1. Field of the Invention
The subject invention pertains to the field of video displays and more particularly to a device that provides compatibility between a computer adapted for use with a first video display format defined by a first set of parameters and software originally designed for use with a computer having a second video format defined by a second set of parameters.
2. Description of the Prior Art
A wide variety of computers and particularly personal computers have been introduced over the past several years. Of the many that have been sold and manufactured, several have emerged as defacto standards due to their very large popularity. The high popularity of particular computers has in turn encouraged various third party software vendors to write many programs adapted to the popular machines.
Heretofore, computer manufacturers have had to decide whether to design a totally new machine having new and improved features while sacrificing some or all compatibility with the many software packages designed for a standard machine or to design essentially a copy of the standard machine, capable of running most if not all of the software previously designed for the standard machine. If the choice was made for compatibility, the resulting computer generally had few if any advantages or improved capabilities over the machine which it emulated.
Recently, computer users have demanded improved graphic capabilities in their machines to provide for more readable, informative and eye catching displays. The improved displays make better presentations of data and decrease eye strain. The improved displays thus encourage the use of a particular vendor's software as well as the particular computer with which the software is used. Therefore, one area in which manufacturers have striven to make improvements is in the computer display. The displays used with most computers are cathode-ray tube (CRT) based. Their resolution is usually specified in the number of vertical and horizontal pixels (dots or picture units) that they can distinctly display.
One personal computer that has enjoyed a large degree of consumer acceptance and therefore has had a large quantity software written for it is the IBM PC, manufactured by International Business Machines Corp., Armonk, N.Y. Due to the tremendous acceptance of the IBM PC, particularly by commercial users, many manufacturers have found it desirable to design a computer compatible with the IBM Pc yet have striven to improve the machine in several areas. One such area is in its display.
The IBM PC uses a display that can show 640.times.200 pixels. This has been shown to be just barely adequate for office graphics. Further, this machine, like others, displays a character formed from a matrix of 8.times.8 pixels. In order to obtain letters having true descenders and yet still have sufficient space between consecutive lines of text on the display, it is necessary, when using a 8.times.8 letter, to use a very small, low resolution letter since in order to provide for the true descenders, less than the full 8.times.8 pixel matrix is used.
It is thus preferable to use a pixel matrix for each character having more than 8.times.8 pixels. One such matrix that may be used is an 8.times.16 matrix having 8 pixels in the horizontal direction and 16 pixels in the vertical direction. As will be clear to those skilled in the art, such a matrix can retain compatibility with an 8.times.8 pixel matrix merely by scanning consecutive pairs of rows identically.
Most personal computer display screens use an 80.times.25 character screen. That is, up to 80 columns of characters and 25 lines or rows of characters can be displayed on a screen.
For 8.times.8 characters, 640.times.200 pixels can be displayed on the screen. To use an 8.times.16 character, the computer must be capable of displaying 640.times.400 pixels.
Generally, there are 3 levels of interface with a computer's display circuitry. These are (1) hardware, (2) the computer's basic input/output system and (3) the computer's operating system.
In a machine such as the IBM PC, the operating system does not support all screen functions well. It does not allow instructions to the screen since once the screen parameters are set they usually cannot be reset. Consequently, much software writes directly to the hardware or to the basic input/output system and bypasses the operating system.
While a program which utilizes only the operating system in order to write to the computer screen would be usable with a compatible machine having a higher resolution, any program that directly programmed the hardware would show a scrambled display.
Heretofore, a manufacturer of a computer who wished to make a product compatible with software designed for an IBM PC and its standard 640.times.200 pixel screen had to sacrifice a degree of compatibility if he used a higher resolution screen. Typically, manufacturers of such compatible computers would just simply choose to forego the advantages offerred by a higher resolution screen in order to maintain compatibility.
It is thus an object of the present invention to provide a device which permits the use of software designed for a standard computer having a relatively low resolution screen with another computer having a higher resolution screen.
It is a further object of the invention to provide a device which permits the use of the higher resolution screen with software designed for it, yet still remain completely compatible with software requiring a lower resolution screen.
It is still another object to provide a personal computer having finely detailed characters when used with software operating from the computer operating system.
These and other objects of the invention will be more apparent upon reference to the following specification and the annexed drawings.