WHAT IS COLUMBIA? A file compressor. It aims to reduce tape or disc file space. The principle is that in any file, there is repetition. E.G. a text file, only a few characters are used: letters, punctuation, numbers. Yet the CPC stores each in the space that has a capacity for up to 256 values. So, for example, by replacing each usage of the word "the" with the otherwise unused value 128, you will save 2 characters (bytes) per occurrence of the word. In goes the original file: out comes the compressed file, either an "archive" or an "executable" file. The "executable" is for machine code programmers: it outputs a RUNable file as the original program, immediately decompressing into memory. The "archive" option is something that all will find useful: the file has to be decompressed by Columbia before it can be used again, but takes up a lot less space on the disc. E.G. - say you have a disc on which are all your personal correspondence. You are unlikely to read the letters again, later you might need to find out what you wrote. Compress them all with Columbia and erase the originals: you've now saved much disc space, should you need the letter again, all you have to do is decompress it with Columbia before loading it into your WP. HOW TO USE COLUMBIA If you only want to create archive files, this is all you need to know. 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. 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. 64k users - you are STRONGLY advised to alter the "compression type" setting, use 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 graphics). On 64k machines,this is a long-winded process that requires reading the file in twice - especially arduous on tape machines. If you've 128k, or are only decompressing files, you needn't worry about this. COLUMBIA STRENGTHS/WEAKNESSES. Columbia uses an advanced compression algorithm known as LZW (Lempel-Ziv-Welch), like hard disc compression on Macs and PC's. LZW is very good at compressing text files - the longer the better. One sample 36k file reduced to 18k after Columbia compression. Graphics should also show significant improvement, although less so. 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 can't 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 RUNable directly, decompressing into memory. The outcome on most machine code programs is a smaller file which works exactly as before, except that after loading, a few seconds elapse while the program decompresses. If you plan to use the executable file option, the following notes may help. First, just because Columbia has compressed a file doesn't necessarily mean it will RUN correctly! As standard, an executable file is loaded so that it takes up memory to &A200 (near HIMEM). If your program began near 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 often 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 it loads lower or higher in memory. Another point to note is decompression requires just over 12k of workspace in memory. When loading an executable file, you will usually see junk on screen for a few seconds: this is where the workspace is located by default. However, if this is not suitable (e.g. 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 a 0 execution address will return after calling the decompression routine, 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) Special permission has been obtained to Robot PD Library allow distribution of COLUMBIA with ATM. January 1995 For ROBOT SOFTWARE Phone 01353 777006 *