XMODEM
| Purpose | file transfer protocol |
|---|---|
| Developer(s) | Ward Christensen |
| Introduced | 1977 |
| Influenced | YMODEM, many others |
| Hardware | modems |
XMODEM is a simple file transfer protocol developed as a quick hack by Ward Christensen for use in his 1977 MODEM.ASM terminal program. It allowed users to transmit files between their computers when both sides used MODEM. Keith Petersen made a minor update to always turn on "quiet mode", and called the result XMODEM.
XMODEM became extremely popular in the early bulletin board system (BBS) market, largely because it was so simple to implement. It was also fairly inefficient, and as modem speeds increased this problem led to the development of a number of modified versions of XMODEM to improve performance or address other problems with the protocol. Christensen believed his original XMODEM to be "the single most modified program in computing history".Chuck Forsberg collected a number of modifications into his YMODEM protocol, but poor implementation led to a further fracturing before they were re-unified by his later ZMODEM protocol.
XMODEM, like most file transfer protocols, breaks up the original data into a series of "packets" that are sent to the receiver, along with additional information allowing the receiver to determine whether that packet was correctly received.
The original XMODEM used a 128-byte data packet, the basic block size used on CP/M floppy disks. The packet was prefixed by a simple 3-byte header containing a <SOH> character, a "block number" from 0-255, and the "inverse" block number—255 minus the block number. Block numbering starts with 1 for the first block sent, not 0.
The packet was also suffixed with a single-byte checksum of the data bytes. The checksum was the sum of all bytes in the packet modulo 256. The modulo operation was easily computed by discarding all but the eight least significant bits of the result, or alternatively on an eight bit machine, ignoring arithmetic overflow which would produce the same effect automatically. In this way the checksum was restricted to an eight bit quantity which was able to be expressed using a single byte. For example, if this checksum method was used on a tiny data packet containing only two bytes carrying the values 130 and 130, the total of these codes is 260 and the resulting checksum is 4.
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