Simpleton Geek Teaches Computer Programming Book 2 Chapter 6D

6.4 Computer I/O explained DATE

“So, if the construct Init-Input-Output-Data is rather common, maybe I should make a template for it.” says Peter. He and the teacher was in the office during consulting hour.

“Actually, a more common construct is Input-Function-Output. The Data block is either usually being loaded on the Init phase, in case of the scope of the program, or the Input phase, in case of subroutines. You do know what a subroutine is, right?”

“I know what a function is.”

“You can think of subroutine as a function, but in truth, a subroutine is just a block of code. No more, no less. What is commonly done, is you assign values to all the variables that you need for that subroutine, then call the subroutine, then read the variables that is assigned on that subroutine. Therefore, the structure is Input-Function-Output.

“Furthermore, you can have multi-level of it. Some of them can be nested quite deep. Any question?”

“Will that be a GOTO or CALL command?”

“GOSUB command. There's a GOSUB-RETURN pair construct. GOSUB tells the program to go to a certain label that is the subroutine. RETURN returns the program execution to the original place of the call.”

“And about GOTO?”

“Goto is simply changing the execution of the program to another place. You know about the cursor of character text screen and the cursor for the DATA?”

“Yes.”

“GOTO command changes the 'cursor' of the program execution to the defined label.”

“Seems like there's quite a lot of cursor going on.”

“You wouldn't believe it. Every time GOSUB is called, there's the pointer for the stack changed. Yes, certainly there's a lot of things going on around.”

“So, how come none of the computer programming book mention this? I would think that at least some of them would have mentioned it.”

“Hmmm. Good question. I think part of it is the notion that a good high level language hides the complexity of the computer. The higher the language, the more the complexity is hidden. Another reason is that the program execution 'cursor' isn't called program cursor. It's called 'Program Counter'.”

“Program Counter? That doesn't seem to fit.”

“The original program is machine language, remember? All it is, is a number, denoting which line of the program, the CPU needs to execute next. That's all it is, a number.

“You can think of it as a pointer that points to the memory address to be fed to the CPU.”

“So a program counter is a pointer.”

“Correct. Just like a variable is a pointer.”

“Huh? How does that work?” Peter was confused.

“Do you know why we specify X,Y,WORD$? They act as a placeholder for the actual values. The actual values aren't X and Y. They're numbers!”

“Yes, I see.”

“Numbers, which resides in a certain place in memory. Do you know which of the memory address contain the values? Well, you look up the entry at X, and that tells us not the value of X, but the memory address containing the value of X! So pointers are look up address, or references! Just for fun, it may be fun to look up all internal variables, such as TIME$, as well.”