Default command line in map file

The first sector of the map file is reserved for a default command line. Unless the user invokes the boot prompt by pressing a shift key or unless an externally provided command line is present, the command line in the map file is interpreted as if it had been typed on the keyboard. The first two bytes of the first sector of the map file have to contain the magic number <#519#> DC_MAGIC<#519#> (0xF4F2) in little-endian byte order. They are followed by a NUL-terminated string with a maximum length of 510 bytes, including the NUL. Note that the boot loader limits command lines to 78 characters after removing duplicate spaces. The command line is disabled by either clobbering the magic number or by using an empty string (i.e. only a NUL byte) as the command line.