DOS Game Resolutions
Introduction
Until VGA arrived in 1987, DOS games typically ran in 320 x 200 resolution at whatever colour depth the graphics card could support. CGA could reach 620 x 200 but only in two colours, so 320 x 200 was commonly chosen to get the maximum of 4 colours. EGA was better, reaching 640 x 350 in 16 colours but there are some curiosities with this number of lines given that the 350 lines doesn't scale well compared to the 200 lines of CGA.
Even most VGA games up to around the mid-90s continued to run in the 320 x 200 graphics mode, as it offered higher performance (refresh rates) compared to 640 x 400, especially when having to display 256 simultaneous colours. This was called "mode 13h". In DOS, these ran with a vertical refresh rate of 70 Hz.
Another common graphics mode was 320 x 240, called "mode X", with a refresh rate of 60 Hz.
Refresh Rates
The refresh rate is the number of times the display's content is refreshed per second. There are two refresh rates a monitor and graphics card manages - horizontal and vertical. The horizontal refresh rate, also known as the horizontal scan rate, is the rate at which an entire line is drawn, i.e. the electron beam travels from the far left to the far right and then back again ready to begin the next line. It is measured in kilohertz (kHz) with some common values being 15.75 Khz, 18.432 kHz, 21.85 kHz and 31.5 kHz.
The vertical refresh rate, also known as the vertical scan rate, is the time it takes for the electron beam to draw the entire screen's content and return to the top-left corner ready to start drawing the next frame. It is measured in hertz (Hz) and is typically 60 Hz or higher - anything lower than 60 Hz and your eyes will detect the gaps between refreshes resulting in a flicker which can be uncomfortable.
Naturally the more that needs to be drawn, the longer it takes. This is why the horizontal refresh rate is a big determining factor in a monitor's maximum resolution.
Aspect Ratio
As you probably already know, the aspect ratio of an image is the ratio of the number of pixels on the X-axis compared to the number of pixels on the Y-axis. DOS games that run in 320 x 200 therefore have an aspect ratio of 16:10. "Yes, but analogue monitors had a 4:3 aspect ratio, didn't they?", I hear you ask. Well, yes, so the monitor does actually display all of the pixels in the 320 x 200 image, but the displayed image is often squashed to fit - this means that each pixel isn't perfectly square. Remember we're in the era of analogue CRT monitors here, which have the advantage of being able to "size" the horizontal and vertical size the area of where the electron gun fires. This capability disappeared with the introduction of LCD displays, since they have one dot per pixel, which is either 'lit' or 'not lit' - you couldn't change the size of each dot, meaning scaling to a imperfect ratio [one that's not exactly divisible by the other] would often produce a messy image. In some cases, the display would add black bars on the left or right of the image in order to show perfectly square pixels. The 4:3 aspect ratio was the most common until the early 2000s, when 16:10 (e.g. 1280 x 800) and even later, 16:9 (1920 x 1080) became the norm.
Scan Doubling
When displayed on a CRT monitor, the 320 x 200 image would typically be "scan-doubled" in order to fill a 640 x 400 portion of the screen. Essentially, every pixel in the 320 x 200 array in the graphics card's frame buffer is drawn twice on the X-axis and twice on the Y-axis. If the graphics mode was 320 x 240, this would be scan-doubled to fill a 640 x 480 portion of the screen.
Because these were precisely twice the width and twice the height, the scaling produced a perfectly accurate picture of the original (just more blocky). You could certainly get away with this on CRT monitors because of the naturally blurry/soft characteristics of a CRT.
Some DOS games run in the maximum VGA resolution of 640 x 480, typically at 60 Hz, without any scan-doubling, so the image represented contains four times the content (twice on the X-axis and twice on the Y-axis).
Odd Resolutions
There are also games that use either a combination of resolutions, for example the main menu could be in 650 x 350 but the main game is in 320 x 200 (with scan-doubling that fills the screen). Furthermore, as games became more advanced, they moved away from the standard 'modes' in the VGA standard, creating their own unique resolutions. You may find that your graphics card can support these, but other cards might struggle to work properly in these non-standard graphics resolutions.
The following DOS games are known to have odd screen resolutions:
| Game | Resolution |
|---|---|
| Earthworm Jim 2 | 320 x 224 |
| Gauntlet | 160 x 200, 256 x 200 |
| Incredible Machine / Even More Incredible Machine | 640 x 471 and 640 x 448 |
| Jazz Jackrabbit | 320 x 199 |
| Jurassic Park | 320 x 184 |
| Lemmings / Oh No! More Lemmings | 640 x 350 w/ palette switching mid-frame |
| Linewars II | 320 x 480 |
| Pinball Dreams / Pinball Illusions* | 360 x 350 / 320 x 400 |
| Prehistorik 2 | 312 x 200 |
| Quake* | 320 x 350, 320 x 400, 320 x 480, 360 x 200, 360 x 240, 360 x 350, 360 x 400, 360 x 480 |
| Scorched Earth* | 320 x 400, 320 x 480, 360 x 480 |
| System Shock (CD version)* | 320 x 400 |