(11) 450 *REFRESH 460 fps%=fps%+1 470 IF TIME>fpst% THEN lfps%=fps%:fps%=0:fpst%=TIME+100 480 UNTIL INKEY(0)<>-1 490 *REFRESH ON 500 END
I have also added support for the COLOUR statement (for changing the text foreground and background colour) and copy key editing.
Copy key editing, as demonstrated in the screenshot on the right, lets you break the text input cursor into two parts - a write cursor (which is left behind on the line you were editing) and a read cursor, which can be positioned anywhere on the screen. Pressing the copy key (in this case, XTθn) reads a character under the read cursor and writes it to the write cursor, then increments both.
One feature that's a bit more fun is the support of device files. This is a way of accessing external devices as if they were files. For example, by opening the file AT.DEV you can read and write bytes using the AT protocol (used by AT and PS/2 keyboards and mice) using BBC BASIC's built-in file manipulation routines.
You could use this to do something useful, or could just use this to flash the LED on a keyboard back and forth.
10 keyb%=OPENOUT"AT.DEV" 20 DATA 2,4,1,4,-1 : REM LED flash pattern (-1 terminated). 30 REPEAT 40 READ l% 50 REPEAT 60 PROC_setled(l%) 70 PROC_pause(30) 80 READ l% 90 UNTIL l%=-1 100 RESTORE 110 UNTIL FALSE 120 END 130 : 140 DEF PROC_flushin 150 REPEAT 160 IF EXT#keyb% d%=BGET#keyb% 170 UNTIL NOT EXT#keyb% 180 ENDPROC 190 : 200 DEF PROC_setled(l%) 210 BPUT#keyb%,&ED 220 PROC_flushin 230 BPUT#keyb%,l% 240 PROC_flushin 250 ENDPROC 260 : 270 DEF PROC_pause(t%) 280 start%=TIME 290 REPEAT UNTIL TIME >= start%+t% 300 ENDPROC