Radio Shack TRS-80 Model II Reference Manual page 197

Technical Information
Scroll Mode
In the Scroll Mode, the Display can be thought of as a sequence of 1920
display positions, as illustrated below:
Line 0
Line 1
Line 2
Line 3
Line 4
LineS
Line 6
Line7
Line 8
Line 9
Line 10
Line 11
Line 12
Line 13
Line 14
Line 15
Line 16
Line 17
Line 18
Line 19
Line 20
Line 21
Line 22
Line 23
DISPLAY POSITIONS, SCROLL MODE
Note: The Display has two character sizes: 80 characters per line and 40
characters per line. The illustration above shows the 80 character per line
mode.
In the scroll mode, each time an acceptable display character is received, it is
displayed at the current cursor position, and the cursor advances to the next
higher numbered position.
When the cursor is on the bottom line and a line-feed or carriage return is
received, or when the bottom line is filled, the entire Display is "scrolled":
1. Line 0 is deleted
2. Lines 1-23 are moved up on one line
3. Line 23 is blanked
4. The cursor is set to the beginning of line 23.
Note: Lines 0 to 22 on the Display can be protected from scrolling via the
SCROLL
function call.
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