The first widely used file transfer protocol for dial-up modems on personal computers. Developed by Ward Christensen for CP/M machines, Xmodem programs supported the earlier checksum method and the subsequent CRC method of error detection. Xmodem transmitted 128-byte blocks. Xmodem-1K improved speed with 1KB blocks. Xmodem-1K-G transmitted without acknowledgment for error-free channels or when modems were self correcting, but transmission was cancelled upon any error. See
Zmodem.