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XMODEM
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===Minor problems=== XMODEM was written for [[CP/M]] machines, and bears several marks of that [[operating system]]. Notably, files on CP/M were always multiples of 128 bytes, and their end was marked within a block with the <kbd><EOT></kbd> character. These characteristics were transplanted directly into XMODEM. However, other operating systems did not feature either of these peculiarities, and the widespread introduction of [[MS-DOS]] in the early 1980s led to XMODEM having to be updated to notice either a <kbd><EOT></kbd> ''or'' <kbd><EOF></kbd> as the end-of-file marker. For some time it was suggested{{Weasel inline|date=April 2025}} that sending a <kbd><CAN></kbd> character instead of an <kbd><ACK></kbd> or <kbd><NAK></kbd> should be supported in order to easily abort the transfer from the receiving end. Likewise, a <kbd><CAN></kbd> received in place of the <kbd><SOH></kbd> indicated the sender wished to cancel the transfer. However, this character could be easily "created" via simple noise-related errors of what was meant to be an <kbd><ACK></kbd> or <kbd><NAK></kbd>. A double-<kbd><CAN></kbd> was proposed to avoid this problem, but it is not clear if this was widely implemented.
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