Further PC Assembler Sample: Processing the Command Tail

Reminder: The Command Tail

the characters typed on the command line following the command/program name in an MS-DOS environment.

• 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

  : 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 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), 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.

Coding General Usage Routines

As already seen in the prior programming example, subroutines are written in PC Assembler as PROC's (procedures). NEAR PROC's are included in the same code segment as the statement that calls them; FAR PROC's (which we are not using here) are in other code segments.
Assembler coding can be cryptic and challenging. Good programming technique therefore dictates that we develop Assembler routines which are internally self-documented and which are designed for re-usability.

• Sample code:

  • ;GetChar
    ;   Return next character at ES:[SI] in AL
    ;   unless the next character is a blank
    ;   in which case return a 0 in AL
    ;   or unless the next character is a
    ;   [carriage return] in which case return
    ;   255 in AL
    ;   Advance SI one byte location unless the
    ;   next character is a [carriage return]
    ;   in which case leave SI pointing to the
    ;   [carriage return]
    
    GETCHAR PROC    NEAR
            MOV     AL,ES:[SI]
            CMP     AL,0Dh
            JZ      END_OF_LINE
              INC     SI        ;advance ptr unless at EOL
              CMP     AL,20h
              JNZ     DONE_GET  ;if not a blank, its a normal char
                MOV     AL,0    ;return 0 in AL if it is a blank
              JMP     DONE_GET
    END_OF_LINE:
              MOV     AL,255  ;return 255 in AL if it is a [carriage return]
    DONE_GET:
            RET
    GETCHAR ENDP