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This is a re-post of a blog entry I made, back in 2022. I have included it here for convenience, as some readers may not go into my blog section to find it.

Half Fast 128 is a utility program for the C128, which allows you to control the CPU speed when running in C64 Mode. According to the instruction manual, Half Fast allows you to access the 2Mhz processing speed that the C128 is capable of, when in C64 mode. You use the + and - keys, on the C128's numeric keypad to control the speed.

From the manual:

By default, Half-Fast will put your computer into FAST mode only during the screen's vertical blanking interval, so the whole 40-column screen will be visible… Pressing the numeric keypad “+” your computer will run faster, but part of the screen will fuzz into colored rectangles. With the whole screen fuzzed, your computer will be processing about twice as fast as it would normally. To reduce the computer's speed back to normal, press the numeric keypad “-” key. More of the 40 column screen will become visible as you do this, and your computer's processing speed will slow down.

Below, I've included a video of me using Half Fast on the VICE C128 emulator. Although the screen doesn't display the fuzzy rectangles described, it does show some wiggly lines in the top and bottom margins of the screen. In the video, after I load and run Half Fast, I then load up a simple “screen saver” program I wrote when I was learning BASIC. It prints a selection of characters across the screen, over and over again.

Now, here's the caveat with Half Fast 128: you will not be able to send or receive data to disk drives or printers when operating at the faster speeds. You need to return to “regular” speed when ever these devices need to be accessed.

When I run my program with Half Fast you can see the speed difference when I use the + and - keys. When the wiggly lines disappear, I'm running at the regular 1Mhz speed. The more frantic the wiggly lines, the faster the CPU is running.

So, Half Fast does what it's supposed to do. How you can take advantage of this, I have no idea. If you have a C128 and want to write faster BASIC programs, you might as well just use BASIC 7.0 and the C128's FAST mode. I suppose, if you had a particular BASIC program you use for the C64, and you have a C128, then this might be useful to you. If there's something you could use this utility program for, please let me know. I'd enjoy hearing about it.

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