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 header was followed by the 128 bytes of data, and then a single-byte checksum. The complete packet was thus 132 bytes long, containing 128 bytes of payload data, for a total channel efficiency of about 97%.