Further Programming Information; Cursor Locations And Control Code Extensions; Control Characters - AMSTRAD CPC464 User Manual

9 Further programming
information
Subjects covered in this chapter
* Text Legal Position

* Control Characters

* The Machine Operating System
* Interrupt structures

9.1 Cursor locations and control code extensions

In a variety of applications programs, the text cursor may be positioned outside the current window.
Various operations force the cursor to a legal position before they are performed, these are :
? ?writing a character
? ?drawing the cursor ' block'
? ?obeying the control codes marked with an asterisk in the list on the following
pages.
The procedure for forcing the cursor to a legal position is as follows :
a. If the cursor is to the right of the right hand edge, then it is moved to the leftmost column of the
next line down.
b. If the cursor is to the left of the left hand edge, then it is moved to the rightmost column of the next
line up.
c. If the cursor is above the top edge, then the window is rolled down a line and the cursor is set to
the top line of the window.
d. If the cursor is below the bottom edge, then the window is rolled up a line and the cursor is set to
the bottom line of the window.
The tests and operations are done in the order given. The illegal cursor positions may be zero or
negative, which are off to the left or above the window.
Character values (see appendix III) in the range 0....31 sent to the text screen do not produce a
character on the screen (and should not be injudiciously applied) but are interpreted as
CONTROL
, Some of the codes alter the meaning of one or more of the following characters, which are
CODES
the code' s parameters.
The codes marked * force the cursor to a legal position in the current window before they are obeyed
- but may leave the cursor in an illegal position. The codes and their meanings are described with
first their value (&XX), then the decimal equivalent.
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