Editing XMODEM (section)

==Packet structure==

The original XMODEM used a 128-byte data packet, the [[Block (data storage)|block size]] used on [[CP/M]] [[floppy disk]]s. The packet was prefixed by a simple 3-byte header containing a <kbd><[[C0 and C1 control codes|SOH]]></kbd> character, a "block number" from 1-255, and the "inverse" block number—255 minus the block number. Block numbering starts with 1 for the first block sent, not 0. The header was followed by the 128 bytes of data, and then a single-byte [[checksum]]. The checksum was the sum of all 128 data bytes in the packet [[modulo operation|modulo]] 256. The complete packet was thus 132 bytes long, containing 128 bytes of [[Payload (computing)|payload data]], for a total [[channel efficiency]] of about 97%.

The file was marked "complete" with a <kbd><[[End-of-transmission character|EOT]]></kbd> character sent after the last block. This character was not in a packet, but sent alone as a single byte. Since the file length was not sent as part of the protocol, the last packet was padded out with a "known character" that could be dropped. In the original specification, this defaulted to <kbd><nowiki><SUB></nowiki></kbd> or 26 decimal, which CP/M used as the end-of-file marker inside its own disk format. The standard suggested any character could be used for padding, but there was no way for it to be changed ''within the protocol'' itself – if an implementation changed the padding character, only clients using the same implementation would correctly interpret the new padding character.