$X with Terminal I/O
The following table shows the effects of different characters on $X.
||Effect on $X
|Any printable ASCII character
|Nonprintable characters (such as escape sequences)
||See Using ObjectScript.
The S(ecret) protocol of the OPEN and USE commands turns off echoing. It also prevents $X from being changed during input, so it indicates the true cursor position.
WRITE $CHAR() changes $X. WRITE * does not change $X. For example, WRITE $X,"/",$CHAR(8),$X performs the backspace (deleting the / character) and resets $X accordingly, returning 01. In contrast, WRITE $X,"/",*8,$X performs the backspace (deleting the / character) but does not reset $X; it returns 02. (See the WRITE command for further details.)
Using WRITE *, you can send a control sequence to your terminal and $X will still reflect the true cursor position. Since some control sequences do move the cursor, you can use the SET command to set $X directly. For example, the following commands move the cursor to column 20 and line 10 on a Digital VT100 terminal (or equivalent) and set $X and $Y accordingly:
ANSI standard control sequences (such as escape sequences) that the device acts on but does not output can produce a discrepancy between the $X and $Y values and the true cursor position. To avoid this problem use the WRITE * (integer expression) syntax and specify the ASCII value of each character in the string. For example, instead of using:
use this equivalent form:
As a rule, after any escape sequence that explicitly moves the cursor, you should update $X and $Y to reflect the actual cursor position.
You can set how $X handles escape sequences for the current process using the DX()Opens in a new tab method of the %SYSTEM.ProcessOpens in a new tab class. The system-wide default behavior can be established by setting the DXOpens in a new tab property of the Config.MiscellaneousOpens in a new tab class.
$X with TCP and Interprocess Communication
When you use the WRITE command to send data to either a client or server TCP device, InterSystems IRIS first stores the data in a buffer. It also updates $X to reflect the number of characters in the buffer. It does not include the ASCII characters <RETURN> and <LINE FEED> in this count because they are considered to be part of the record.
If you flush the $X buffer with the WRITE ! command, InterSystems IRIS resets $X to 0 and increments the $Y value by 1. If you flush the $X and $Y buffers with the WRITE # command, InterSystems IRIS writes the ASCII character <FORM FEED> as a separate record and resets both $X and $Y to 0.