MS-DOS copies the two default file control blocks pointed to by the EXEC parameter block into the child program's PSP at offsets 005CH and 006CH. To emulate the function of COMMAND.COM from the child program's point of view, the parent program should use Int 21H Function 29H (the system parse-filename service) to parse the first two parameters of the command tail into the default file control blocks before invoking the EXEC function.
File control blocks are not much use under MS-DOS versions 2 and 3, because they do not support the hierarchical file structure, but some application programs do inspect them as a quick way to get at the first two switches or other parameters in the command tail. Chapter 8 discusses file control blocks in more detail.