Can someone tell me please what this code does.
I need it for my studies. Thank you.

Line 1, JSR CrLf, calls a subroutine which no doubt outputs a carriage-return character and a line-feed charcter to whatever output device is selected.

LDA #00 and TAX puts 0 in both the accumulator and the X index register. Since you're going to fill a page of memory with 0's, the 65c02 could replace these two lines with LDX #00 and make the next line STZ (STore Zero) instead of STA.

Then you have a loop to zero-out 256 bytes of memory starting at address $0800. "Init" is a label marking the top of the loop where the BNE three lines down will branch up to repeatedly until the index register X rolls over back to 0.

Actually the CPX #0 is a wasted line, since this action happens automatically anyway with the INX. CPX #0 can be omitted to speed things up a little while still doing exactly the same thing.

The next loop will run as many as $3C times, with Y values from 0 to $3B. Y is used as the loop counter, being initialized by LDY #0. "Eingabe" labels the top of the loop for the BNE below to branch up to until finished.

Read a character and see if it's a carriage-return ($0D). If so, branch out of the loop to label VorAus. Otherwise keep going and write the charcter. What you're writing to is not clear here. The loop records how many times each character occurs; so after writing the character, you're going to increment the corresponding memory byte, whose address is $0800 plus the character's value which has been transferred to X by using TAX.

Increment Y (the loop counter), and loop back up again only if you are not up to the maximum number of times you wanted to run the loop. When you repeat the loop, you will read the next character.

If the Y value is not needed for anything in subroutine WriteChr, then it would be more efficient to initialize Y to $3C and then do DEY instead of INY. In other words, you're going to count down instead of up. Then you can get rid of the CPY line, and the "BNE Eingabe" will still work correctly because of the automatic compare-to-zero that happens with an increment or decrement instruction.

Get a good grade in your class and join us often on the forum.

Garth

Pete,

Just curious, where are you going to school that's teaching 6502 assembly? You'd be surprised at the number of homework questions that get e-mailed to 6502.org.

Regards,
Mike

_________________
- Mike Naberezny (mike@naberezny.com) http://6502.org

Hi Mike

I am studying economic computer science in Wedel ,Germany. There we have weekly tasks in e.g. 6502 assembly.

I hope I it wouldnt be a problem to ask something about 6502 in the future.

Regards Pete