The standard screen was 80 by 25. There were two addresses you needed to know 0xb000 for monochrome displays and 0xb800 for color. For monochrome you could just blast characters/attributes to the address and everything looked great. For color you had to add a little bit of assembly so writes didn't happen when the monitor was doing certain things (or else you would get some flickering). The little hacks were all well known. Then you could build your own 'windowing' system by just maintaining separate screen buffers and having a little bit of code to combine for buffers when writing the actual hardware. In the early days everyone code was synchronous and code would start listening for keyboard events and react and repaint in a very ad hoc fashion. Mouses made things a bit more complicated as you needed to maintain a persistent model of the UI to process their events. So the UI code was simple and easy to work on, but you had to squeeze these programs into tiny memory footprints so you would spend a lot of time trying to find more memory. One of the bigger projects I worked on had a memory manager that relocated blocks to create contiguous space but since there was no OS support for things that like the code was actually updating pointer in the heap and stack - which was a great source of complicated bugs. Whoa onto anyone that tried to use a linked lists in such an environment. But yeah, it was a fun time.