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Computer organization Notes

Assembly langugae Input-Output Programming

Users of the computer write programs with symbols that are defined by the programming language employed. The symbols are strings of characters and each character is assigned an 8-bit code so that it can be stored in computer memory. A binary-coded character enters the computer when an INP (input) instruction is executed. A binary-coded character is transferred to the output device when an OUT (output) instruction is executed. The output device detects the binary code and types the corresponding character. Table (a) lists the instructions needed to input a character and store it in memory. The SKI instruction checks the input flag to see if a character is available for transfer. The next instruction is skipped if the input flag bit is 1. The INP instruction transfers the binary-coded character into AC (0-7). The character is then printed by means of the OUT instruction. A terminal unit that communicates directly with a computer does not print the character when a key is depressed. To type it, it is necessary to send an OUT instruction for the printer. In this way, the user is ensured that the correct transfer has occurred. If the SKI instruction finds the flag bit at 0, the next instruction in sequence is executed. This instruction is a branch to return and check the flag bit again. Because the input device is much slower than the computer, the two instructions in the loop will be executed many times before a character is transferred into the accumulator.
Table (b) lists the instructions needed to print a character initially stored in memory. The character is first loaded into the AC. The output flag is then checked. If it is 0, the computer remains in a two-instruction loop checking the flag bit. When the flag changes to 1, the character is transferred from the accumulator to the printer.

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