DECOM Origin Mode

CSI ? 6 h sets origin mode and CSI ? 6 l resets it. It is reset by default, and by a reset of the terminal.

Reset, positions count from the top-left corner of the screen. Set, they count from the top and left margins, the corner of the scrolling region: line 1 of CUP is the top margin, and column 1 the left margin. DEC STD 070 also says that in origin mode “the Active Position cannot be moved above the Top Margin or left of the Left Margin”, and the VT510 manual that it “cannot move outside of the margins”, so a position past the bottom or right margin is clamped to it. The margins are set by DECSTBM and DECSLRM.

Setting or resetting the mode moves the cursor to the new home position: the top and left margins when set, the first line and column when reset. Origin mode affects every function that takes a position, and the reports that give one back, such as the cursor position report from DSR. It does not limit erasing: ED still erases the whole screen.

A host can ask whether it is set with DECRQM, CSI ? 6 $ p.

Validation

DECOM-1: Setting it homes the cursor to the margins

\e[2;3r   # region is lines 2 and 3
\e[3;4H
\e[?6h

|_____|
|_____|
|_____|
|_____|
cursor 2,1

DECOM-2: Resetting it homes the cursor to the corner

\e[2;3r
\e[?6h
\e[2;2H
\e[?6l

|_____|
|_____|
|_____|
|_____|
cursor 1,1

DECOM-3: Counted from the left margin, clamped to the right

\e[?69h   # allow left and right margins
\e[2;4s   # columns 2 to 4
\e[?6h
\e[1;1H
X
\e[1;9H   # clamped to the right margin
Y

|_X_Y__|
|______|

DECOM-4: Clamped to the bottom margin

\e[2;3r
\e[?6h
\e[9;9H
X

|_____|
|_____|
|____X|
|_____|

DECOM-5: Still counted from the margins after they move

\e[2;3r
\e[?69h
\e[2;4s
\e[?6h
\e[1;1H
X         # at line 2, column 2
\e[?69l   # the left and right margins go
\e[r      # and the top and bottom ones
\e[1;1H
Y         # origin mode is still set, now at the corner

|Y_____|
|_X____|
|______|
|______|

DECOM-6: Asking about it

\e[?6h
\e[?6$p
\e[?6l
\e[?6$p

|_____|
reply \e[?6;1$y\e[?6;2$y