hoặc Descriptor Bảng toàn cầu hoặc một Bảng Descriptor địa phương. Các bảng mô tả, trong đó bao gồm mục 8-byte, xác định các điểm bắt đầu phân khúc (được biết đến như là cơ sở), kích thước các phân khúc (được gọi là giới hạn) và các thuộc tính phân đoạn | 536 The Giant Black Book of Computer Viruses these techniques work perfectly well they are in principle just the same as using assembler and assembler is more versatile. The reader who is interested in such matters would do well to consult some of the material available on The Collection On the face of it writing destructive code is the simplest programming task in the world. When someone who doesn t know the first thing about programming tries to program the first thing they learn is that it s easier to write a destructive program which louses something up than it is to write a properly working program. For example if you know that Interrupt 13H is a call to the disk BIOS and it will write to the hard disk if you call it with ah 3 and dl 80H you can write a simple destructive program mov dl 80H mov ah 3 int 13H You needn t know how to set up the other registers to do something right. Executing this will often overwrite a sector on the hard disk with garbage. Despite the apparent ease of writing destructive code there is an art to it which one should not be unaware of. While the above routine is almost guaranteed to cause some damage when properly deployed it would be highly unlikely to stop a nuclear attack even if it did find its way into the right computer. It might cause some damage but probably not the right damage at the right time. To write effective destructive code one must pay close attention to 1 the trigger mechanism and 2 the bomb itself. Essentially the trigger decides when destructive activity will take place and the bomb determines what destructive activity will happen. We will discuss each aspect of destructive code writing in this chapter. 1 Consult the Resources section in this book for more information. Destructive Code 537 Trigger Mechanisms Triggers can cause the bomb to detonate under a wide variety of circumstances. If you can express any set of conditions logically and if a piece of software can sense these conditions then they can
