Harvard vs. vonNeuman

Ron Hudson wrote: ... Back in high school I learned to program on a Wang 2200, which has only BASIC, interpreted in microcode. Just for the entertainment value, I wrote a program that would allow you to enter an arbitrary function as a string and then would graph that function over the range specified. It would have been possible to interpret that string to evaluate the function at each point, but I did something relevant to the above question. The 2200 had a block oriented tape system that allowed programs to read, write, and step through blocks arbitrarily. I took the string that the user hand entered, tokenized it, and wrote it out as a one line program to the three blocks on the tape immediately following the graphing program (program header block, program block, program trailer block). The program would then OPEN the program just written, which would change line 1000 which was something like 1000 DEFFNY(X)=X but with the right hand side replaced with the desired function. Although writing out the function and reading it back in took four or five seconds, the speed of the graphing more than made up for it. This was about the time that I figured out that the only difference between "protected" programs (can be run but not saved, listed, or modified) was a single bit at the start of the block header. It took about five lines of code to open a block, flip the bit, and write it back, thereby defeating the protection.