C o l u m b i a help file WHAT IS COLUMBIA? Columbia is a file compressor. It aims to reduce the amount of space a tape or disc file takes up. This works on the principle that in any file, there is a certain amount of repetition. For example, in a text file - a letter created on your word processor, say - only a certain number of characters are used: letters, punctuation, numbers. Yet the CPC stores each of these in the space that has a capacity for up to 256 values. So, for example, by replacing each occurrence of the word "the" with the otherwise unused value 128, you will save 2 characters (bytes) per occurrence of the word. In one end, you feed the original file: out of the other end comes the compressed file, which can be either an "archive" or an "executable" file. The "executable" option is meant for machine code programmers: it produces a file which can be RUN like the original program, and decompresses into memory immediately. The "archive" option, on the other hand, is something that everyone will find useful: the file has to be decompressed by Columbia before it can be used again, but it takes up a lot less space on the disc. For example, say that you have a disc on which you store all your personal correspondence. You are unlikely to read the letters ever again, but you might need to find out what you wrote at some point. So compress them all with Columbia and erase the originals: you've now saved a lot of disc space, but should you want to look at a letter again, all you have to do is decompress it with Columbia before loading it into your word-processor. HOW TO USE COLUMBIA If you're only interested in creating archive files, this is all you need to know. There are two versions of Columbia on your covertape: the tape version and the disc version. If you select Columbia from the menu on tape, the tape version will load. If you transfer the tape to disc, only the disc version will end up on your disc. Once Columbia has loaded, you will see five icons at the foot of the screen. You can select one of these with cursor keys and SPACE, COPY or ENTER. In order, they are: compress a file, decompress a file, set options, display file information, and display information about Columbia itself. To compress a file from tape, select the first icon and type in the filename to be compressed. Then type in the name of the new compressed file to be created. (It is recommended that you use two separate tapes.) Whenever you see "Press PLAY then any key", insert the tape with the original file on. "Press REC and PLAY then any key" means you should insert the tape for the new file. Decompression is very similar: "Press PLAY" means insert the tape with the compressed file on, and at "Press REC and PLAY", you should insert the tape to put the new uncompressed file on. Disc users are presented with an up/down scrolling list of all the files on the disc. Press COPY or SPACE to highlight the ones you want to compress, and then press ENTER. Make sure that you have enough space on the disc to store the compressed files. B-drive owners can select the source and destination drives using the "options" icon: use cursor keys (up, down, left, right) to change the options, and COPY, SPACE or ENTER to finish. If you only have 64k, you are STRONGLY recommended to change the "compression type" setting, using the "options" icon, to "archive". The standard setting is "automatic": when compressing data, this will create executable files from any machine code you feed into Columbia (this includes screens and other graphics). With 64k machines, this is a long-winded process that requires reading the file in twice - especially arduous on tape machines. If you have 128k, or you are only decompressing files, you needn't worry about this. COLUMBIA'S STRENGTHS AND WEAKNESSES Columbia uses an advanced compression algorithm known as LZW (Lempel-Ziv- Welch), which is similar to that used on PCs and Macintoshes for hard disc compression. LZW is very good at compressing text files - the longer the better. One sample 36k file was reduced to 18k after compression with Columbia. Graphics should also show a significant improvement, although less so. Although the saving on machine code programs is not so dramatic, it is still on a par with other CPC compression programs. You can only create executable machine code files with Columbia. You cannot take a BASIC program and create an executable file. CREATING EXECUTABLE FILES If a machine code file is compressed when Columbia is set to "executable" or "automatic" compression, Columbia will create a file which can be RUN directly and decompresses into memory. The result of this on most machine code programs will be a smaller file which works exactly the same way as before, except that immediately after loading, a few seconds elapse while the program decompresses. If you have transferred Crazy Cars 3 to disc from a past covertape, try compressing the file "CC3.BIN" for an example. If you plan to use the executable file option, the following notes may help. First of all, just because a file has been compressed by Columbia does not necessarily mean that it will RUN correctly! As standard, an executable file is loaded so that it takes up memory to &A200 (near HIMEM). However, if your program originally approached this address, the file may not run correctly, as the decompression code will be overwritten by the program code, causing a crash. Problems will also frequently occur if Columbia could not make a saving on the file. To solve this, you can change the "memory limit" before compressing a file, so that it loads lower or higher in memory accordingly. Another point to note is that decompression requires just over 12k of workspace in memory. When loading an executable file, you will usually see junk on the screen for a few seconds: this is where the workspace is located by default. However, if this is not suitable (for example, if you have compressed a screen!), you can move the workspace somewhere else in memory using the "buffer location" option. If the "buffer location" is set to "screen", the buffer will be located in screen memory, and the screen will be cleared after decompression. A file with an execution address of 0 will return after the decompression routine is called, rather than jumping to the execution address. You may find the "file information" icon useful when creating executable files. INCORPORATING DECOMPRESSION ROUTINES INTO YOUR OWN PROGRAMS Machine code programmers may want to use the decompression routine in their own programs, so that they can access archive files without having to use Columbia. Fully documented source code (Maxam format) is provided for this purpose. Richard Fairhurst (CRTC) Robot PD Library January 1995 *