IRM Insertion Replacement Mode
Insert characters instead of writing over them.
CSI 4 h/l
- ECMA-48
- §7.2.10 IRM – Insertion Replacement Mode, p. 24
- DEC STD 070
- §5.11 Insert/Replacement Mode, p. 5-138
- VT220
- §4.6.4 Insert/Replace Mode (IRM)
- VT510
- IRM—Insert/Replace Mode
- xterm
- Functions using CSI
Mode 4 decides what a printed character does to the one under the cursor.
| State | ECMA-48 calls it | Meaning |
|---|---|---|
Reset, CSI 4 l | REPLACE | the character takes the place of the one under the cursor |
Set, CSI 4 h | INSERT | the character is inserted, and the rest of the line moves right |
Replace is the default. In insert mode, the character under the cursor and everything right of it move one column right to make room, and whatever is pushed past the right margin is lost. Only printing is affected: the controls and the erasing functions work as they always do.
xterm keeps the mode (ansi_modes), and so does libghostty-vt, and both answer DECRQM with its state.
Support
| Terminal | Version | Support | Notes |
|---|---|---|---|
| VT220 | manual, 2nd ed., 1984 | Yes | §4.6.4 Insert/Replace Mode (IRM) |
| VT510 | manual, 1st ed., 1993 | Yes | IRM |
| xterm | patch 412, xterm-411a | Yes | |
| libghostty-vt | 0f171f6 | Yes | |
| kitty | 0.49.1, run in a VM | Partly | fails IRM-4 of the 6 cases that show it working |
| foot | 1.28.0, run in a VM | Partly | fails IRM-4, IRM-5, IRM-6 of the 6 cases that show it working |
| pyte | 0.8.2, run as a library | Partly | fails IRM-4 of the 4 cases that show it working |
| xterm.js headless | 6.0.0, run as a library | Partly | fails IRM-4 of the 6 cases that show it working |
| alacritty_terminal | 0.26.0, run as a library | Partly | fails IRM-4 of the 6 cases that show it working |
| ConEmu | build 230724, its documentation | No | not in its ANSI escape codes |
| rxvt-unicode | 9.31, run in a VM | Partly | fails IRM-4, IRM-5, IRM-6 of the 6 cases that show it working |
| VTE | 0.84.1, run as a library | Yes | passes the 6 cases that show it working |
Validation
IRM-1: Insert mode
abc
\e[1;1H
\e[4h # IRM
X
|Xabc__|
cursor 1,2
IRM-2: What is pushed off the line is lost
ABCDEF # fills the line
\e[1;2H
\e[4h # IRM
X
|AXBCDE|
|______|
cursor 1,3
IRM-3: Back to replace mode
abc
\e[1;1H
\e[4h # IRM: insert
X
\e[4l # replace
Y # over the a
|XYbc__|
IRM-4: Insert stops at the right margin
\e[1;5H
abcdef # columns 5 to 10
\e[?69h # allow left and right margins
\e[5;10s # margins at columns 5 and 10
\e[1;7H
\e[4h # IRM
X # the f is pushed past column 10
|____abXcde__|
IRM-5: Reset by default
\e[4$p
|_____|
reply \e[4;2$y
IRM-6: Set
\e[4h
\e[4$p
|_____|
reply \e[4;1$y