Printing characters not at expected position

Question

Simple program...

10 print chr$(147);chr$(5)
20 poke 53280,4: poke 53281,0
30 y=0 : for x = 1 to 35
40 poke 214,y : print : poke 211,x
50 print chr$(171);chr$(177);chr$(179);
60 next x : print : print

This is a program from a book 'Step by step programming: Commodore 64' - page 38

I can't understand why the characters are printed on the third line (index = 2) when y = 0 in the program, and there is a single print between the pokes before the characters are printed - which would move the cursor down a single line to line 2 (index 1). Can anyone explain why it behaves like this?

Just to clarify the Y position, here's another screenshot showing the cursor, and also with the print statement removed. So shouldn't that be now on line 1 (index = 0).

And here's a simpler program that I think hones in on the issue:

10 print chr$(147);chr$(5)
40 poke 211,0: poke 214,0
50 print chr$(171);chr$(177);chr$(179);

This program doesn't show anything, which is unexpected.

If I change this code to poke 214,1 (Y-ord) I get an expected output..

So this is correctly at <0,1>, but I can't use y = 0 here. Maybe this is related to the question. There is something off about all this.

Answer 1

It's not enough to poke the values into those locations - there's other information held elsewhere that also needs to be updated. See, for example, the following (slightly paraphrased) text from this page:

By using locations 211 and 214, we can position the cursor on the screen very easily. POKE the column into 211, the line into 214.

Of course, that isn't enough; the operating system doesn't know that the cursor has been moved. There's a ROM routine that will take care of the whole business for us. You can call it with SYS 58732, as per:

POKE 211, COLUMN : POKE 214, LINE : SYS 58732

The code at that 58732 location is needed because:

• the C64 screen memory lines are spread across different pages of screen memory and are not always in a fixed spot; and
• the physical lines on the screen are 40 columns but the logical lines (as used by the screen editor and output routines) can be longer.

This means there's more information that needs updating, over and above just the lower-level physical line/column bytes.

Specifically, a data table contains, for each physical line, enough information to work out its memory location and whether it is the first/only or subsequent physical line of a logical line. This table is used to update all the other information, by calling the ROM routine at $E56C (58732).

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If you really want to understand, there's a great site with analysis of the C64 ROM (the specific code mentioned in this answer is here), which goes into a stunning level of detail on how things work.

But, if you just want to get it working, make sure you call that ROM routine before you try to PRINT anything, after having used POKE to change the line and/or column values.

Answer 2

In your approach, the commands poke 214,y:print:poke 211,x position the cursor one line below the given y value. The reason is that the following carriage return (issued by the single print command) is positioning the cursor but also entering the next line. Since poke 214,-1 is not possible, this means you can only position the cursor to lines below the top one.

An alternative is to use poke 214,y:poke 211,x:sys 58732, which exactly positions the cursor at x,y. But this version with the SYS call is about 50% slower than your approach.

So if you do not need to position into the top line, using poke 214,y:print:poke 211,x is fine as long as you use a number one less than the intended line for y.