Self Modifing Code was RE: Harvard vs. vonNeuman

[David V. Corbin]

>>>  It is interesting that different people have different 
>>> ideas as to what self modifying code is. Actually, the 
>>> ability to load code and execute from a mass storage is 
>>> self modifying.

AGREED

>>> Code that uses a variable value for a table to look up 
>>> execution addresses is self modifying. 

DISAGREE [the contents of a location containing executabgle code are never
changed]

>>> Most every 
>>> multitasking system I've seen are self modifying.

AGREED

>>>  I think what people consider dangerous self modifying is 
>>> when the program alters the actual execution instructions 
>>> in memory and then executes them. It is strange that this 
>>> form gets such a large amount of emotional negative 
>>> response while other types are hardly considered issues.
>>>  No matter how the code is written, one can make bad code 
>>> in most any form. One can also make good code as well. 
>>> There is no reason why self modifying code should be any 
>>> more dangerous than any other type of coding if done with a 
>>> complete understanding of what the limitations are. What is 
>>> the difference between replacing one or two instructions 
>>> and then executing them or replacing an overlay and then 
>>> executing that overlay? Other than size, both are doing the 
>>> same thing. One is forbidden while the other is normal operation.

Since when do peoples reactions and reality coincide?


When I was involved in military systems development [77-92] the restriction
[for certain applications] against self-modifying code were quite clear:
Program memory had to be completely inviolate to change, execution out of
modifiable memory was not allowed, AND this had to be hardware enforced.