Way way back in the day Itried to learn C from a game programming book. If I recall correctly, one of the first things your game "engine" would do would be to switch display modes to render. This involved a bit of asm to switch to a 640x480 display mode (mode 13 maybe?) so you could draw directly to the screen. Something like that.
My question is, what is the modern equivalent of this? I'm interested in writing a command line program that does something similar; drops into some kind of raster mode for me to draw to, but, I do not assume that my program would be running under some kind of window manager like kde, unity,Aqua etc.
Would this be something that OpenGL could provide (or does OpenGL assume a window manager too). My proposed program isn't a game, but would ideally start with a basic clear screen that I can draw primitives (2d lines, circles rects etc)
Cheers!
Modern operating systems don't give programmers as convenient access to low-level graphics routines as they used to. Partially, this is due to the advent of the GPU, which makes utilizing the graphics hardware a much more significant challenge than if you only had a CPU. The other reason is as window managers have gotten more and more complex, the graphical sandbox each operating system gives a programmer is more constrained.
That being said, OpenGL is definitely worth looking at. Its cross-platform, versatile, and automatically utilizes any hardware available (including the graphics card). OpenGL itself doesn't directly provide access to a windowing context, but you can easily create that with the OpenGL utility library (GLUT). OpenGL is however very low-level, you'll have to deal with frame buffers and flushing and bit masks and all sorts of low-level nonsense that can make OpenGL development a nightmare if you haven't done it before.
If I were starting a project, I would probably want a more robust graphics environment that provides drawing functions and windowing out of the box. Both SDL and SFML provide low-level graphics APIs that will be a little more friendly to start with. They are both implemented on top of OpenGL so you can use any of the OpenGL features when you want to, but you don't have to worry about some of the more tedious details.
As a side note, C might not be the best language to get started with graphics programming these days. If you want a really simple graphics environment that is becoming more relevant every day, you might want to check out what the web has to provide. JavaScript and the HTML5Canvas provide a very simple interface for drawing primitives, images, etc.