type a variable name not previously defined then the computer will stop with the report **2 Variable not found, 20:1**. ### Using control codes with PRINT In the beginning of this chapter, we saw the effect that *control codes* 29 and 30 had in adjusting the size of the font that's currently printed on screen. There are more *control codes* that we can use with **PRINT**. **CHR$ 22** and **CHR$ 23** affect printing in the same manner as **AT** and **TAB**. They are rather odd as *control codes*, because whenever one is sent to the screen to be printed, it must be followed by two more characters that do not have their usual effect: they are treated as numbers (their codes) to specify the y and x positions (for **AT**) or the tab position (for **TAB**). You will almost always find it easier to use **AT** and **TAB** in the usual way rather than the control codes, but they might be useful in some circumstances. The **AT** control character is **CHR$ 22**. The first character after it specifies the y-position (be it a line number or y-pixel value according to the *layer* we're currently in) and the second the column number, so that: ``` PRINT CHR$ 22+CHR$ 1 +CHR$ c; ``` has exactly the same effect as: ``` PRINT AT 1,c; ``` This is so even if **CHR$** 1 or **CHR$ c** would normally have a different meaning (for instance if **c=13**); the **CHR$ 22** before them overrides that. The **TAB** control character is **CHR$ 23** and the two characters after it are used to give a number between **0** and **65535** specifying the number you would have in a **TAB** modifier: ``` PRINT CHR$ 23+CHR$ a+CHR$ b; ``` has the same effect as: ``` PRINT TAB a+256*b; ``` As with the character size *control codes*, there are further *control codes* that only apply to *layers* other than 0 and further modify their behaviour. One of those, is **CHR$ 26** or the *Scroll-prompt inhibitor control code*. Set by **CHR$ 26; CHR$** *n*; where *n* is the number of lines that can be scrolled off before the *scroll prompt cursor* appears (as discussed in the *Scrolling* section above) but after the first full screen length has been printed. If *n=0*, the scroll prompt function is inhibited for that *layer/window*. Note that the *n* number of lines is calculated based on an 8 pixel character height. That can lead to some very confusing results if your chosen character height is different. Some are easy to calculate like the standard or double height characters, with the latter in essence halving the amount of lines but others not so easy as with the reduced height and double reduced height characters. In the two last cases you have to calculate how many pixels your program outputs vertically by getting the amount of actual lines *times* the height of the characters and then divide the product by 8 (standard character height) in order to arrive to how many lines you need to instruct the system via the *Scroll-prompt inhibitor control code* to allow. If this sounds unnecessarily complicated that's because it is! In most cases, the average user will either need to disable scroll-prompting by setting *n* to **0** or just set it to a full screen of data by setting *n* to **24** (for all screen modes except **LAYER 1,0** which requires *n* set to **12**). On *Layer 0* you can duplicate that behaviour albeit in a less confusing way since the characters are always 8 pixels high, by employing a bit of **POKE** trickery to inhibit the **scroll?** prompt by doing: