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То есть: пишется обработчик прерывания, вешается
резидентом, запускается программа, а затем читается
лог-файл, куда резидент дублирует траффик?

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Portmon is a utility that monitors and displays all serial and parallel port activity on a system. It has advanced filtering and search capabilities that make it a powerful tool for exploring the way Windows works, seeing how applications use ports, or tracking down problems in system or application configurations.

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Да.

То, что ниже рекомендует tristes_tigres
в принципе то, но под виндовс.
Нужен аналог под dos.

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DOSBox is capable of timing-compatible implementation of the serial ports, and can run older hardware and software dependent on such; however, some USB devices that are supported by the host OS can act as a replacement for older serial port devices when using the emulator.

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