What is a Delphi DCU file?

In addition to David Schwartz's answer, there is one case when a dcu actually is quite different from typical obj files generated in other languages: Generic type definitions. If a generic type is defined in a Delphi Unit, the compiler compiles this code into a syntax tree representation rather than to machine code. This syntax tree representation then is stored in the dcu file. When the generic type then is used and instantiated in another unit, the compiler will use this representation and "merge" it with the syntax tree of the unit using the generic type. You could think of this being somewhat analogues to method inlining. This, btw is also the reason why a unit that makes heavy use of generics takes much longer to compile, although the generic types are "linked in" from a dcu file.

A Delphi Compiled Unit contains object code, and pre-compiled headers, and is therefore somewhat comparable to both an obj file and a .pch / .gch file.

The 'interface' section of a Delphi source file corresponds to the header, and the 'implementation' section creates the object code.

Pre-compiled header files may significantly reduce compilation and link time. The DCU header section provides link information to other referenced units, that does not have to be re-discovered.

In the Delphi / Turbo Pascal environment, pre-compiled headers support strict type checking, which would have required source-code referencing if an Object file format like .coff or .obj had been used. (In C++, name mangling provides a similar but less complete function).

I believe .dcu generally means "Delphi Compiled Unit" as opposed to a .pas file which is simply "Pascal source code".

A .dcu file is the file that the DCC compiler produces after compiling the .pas files (.dfm files are converted to binary resources, then directly processed by the linker).

It's analogous to .o and .obj files that other compilers produce, but contains more information on the symbols (therefore you can reverse engineer the interface section of a unit from it omitting comments and compiler directives).

A .dcu file technically not a "cache" file, although your builds will run faster if you don't delete them and when doesn't need to recompile them. A .dcu file is tied to the compiler version that generated it. In that sense it is less portable than .o or .obj files (though they have their share of compatibility problems too)

Here's some history in case it adds anything.

Compilers have traditionally translated source code languages into some intermediate form. Interpreters don't do that -- they just interpret the language directly and run the application right away. BASIC is the classic example of an interpreted language. The "command line" in DOS and Windows has a language that can be written in files called "batch files" with a .bat extension. But typing things on the command line executed them directly. In *nix environments, there are a bunch of different command-line interpreters (CLIs), such as sh, csh, bash, ksh, and so on. You can create batch files from all of them -- this are usually referred to as "scripting languages". But there are a lot of other languages now that are both interpreted and compiled.

Anyway Java and .Net, for example, compile into something called an intermediate "byte-code" representation.

Pascal was originally written as a single-pass compiler, and Turbo Pascal (originating from PolyPascal) - with different editions for CP/M, CP/M-86 and DOS - directly generated a binary executable (COM) file that ran under those operating systems.

Pascal was originally designed as a small, efficient language intended to encourage good programming practices using structured programming and data structuring; Turbo Pascal 1 was originally designed as a an IDE with built-in very fast compiler, and an affordable competitor in the the DOS and CP/M market against the long edit/compile/link cycles at that time. Turbo Pascal and Pascal had similar limitations as any programming environment back then: memory and disk space were measured in kilobytes, processor speeds in Megahertz.

Linking to an executable binary prevented you from linking to separately compiled units and libraries.

Before Turbo Pascal, there was UCSD p-System operating system (supporting many languages, including Pascal. The UCSD Pascal compiler back then already extended the Pascal language with units) which compiled into a pseudo-machine byte-code (called p-code) format that allowed linking multiple units together. It was slow though,

Meanwhile, c evolved in VAX and Unix environments, and it compiled into .o files, which meant "object code" as opposed to "source code". Note: this is totally unrelated to anything we call "objects" today.

Turbo Pascal up to and including version 3 directly generated .com binary output files (although you could use amend those overlays files), and as of version 4 supported separating code into units that first compiled into .tpu files before linked into the final executable binary. The Turbo C compiler generated .obj (object code) files rather than byte-codes, and Delphi 2 introduced .obj file generation on order to co-operate with C++ Builder.

Object files use relative addressing within each unit, and require what's called "fix-ups" (or relocation) later on to make them run. Fix-ups point to symbolic labels that are expected to exist in other object files or libraries.

There are two kinds of "fix-ups": one is done statically by a tool called a "linker". The linker takes a bunch of object files and seams them together into something analogous to a patchwork quilt. It then "fixes-up" all of the relative references by plugging-in pointers to all of the externally-defined labels.

The second fix-ups are done dynamically when the program is loaded to run. They're done by something called the "loader", but you never see that. When you type a command on the command line, the loader is called to load an EXE file into memory, fix-up the remaining links based on where the file is loaded, and then control is transferred to the entry point of the application.

So .dcu files originated as .tpu files when Borland introduced units in Turbo Pascal, then changed extension with the introduction of Delphi. They are very different from .obj files, though you can link to .obj files from Turbo Pascal and Delphi.

Delphi also hid the linker entirely, so you just do a compile and a run. All of the linker settings are still there, however, in one of Delphi's options panes.