In source-level debugging, the entire source code is available for viewing. An @ sign is displayed in column one of the current line (the line of code that's being executed). The line containing the cursor shows a > sign in column one. (If the cursor is on the current line, then the cursor is hidden by the @ sign.) For terminals that support reverse video, the cursor line is highlighted. Use the arrow keys to move the cursor. Press F10 to access the menu bar and to toggle back to source code from the menu bar.
If your runtime offers mouse support, then you may use a mouse in the area of the screen that displays the source code. The mouse allows you to perform the following common actions:
| Move the cursor line | To move the cursor to a different line, simply click anywhere on the line you want. |
| Scroll the source | To scroll the source up or down, hold the mouse button down and move the mouse off the top or bottom edge of the source window. The source will scroll to track the mouse. The source scrolls slowly, to make it easy to adjust the current display by a small amount. |
| Highlight a variable or procedure | Point the mouse at a variable or a procedure name and click on it to highlight the name and enable the Selection entry on the menu bar (discussed below). Several operations are available under Selection that act on the highlighted item. The highlighted item will also become the default value used by many menu options.
Using variables, you may specify data names that require arguments, such as tables that require indexes. You cannot specify literals. You can use the mouse, F7 (display variable on current line), or the Tab key (highlight variable on current line) to view qualified and indexed data items in the source. As long as a variable and all of its qualifiers and indexes are on one line, the entire expression is evaluated by these keys. If a variable and all of its qualifiers and indexes span multiple source lines, the entire expression is ignored, but component items are still found. |
| Display a variable | To view the value of a variable, double-click that variable. |
| View procedure | To scroll quickly to a paragraph or section, double-click its name. |
| Run to desired line | To set a temporary breakpoint, double-click a verb. This establishes a temporary breakpoint at the line containing the verb. The program runs to that line (unless it encounters another breakpoint before it reaches the line). When it reaches a breakpoint, the runtime returns to the debugger prompt and awaits your next command. |