Linux 2.2 and the Frame-Buffer Console
Linux is a fascinating and fast-paced beast. It seems like only yesterday the hardy developers of the Linux kernel were busily putting the finishing touches on ELF support, loadable modules and SMP (Symmetric Multiple Processing)--things we take for granted today. In those days, more time was spent on the critical hurdles, the ones that would turn Linux into the great server platform it is today. In today's Linux world, more time is spent on the less critical components of the system: new file systems, television and radio cards and parallel-port drives. I feel the increased interest in the operating system by the world's corporations will make even more “non-critical” hardware be supported in the future.
Linux 2.2 is a milestone for Linux's development. No longer is Linux a niche operating system—it is a viable solution for the masses. With support for so many new options, it's no wonder.
Text-mode, Linux's most basic output method, is also one area where Linux has changed little since the “old” days. The text-mode console of Linux 2.0 is pure in its simplicity with one obvious (and maybe striking) fact: no code for graphical primitives is in the kernel. The lowest level interface to the text-mode kernel is as simple as the file stream. Higher level functions, such as would be required for full-screen text console applications (Pine, etc.), are done through a superset of the vt100 terminal protocol. Libraries, such as ncurses, are built on top of this to simplify programming and to provide a sort of terminal abstraction. Applications written for Linux's text-mode console using this abstraction can run on just about any terminal. Scroll back and Linux's famous virtual consoles are not a sophisticated extension of those basic building blocks, but rather an extension provided because of the text-mode driver's close relationship to the VGA hardware on which it was designed.
Other utilities exist, such as SVGATextMode, which allow us to access some of the graphical capabilities of hardware in text mode. These utilities generally allow one to change text resolution, fonts and colors. However, these extensions directly interface VGA features and do not call on any extensions in the kernel. Generally, the Linux console is so modular in design that it does not notice the low-level VGA feature changes. These utilities should not be confused with SVGALib or the frame-buffer console, as they allow manipulation only within the hardware's text modes.
There is obviously quite a bit more to modern hardware than simple text modes. As mentioned before, kernel hooks are not provided for accessing these functions, but many user-space programs and libraries are available that bypass the kernel altogether to access the hardware beneath. (“User space” is a term used to describe the limited and protected “space” in which user programs run. In contrast, “kernel space” routines are generally unprotected and can cause crashes and other problems. User protections can be removed in order to allow user programs to access hardware.) Unfortunately for SVGALib, the most popular alternative, the support API provided is heavily tied to the features of the VGA hardware, making it difficult to port either the SVGA library or the end application to any other type of hardware. The other downside is that this library does not support all VGA hardware at its full potential, but that can be forgiven due to the turbulent nature of hardware design. Limitations aside, SVGALib has proven to be a stable and popular solution to the console problem and is the primary interface used in Quake and other games.
The final and most popular option for accessing the video hardware under Linux is through the X Window System, the most common GUI subsystem for UNIX. The X Window System includes an “X Server” which is similar in purpose to a Windows-style video driver. In addition to “driver” features, the X server includes code for running programs over a network and handling many GUI tasks internally. In this respect, the “driver” portion is not truly separate from the “server” portion. A program that wishes to access the video hardware would do so by communicating with the X server through its API.
The first and most obvious disadvantage of this approach is that it would be difficult, if not impossible, on some setups to run a “console” (full-screen) application via this method. Second, because of the combined driver/server features of the X system, servers tend to be very large, making it difficult to allow programs to run in low-memory situations. The third disadvantage of this approach is that it is fairly common for companies to profit by selling closed-source X servers for new hardware. However, the primary advantage of this system, having a long-standing and cross-platform pillar to base graphical applications on, seems to outweigh the negatives.
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