ASCII 25 — EM

End of Medium (EM) originally indicated the physical end of a storage medium, such as the end of a paper tape reel or the last usable position on a punched card. When a tape reader encountered EM, it knew no more valid data would follow on the current medium. While physical media markers are obsolete today, the concept of end-of-medium signaling persists in modern storage systems and streaming protocols that must distinguish between intentional data completion and error-induced stream termination.

Control characters were defined in the original 1963 ASCII standard to manage telecommunications equipment and terminal devices. Unlike printable characters representing visible symbols, control codes perform actions: initiating transmissions, acknowledging received data, triggering device alerts, and structuring information hierarchically. Of ASCII's 128 code points, 33 are designated as control characters (codes 0–31 plus 127), reflecting the standard's deep roots in telegraphy and serial communication systems. While most control codes have fallen out of daily use, several remain essential to modern computing workflows.

In the ASCII encoding table, End of Medium is assigned code point 25 in decimal (0x19 hexadecimal, 031 octal, 00011001 binary). The 7-bit ASCII standard, first published in 1963 by the American Standards Association, defines exactly 128 characters that remain the foundation of text encoding systems worldwide. UTF-8, the dominant encoding on the modern web, is fully backward compatible with ASCII — every ASCII character is encoded as the identical single byte in UTF-8, guaranteeing that End of Medium works reliably across all operating systems, programming languages, and internet protocols.