* The DOS Program 
 
 The actual DOS program is the file IBMDOS.COM or MSDOS.SYS. It provides a high- 
level interface for user (application) programs. This program consists of file 
management routines, data blocking/deblocking for the disk routines, and a 
variety of built-in functions easily accessible by user programs. 
 When a user program calls these function routines, they accept high-level 
information by way of register and control block contents. When a user program  
calls DOS to perform an operation, these functions translate the requirement 
into one or more calls to IBMBIO.COM, MSDOS.SYS or system hardware to complete 
the request.