Harvard vs. vonNeuman

Dwight> Hi It is interesting that different people have different Dwight> ideas as to what self modifying code is. Actually, the Dwight> ability to load code and execute from a mass storage is self Dwight> modifying. Indeed. Dwight> Code that uses a variable value for a table to Dwight> look up execution addresses is self modifying. Not as the term is usually used. You're changing a pointer, and pointers are data. Ask yourself "do I have to flush the I-cache for this to work?" The answer in this case is NO. Dwight> Most every Dwight> multitasking system I've seen are self modifying. I think Dwight> what people consider dangerous self modifying is when the Dwight> program alters the actual execution instructions in memory Dwight> and then executes them. It is strange that this form gets Dwight> such a large amount of emotional negative response while Dwight> other types are hardly considered issues. No matter how the Dwight> code is written, one can make bad code in most any form. True. Part of the issue, for modern machines, is that the hardware gets in the way. If you change code, the I-cache doesn't change (in machines I'm familiar with) so the old instruction may still execute. This makes instruction modification a tricky business. Debuggers do it, operating systems do it -- and both have to pay attention to all the cache magic you have to perform for it to be right. paul