Direct Access to Port Hardware

Back in the DOS era, PCs had serial and parallel ports controlled by separate controller chips on the motherboard. Like everything else in the machine, these controller chips could be directly accessed by any software running under DOS. By writing bit-mapped control values to the chips and creating custom interrupt service routines, one could create custom ''fine-tuned'' serial interface software, which enabled the plodding 300-character-per-second dial-up modems of that time to work as fast as they were capable. That was routine, but with some cleverness, you could make standard computer hardware do things it was not really intended to do. By studying the hardware controllers for the machine's parallel port, for example, I was able to write a two-way communications system in assembly that moved data very quickly from one computer to another through their parallel ports. (This was actually pre-PC, using CP/M for the Z80 CPU.)

Again, as with video, the requirements of multitasking demand that the operating system manage access to ports, which it does through drivers and code libraries; but unlike video, using drivers for interface to ports is actually much simpler than completely controlling the ports yourself, and I do not mourn those ''bad old days.''

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