When the CPU chip runs faster, it generates more heat. From what I hear, most CoCo 6809's can take the heat of running double-speed, but some might burn out. With that chip fried, the entire machine is dead. So, there is some risk with doing this; if you treasure your CoCo, you might not want to try it. Personally, I've done this on a handful of CoCo's, and never had a problem with it. But your machine may be the "lucky" one, so be forewarned!
Also note that a lot of the timing-dependent things in the CoCo BASIC ROMs won't work right at any speed other than "slow", like reading or writing cassettes and disks, making sounds, and maybe some other things. So you may need to put on the brakes at some point after speeding up.
Also, you won't normally use the "double speed always" setting, because that disables the video and dynamic RAM refresh; it lets the processor use memory faster by letting it take the memory cycles that would normally be used for those things. So while running "fast", you won't get any picture, and you might lose the contents of RAM unless your access pattern is just right (making a program do that would be tricky, maybe impossible). The good news is that in BASIC programs, the "AD" speed works out to be nearly double, because it spends most of its time executing ROM code anyway.
POKE &HFFD7,1 : REM GO FAST (A.D. SPEED) ... POKE &HFFD6,1 : REM BACK TO NORMAL (SLOW) SPEED
REM THESE 2 BITS SHOW THE SETTINGS REM THAT CONTROL THE SPEED ---------+-+ REM | | REM v v ... REM FIRST CASE, GO THROUGH A.D. WHEN SPEEDING UP REM GOING SLOW AT FIRST 0 0 ... REM STEP ON THE GAS POKE &HFFD7,1 : REM A.D. 0 1 POKE &HFFD9,1 : REM FAST-1 1 1 ... REM YEEHAA! ... REM BRAKE POKE &HFFD8,1 : REM A.D. 0 1 POKE &HFFD6,1 : REM SLOW AGAIN 0 0 ... ... ... REM GOING SLOW ... REM SECOND CASE, GO DIRECTLY FROM SLOW TO FAST REM STOMP ON THE GAS POKE &HFFD9,1 : REM FAST-2 1 0 ... REM YEEHAA! ... REM BRAKE REM CANNOT GO FROM FAST-2 DIRECTLY TO SLOW OR A.D. REM MUST GO TO FAST-1 FIRST POKE &HFFD7,1 : REM FAST-1 1 1 POKE &HFFD8,1 : REM A.D. 0 1 POKE &HFFD6,1 : REM SLOW AGAIN 0 0 ...Back to my top-level CoCo page