Contents

What Are Registers?

Registers are typically associated with fast-access memory locations inside a computer’s processor. They are a fundamental part of a computer’s instruction set architecture. More generally, they are a storage location for data and can be found even in mechanical and electro-mechanical calculation machinery that predate programmable electronic digital computers.

The memory function of pocket calculators can be thought of as manipulating a register and programmable desk calculators of the 1970s often had dozens of memory locations referred to as registers. One of the oldest Unix programs is the dc desk calculator (1970). Both numbers and strings can be stored in registers with the s command and their content loaded again with the l command. Registers are named by a single letter, which is similar to how programmable calculators and indeed Emacs work.

Emacs Register Origins

EMACS, in its original form as a TECO extension, got register capabilities by virtue of TECO’s q-registers. TECO was created by Dan Murphy while a student at MIT in 1962 on the PDP-11. Because of resource constraints, interactive editing was not really feasible at the time, which is why TECO was designed to work off of a command tape, more like the Unix sed stream editor a decade later. Programmability was added to TECO quickly and first text variables and then loops were implemented. Text variables were designated with a single character as were integer variables that were used for looping. The q command retrieved the quantity of a register, which is why they were referred to as q-registers.

The TECO manual describes the q-registers (PDP-11 TECO, 1980):

TECO provides 36 data storage registers, called Q-registers, which may be used to store single integers and/or ASCII character strings. The Q-registers have one character names: A through Z and 0 through 9. Each Q-register is divided into two storage areas. In the numeric storage area, each Q-register can store one signed integer. In the text storage area, each Q-register can store an ASCII character string which may be either text or a TECO command string.

EMACS, the TECO-based version that ran on ITS and TWENEX, made heavy use of q-registers. The C-X X and C-X G commands to put text into and get text out of a register, respectively, are directly analogous to TECO’s x and g command. Those commands were carried over as key bindings into GNU Emacs until the prefix changes in Emacs 19 (public release in 1994).

Interestingly, Gosling Emacs, the direct precursor to GNU Emacs, did not have a register feature while GNU Emacs did since the beginning.

Using Emacs Registers

Register Command Prefix

The work on GNU Emacs began in 1984 and before the public release of Emacs 13 (1985), both rectangle and register commands were implemented. The register commands were carried over from the earlier EMACS and used the same command structure as described above. The rectangle commands used the C-x r prefix.

In Emacs 19, C-x key bindings were cleaned up a bit. Besides narrowing and abbrev commands, the register and rectangle commands were unified under C-x r. Those key bindings have not been fundamentally changed in the three decades since.

Register Names And Types

Registers are named with a single character. It does not need to be an ASCII alphanumeric character, it can be any character valid in Emacs, except the ones used for quitting (i.e., C-g.) The placeholder for a register name in the command descriptions is R.

There are two general families of registers, location and content registers (these terms are used in this post for the purpose of explanation, they are not used in the info manual, which is as always the authoritative source.)

Locations are used to record and jump to points in buffers as well as to restore files and display configurations. Content registers are an alternative to the kill ring in that their content does not change after another kill.

Register Command Structure

The key binding to save data in a register is C-x r followed by one letter for the type and then the register name R.

Location Registers

Letter Type Purpose
<SPC> Position Point in current buffer
w Window Window configuration in the current frame
f Frame All frames and their windows
n/a Buffer Buffer name
n/a File File name

In practice, this means to save the position of the point in the current buffer, for example, the command is C-x r followed by the Space key and then a single letter for the register name.

It’s good practice to use a mnemonic for the register name, but considering it can only be a single letter that may prove difficult. Let’s say the window configuration for several IRC channels related to Emacs (e.g., #emacs and #gnus) on Libera.Chat should be saved in order to easily jump back to that view. A good mnemonic for this might be the register name E, so the key sequence to save that window configuration then is C-x r w E.

