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The KMS does only the mode setting. Mode setting is a software operation that activates a display mode (screen resolution, color depth, and refresh rate) for a computer's display controller by using VESA BIOS Extensions or UEFI Graphics extensions (on more modern computers). The display mode is set by the kernel.
The graphics display resolution is also known as the display mode or the video mode, although these terms usually include further specifications such as the image refresh rate and the color depth. The resolution itself only indicates the number of distinct pixels that can be displayed on a screen, which affects the sharpness and clarity of the ...
Note the DE-9 connector, cryptic mode switch, contrast and brightness controls at front, and the V-Size and V-Hold knobs at rear, which allow the control of the scaling and signal to CRT refresh rate synchronization respectively. Various computer display standards or display modes have been used in the history of the personal computer.
The eye's perception of display resolution can be affected by a number of factors – see image resolution and optical resolution. One factor is the display screen's rectangular shape, which is expressed as the ratio of the physical picture width to the physical picture height. This is known as the aspect ratio. A screen's physical aspect ratio ...
Mode 13h provides programmers with a linear 320 × 200 block of video memory, where each byte represents one pixel. This allows ease of programming at the expense of access to other useful features of the VGA hardware. Given the aspect ratio of a 320 × 200 resolution screen for use on a 4:3 display, Mode 13h does not have square pixels. [2]
Furthermore, on a modern x86 system, BIOS calls can only be performed in Real mode, or Virtual 8086 mode. v8086 is not an option in Long mode. This means that a modern operating system, which operates in Protected mode (32 bit), or Long mode (64 bit), would need to switch into real mode and back to call the BIOS - a hugely expensive operation.
Technically, this mode is not a graphics mode, but a tweak of the 80 × 25 text mode. The character cell height register is changed to display only two lines per character cell instead of the normal eight lines. This quadruples the number of text rows displayed from 25 to 100. These "tightly squeezed" text characters are not full characters.
Normal mode frames the 4:3 video to the 16:9 picture area by displaying it in its original aspect ratio, with vertical gray or black bars on both sides of the screen. The disadvantage of this method is the fact that the image is small by virtue of not using the entire width of the screen. This is also known as the 4:3 mode.