Editing TRSDOS (section)

==Example code==
The following [[Z80]] [[assembly language|assembler]] source code is for a subroutine named {{mono|HELLO_WORLD}}. This is an introductory program which will print a message to the video display and then exit. Though simple, it demonstrates how an assembler source code is formatted and interacts with the display hardware through TRSDOS. The example uses TRS-80 Model 4 running TRSDOS/LS-DOS 6.x, produced by Tandy/Radio Shack and Logical Systems, with most any assembler sold by Radio Shack or Misosys Inc. (Series I/EDTASM, ALDS, EDAS or MRAS with {{code|-GC}} switch).
{|
|-valign="top"
|<syntaxhighlight lang="text" highlight="13" style="font-size:85%">
            
            
            
            
            
            
            
            
            
    
  
  
  
  
  
3000 3E 69
3002 EF 
3003 21 14 30
3006 3E 0A
3008 EF
3009 0E 0D
300B 3E 02
300D EF
300E 21 00 00
3011 3E 16
3013 EF
3014 48 65 6C 6C
3018 6F 2C 20 77
301C 6F 72 6C 64
3020 21 0D 0D
3023
</syntaxhighlight>
|<syntaxhighlight lang="tasm" highlight="13" style="font-size:85%">
 ; hello_world_TRS-80 Model 4
 ; Print "Hello, world!" then exit to TRSDOS Ready
 ;
 ; Entry registers: none
 ; Return registers: none
 ;
 ; values below are decimal unless suffixed by H
 ; the term "pseudo-op" means same as "assembler directive", is not Z80 code
 ;
 @CLS         EQU     105                 ; EQU pseudo-op sets @CLS text label = TRSDOS Supervisor Code
 @DSP         EQU     2                   ; set @DSP text label = TRSDOS SVC to send char to display
 @DSPLY       EQU     10                  ; set @DSPLY text label = SVC for text string display
 @EXIT        EQU     22                  ; set @EXIT text label = SVC for returning to TRSDOS Ready
;
              ORG     3000H               ; ORG pseudo-op sets standard start address under TRSDOS 6.x
START         LD      A,@CLS              ; CLS = Clear Screen, erase display and home cursor
              RST     40                  ; Z80 Restart instruction for system SVC processor, execute @CLS
              LD      HL,MSG              ; point to message string with HL (required by @DSPLY SVC)
              LD      A,@DSPLY            ; send text string to *DO Display Output device (video screen)
              RST     40                  ; execute @DSPLY SVC (13d byte needed to terminate string)
              LD      C,13                ; now send another carriage return, needed in reg C 
              LD      A,@DSP              ; to set off Hello message from TRSDOS Ready prompt
              RST     40                  ; once we exit to system
              LD      HL,0                ; indicate no error condition to Command Interpreter
              LD      A,@EXIT             ; return to TRSDOS Ready
              RST     40                  ; (stack integrity maintained, could as well have used a RET!)
 MSG          DB      'Hello, world!',13  ; DB = Define Byte pseudo-op assembles ASCII string to Z80 memory
 CR           DB      13                  ; 13d byte is ASCII carriage return char, moves cursor down
 
              END     START               ; END pseudo-op required by assembler 
                                          ; to produce transfer record to program entry point

</syntaxhighlight>
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