Underneath the key binding, the Elisp function that is used to save registers is called set-register. It takes a register name and the information that is supposed to be saved. In the case of a file or a buffer name, there is no default key binding and that function needs to be called explicitly. The example from the info manual for saving a file name in a register (in this case the register name z) is:

(set-register ?z '(file . "/gd/gnu/emacs/19.0/src/ChangeLog"))

Bookmarks are a better way to record files and their point location.

These “location” registers are identical in how their content can be recalled. In order to jump to a location in a register, the easy to remember key binding is C-x r j followed by the register name.

A special key binding to store keyboard macros is available as C-x C-k x followed by the register name; executing the macro in a register is accomplished by “jumping to it” with C-x r j.

Register Preview

When jumping to a register (or inserting its content, see below), a window with the current register assignment will pop up after a default time of one second before entering the register name. That time can be customized and this preview can also be requested without jumping to a register. The info manual has all the additional information.

It’s important to note that the register preview will show all registers, regardless of what they contain. That means that if a register contains text, but the preview appears after a C-x r j, that content cannot be inserted and an error will be displayed. The same is true in the opposite scenario when a location register is selected after a C-x r i.

Content Registers

Letter Type Purpose
s Text (String) Current region
r Rectangle Current rectangle
n Number Number

Storing text and numbers is why registers in TECO and then Emacs were implemented. Text (strings) between the mark and the point are saved with C-x r s and when used with the universal C-u prefix the selected text is also deleted.

The rectangle between the mark and the point is saved into a register with the C-x r r command followed by a register name. The command to insert a register’s content into the current buffer at point is C-x r i followed by the register name.

C-x r And Rectangles

Because register and rectangle commands were part of the prefix clean-up in Emacs 19, there are many additional key bindings that start with C-x r. Most of the key bindings are fairly mnemonic and the overlap doesn’t really contribute to confusion. The info manual is the authoritative source.

One obvious conflict is C-x r s to store text in a register and C-x r t to fill a rectangle with a string. Once that inconsistency is apparent though, remembering the difference is not difficult.

The number to be stored in a register is assigned with the universal prefix C-u. E.g., to store the number 42 in register H, the key combination is C-u 42 C-x r n H. Without the prefix, the number 0 is stored in the register.

The increment command C-x r + applies to both text as well as number registers. In the case of number registers, the content will be increased by 1 unless there is a prefix.

For text registers, the increment is a an append function. A prepend function also exists and the append can be customized. The append function can be used to build up text segments from different buffers for later, potentially repeated inserts.

Clearing Registers

Somewhat surprisingly, there is no default key binding to clear individual registers. They can be set to nil with the set-register function.

An easier way to manage register content is the register-list package available on ELPA, which offers register editing, deletion, and concatenation commands in an overview buffer similar to what is available by defaults for Bookmarks.

Bookmarks

Registers are saved in the default file ‘~/.emacs.d/.emacs.desktop and restored for the next Emacs session when desktop-save-mode is enabled. Bookmarks are a better way to store and manage file locations longer-term.

The main bookmarks commands are:

  • C-x r m Sets the bookmark for the current file
  • C-x r b Jump to bookmark
  • C-x r l List all bookmarks

Bookmark names default to the filename, but can be given any other name. At exit time of Emacs, bookmarks are by default saved in ‘~/.emacs.d/bookmarks. Bookmarks can be annotated, automatically saved, and customized in other ways.

The bookmarks list command (C-x r l) shows all bookmarks and offers comprehensive functions similar to dired. The mode description (?) shows the mode commands, which are also available on the menu bar.

Summary

Registers are a great way to temporarily store numbers and text, point locations, and window and frame configurations. Text registers can function as an alternative to the kill ring to build up text snippets, number registers can be used as counters.

The key bindings are all prefixed with C-x r, which is also used for rectangles. The command names are easy to remember and well organized.

Together with the Desktop feature, registers are saved across sessions. Bookmarks are a better way to save filenames and point locations and come with a powerful editing mode.

  1. D. Murphy, “The Beginnings of TECO,” in IEEE Annals of the History of Computing, vol. 31, no. 4, pp. 110-115, Oct.-Dec. 2009, doi: 10.1109/MAHC.2009.127. ↩