In 1984, John Henry Software released "Christmas Carols", which is 18 Commodore 64 SID renditions of Christmas songs, complete with themed bitmap graphics, and karaoke-style onscreen lyrics. Some of the graphics are also animated, and the package even included a song-booklet, to encourage singing along with your family, and dog. While the Commodore 64 is capable of displaying multiple graphic modes at once, such as bitmap graphics and text mode, it was often a challenge to work out precisely how to do this "raster split" and many C64 programs from the 1980s do this in a less than optimal way, resulting in a flickery or glitchy seam between the two graphics modes. We’ll try to improve this with some tighter 6510 machine code that results in a rock solid display.
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Index:
0:00 Christmas Carols: The Box
2:35 Inside the box: disk and manuals
6:05 Loading the program
9:22 Angels We Have Heard On High, Away In A Manger
13:18 Jolly Old St. Nicholas, Jingle Bells, We Three Kings
18:06 A Couple Neat Details
21:38 Finding the raster interrupt code
24:50 Examining the raster code
35:03 Finding a de-protected copy
38:25 Looking at the files on the disk
41:15 Entering and examining the raster patch
45:45 Patching the main T1 program
51:48 Patching self-running boot file C
59:55 Testing our fix
1:02:47 Merry Christmas, thanks!
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