Microsoft has made the MS DOS source code available on GitHub. At the time of this writing, the 1.25, 2.0, and 4.0 versions are available. For those that ever ran it, browsing through it shows some interesting attributes. 85% of the source code is x86 assembler and 13.1% is in C. Typically, programs on MS DOS ran in the same memory space (no surprise, since these versions of MS DOS did not support multi-tasking). In modern development the mechanism that a program used for calling an operating system is abstracted away. With a lot of this source code being written in x86 assembler, the communication between a user program and operating system functions is laid bare. Communication and function calls often occurred through dedicated processor interrupts. Though modern processors still use interrupts (especially for hardware events) much of our communication with modern operating systems don’t have to rely on these. Chances are that you don’t have the equipment to write the compiled images to a disk (do you even have a floppy disk?). But I think it is worth looking through if you ever ran it at any point in your life.

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