Mode 13h (the "h" here stands for "hexadecimal," so this is actually VGA mode 19) is a standard 256 colour mode on IBM's VGA graphics hardware. It features a resolution of 320×200 pixels and was used extensively in computer games and art/animation software of the late 1980s and early- to mid-1990s. Mode 13h provides a straightforward manner of access (nicknamed chunky graphics) to video memory at the expense of some useful features the VGA hardware was capable of providing. Due to the aspect ratio of a 320×200 resolution, Mode 13h does not have square pixels. Video Graphics Array (VGA) is a computer display standard first marketed in 1987 by IBM. VGA belongs to a family of earlier IBM video standards and largely remains backward compatible with them. ... International Business Machines Corporation (IBM, or colloquially, Big Blue) (NYSE: IBM) (incorporated June 15, 1911, in operation since 1888) is headquartered in Armonk, New York, USA. The company manufactures and sells computer hardware, software, and services. ... Video Graphics Array (VGA) is a computer display standard first marketed in 1987 by IBM. VGA belongs to a family of earlier IBM video standards and largely remains backward compatible with them. ... A pixel (a contraction of picture element) is one of the many tiny dots that make up the representation of a picture in a computers memory. ... The 1980s decade refers to the years from 1980 to 1989, inclusive, informally sometimes including the years 1979, 1990 and 1991. ... See also 1990s, the band The 1990s decade refers to the years from 1990 to 1999, inclusive, sometimes informally including popular culture from 2000 and 2001. ... In chunky or packed pixel frame buffer organization, the bits defining each pixel are grouped together. ... The aspect ratio of an image is its displayed width divided by its height (usually expressed as x:y). For instance, the aspect ratio of a traditional television screen is 4:3, or 1. ...

Mode 13h is something of a curiosity, because the VGA is a planar device from a hardware perspective, and somewhat unsuited for chunky graphics operation. It was made possible by a hardware trick. In order to understand how Mode 13h was possible, one must consider the VGA's memory layout: Something is called planar if it is made up of flat planes, or pertaining to planes. ...

The VGA's 256K of video memory is not directly available to the programmer and is accessed via a 64KB 'window' in the PC's real mode address space. The Video RAM is actually a bank of 4 planes, 64KB each. By manipulating the VGA's control registers the programmer can decide how some or all of these 4 planes will be affected by each write or read operation. In 256 colour mode each plane represents a pixel. For example, on the first line of the screen pixels 0, 4, 8, 12 etc. are all contained in memory plane 0, while pixels 1, 5, 9, 13 etc. are contained in memory plane 1 and so on. A single byte written to the 64K window therefore can address up to 4 pixels at once. The programmer through a combination of the 64K window available, and the selection of any of the four planes (an effective extra two bits of address space) allows the manipulation of the total 256K video memory. Real mode (also called real address mode in Intels manuals) is an operating mode of 80286 and later x86-compatible CPUs. ...

Mode 13h, however, allows the programmer to access the VGA in "chunky" fashion, where every byte represents a single pixel. This is made possible because in Mode 13h the hardware uses the last two bits of the address to select the plane to write to. The upshot is that the programmer has a simple access model; the downside is that three quarters of the video memory is now inaccessible. A simple bank switching method could have made the remaining memory available, and indeed bank switching was made available on Super VGAs to access the larger VRAMs these devices sported. A bit refers to a digit in the binary numeral system (base 2). ... Bank switching (also known as paging, but only loosely related to the ordinary meaning of paging in computing) was a technique common in 8-bit microcomputer systems, to increase the amount of addressable RAM and ROM without extending the address bus. ... Super Video Graphics Array, almost always abbreviated to Super VGA or just SVGA is a broad term that covers a wide range of computer display standards. ...

Although never documented by IBM, an alternative planar 256 colour mode is available by "unchaining" the video ram; video modes created using this technique are collectively called Mode X. Mode X is an undocumented video graphics display mode of the IBM VGA graphics hardware that was popularized by Michael Abrash, first published in July 1991 in Dr. Dobbs Journal, republished in chapters 47-49 of Abrashs Graphics Programming Black Book, which is now freely available online in...

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