Further PC Assembler Sample: Processing the Command Tail

As a further exercise in IBM PC Assembler programming, we examine a simple program to process the "command tail" as supplied by MS-DOS to all programs.
Although this example is artificially oversimplified compared to "real" applications, it demonstrates a number of methodologies including how to access data in an area not pointed to by the DS: register.

The Command Tail

The "command tail" is the string of characters typed on the command line following the command/program name in an MS-DOS environment. It is located in the Program Segment Prefix - that 0x100-byte prefix that precedes all .COM programs (and is a separate segment in .EXE programs).

Part of the PSP: starts at a byte offset of 0081h from the start of the PSP.
Initially DS: and ES: both point to PSP: For .COM programs, the PSP is part of the only segment; so all segment registers point to it. For .EXE programs, the PSP is outside of all programmer defined segments.
Terminated with a "Carriage Return" code: The text of the command tail ends with a CR character. Alternatively, the length (excluding the "Carriage Return") is contained in the byte at offset 0080h within the PSP.

Accessing Data Using the ES: Register in a .EXE program

In a typical .EXE environment, the DS: register is modified quite early in the program to point to a Data segment (a segment containing data values).

BEGIN:  MOV     AX,DATASEG
        MOV     DS,AX
        ASSUME  DS:DATASEG

In order to access values in the PSP (and particularly in the command tail) in an .EXE program, it is necessary to use the ES: register, since the ES: register is the only thing left which gives the location of the PSP.

   MOV     SI,81h      ;initialize pointer to start of command tail
   MOV     AL,ES:[SI]  ;over-ride default DS: segment usage

The Sample Problem

Display the "words" found on the command tail, one word per line.

The entire sample program can be found at tail.asm.