How to use the Commodore C128 in 2MHz FAST mode

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Commodore LogoThe C128 could run twice as fast as its predecessor, the legendary C64. All you had to do was issue the FAST command, or switch back with the SLOW command to its default speed (1MHz).

One drawback of using FAST mode was that the VIC which powered the 40 column (standard) display wasn’t capable of such a high frequency and therefore would go blank when use go FAST. You had to work on the 80 column display to reap the benefits of a faster computer.

Here’s how to first switch into 80 column mode, then go fast:


and to switch back into 40 column slow mode:


Switching GRAPHIC modes was not necessary to perform long running calculations though – as long as that blank screen isn’t a problem for the user. Consider this:

10 fast : rem screen goes blank
20 gosub 1000 : rem super long calculation
30 slow : rem screen comes back
40 print result$ : rem show result to user

During the “screen outage” keyboard commands were accepted as usual too, so you could type SLOW on the blank screen and see your 40 column display come back to life.

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