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Another character, the bell character (BEL), falls in between binary and text characters When displayed or printed, it signals that a tone should be sounded Back in the old Teletype days, the BEL character caused the teletype machine to ring its bell which was literally a mechanical bell struck by a little hammer BEL characters are allowed in text files, but are little used these days and considered sloppy practice Many modern printers and most displays don't handle them correctly anyway; like the CR/LF pair, they are a barely surviving remnant of an increasingly fossilized past Another one of these fossilized characters will eventually cause you some trouble: the end-of-file (EOF) marker character Unlike EOL, EOF is a single character, ASCII character 26, sometimes written as Ctrl+Z because you will generate the EOF character by holding the control key down and pressing the Z key The EOF character, properly, is not a DOS convention at all DOS inherited EOF from the even older days of CP/M-80, which reigned between 1976 and 1982 In CP/M's archaic file system, there was no precise count of how many bytes were present in a text file The operating system counted how many disk sectors were allocated to a text file, but within the last sector CP/M could not simply count its way to the final byte Instead, CP/M insisted on there being an end-of-file marker at the very end of the significant data and would ignore anything after that marker DOS and Windows, by contrast, keep a precise count of how many characters are present in a text file, and therefore do not require any sort of EOF marker at all However, some older DOS utilities
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recognize EOF, as a nod to older CP/M text files that were sometimes carried forward into the DOS world As character 26 (Ctrl+Z) is not a displayable character and not true white space, this ordinarily did no harm However, some editors and other utilities will not display or manipulate text past an embedded Ctrl+Z Some DOS utilities recognize EOF, and some do not If you find a text file that seems to end prematurely, use a binary viewer such as DEBUG (more on which shortly) to see if a Ctrl+Z character has found its way into the interior of the file Ctrl+Z is not otherwise useful in any text files I'm aware of, so removing it will not damage the file Keep in mind that this only applies to text files Binary files may contain any character values at all, and thus may be shot full of Ctrl+Z characters, any or all of which may be vital to the file's usefulness We return to the issue of inspecting and changing the contents of binary files in a little while
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Manipulating a text file is done with a program called a text editor A text editor is a word processor for program source code files In its simplest form, a text editor works like this: You type characters at the keyboard, and they appear on the screen When you press the Enter key, an EOL marker (for DOS, the two characters CR and LF) is placed at the end of a line, and the cursor moves down to the next line The editor also allows you to move the cursor back up into text you've already entered, in order to change it You can delete words and whole lines and replace them with newly entered text Ultimately, when you decide that you're finished, you press some key like F2, or some combination of keys like Ctrl+K+D, and the text editor saves the text you entered from the keyboard as a text file This text file is the source code file you'll eventually present to the assembler for processing Later on, you can load that same text file back into the editor to make repairs on faulty lines that cause errors during assembly or bugs during execution It's possible to use a word processor as a program text editor In older times, many programmers used WordStar, WordPerfect, Microsoft Word, and other available word processors to edit their program text This works as long as you remember to write your text file to disk in "non-document mode" or "ASCII text mode" Most true word processors embed countless strange little codes in their text files, to control such things as margin settings, font selections, headers and footers, and soft page and line breaks These codes are not recognized ASCII characters but binary values and actually turn the document file from a text file to a binary file The codes will give the assembler fits If you write a program source code file to disk as a document file, it will not assemble See the word processor documentation for details on how to export a document file as a pure ASCII text file Software was expensive in years past, and programmers (who tend to be cheap, yours truly not excluded) understandably wanted to get the most bang for their software budget and used word processors for everything they could These days, software has become cheap or (increasingly) even free, and there are a multitude of plain ASCII text editors available freely for download from the Internet I'll even go you better than that On the CD-ROM associated with this book I've arranged to distribute a programming text editor specifically designed for assembly language programmers in fact, specifically designed to work seamlessly with the assembly that I teach in this book (which is also on the CDROM what a deal!) NASM-IDE was written in Turbo Pascal, and its editor works a great deal like the editors you may have used in Borland's DOS-based programming products I explain how to use NASM-IDE in great detail in the next chapter In earlier editions of this book I spoke of something called JED, which was a simple assemblyprogramming editor that I had written for my own use also in Turbo Pascal JED is history, and while you can still use it if you have it, it doesn't interface well with NASM, the assembler I teach throughout this book NASM-IDE is a great deal like JED but much more sophisticated and obviously, it was created to work with NASM If for some reason the CD-ROM didn't come to you with the book, both NASM-IDE and NASM itself can be downloaded from the Internet without charge, along with the listing files See Appendix C, "Web URLS for Assembly Programmers"
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If you have a text editor that you've used for some time and prefer, there's no reason not to use it It just won't make following along with the text quite as easy
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