Listing 4

; Procedure DoEnviron
; Purpose Convert a string to an environment variable
; Input   String in "string"
; Output  String in "WHAT" environment variable;
;       AX has 0 for success, nonzero for failure

DoEnviron PROC
     call GetEnv    ; Get environment size, length, address
     mov  dx,ax     ; Save size and length
     mov  bx,cx

; Find "WHAT="

     sub  di,di     ;Point to start
     sub  al,al     ;Search for zero
     mov  si, OFFSET what     ; Point source at "WHAT="


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findwh:   repne     scasb     ; Search
     cmp  BYTE PTR es:[di],)  ; If double null, end of environment         
     je   gotend
     jcxz noroom              ; Error if not found
     push di                  ; Save
     push cx
     mov  si,OFFSET what      ;Load address and length of "what"
     mov  cx, lwhat           ; for comparison
     repe cmpsb               ; Compare
     mov  si,di               ; Make copy
     pop  cx                  ; Restore
     pop  di
     jnz  findwh

; Find end of "WHAT" variable

     xchg      di,si
     repne     scasb          ; Find end of environment variable
     xchg      si,di          ; Point source to next variable

; Calculate characters left to write
     
     mov  cx,bx               ; Load total characters
     sub  cx,si               ; Subtract finished to get left

; Move everything back to overwrite "WHAT="

movenv:   push ds             : Save DS 
     mov  ax,es               ; Copy to ES
     mov  ds,ax
     rep  movsb               ; Copy
     mov  BYTE PTR es:[di],0  ; Put null at end in case of error
     pop  ds   ; Restore

; Check environment space

gotend:   mov  al,actual      ; Load length of string
     sub  ah,ah               ; Clear top
     add  ax,lwhat            ; Add length of name
     add  ax,di               ; Add position to get final length
     cmp  ax,dx               ; Is it longer than environment?
     jge  noroom              ; Yes? Quit

: Put WHAT= at end

     mov  si,OFFSET what      ; Load address and length of what
     mov  cx,lwhat
     rep  movsb

; Put new string at end

     mov  si,OFFSET string    ; Load address and length of string
     mov  cl,actual
     rep  movsb
     mov  WORD PTR es:[di],0  ; Put double null at end


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     sub  ax,ax               ; Return 0 for success
     ret

noroom:   inc  ax             ; Return nonzero for fail
     ret
DoEnviron ENDP

; Procedure GetEnv
; Purpose   Find and measure the environment
; Input     None
; Output    Segment of environment in ES, size in AX, length in CX

GetEnv    PROC
     mov  dx,es:10h ; Load segment of COMMAND.COM 
     mov  es,dx     ; into ES
     mov  ax,es:2Ch ; Load COMMAND.Com's environment
     or   ax,ax     ; Is it 0?
     jnz  secondry  ; No? This is a secondary command
                    ; and we have its environment in AX
     dec  dx        ; Yes? This is original COMMAND.COM
     mov  es,dx     ; so point ES to paragraph before PSP
     add  dx,es:03  ; Offset of environment is 3 bytes in
     add  dx,2      ; Adjust it back to PSP
     mov  ax,dx     ; Put it in AX
secondry:

; Note:
; CodeView cannot debug the previous section of code, because the PSP
; addresses checked by the code are those passed from DOS to CodeView,
; not addresses passed from DOS to the program.  To debug with CodeView,
; find the actual address of the environment:

;    S 500:0 L FFFF "COMSPEC="

; When you find the actual address, hard code it into your program:          

;    mov  ax,110Ch  ; Debug line

; Comment the line out for final assembly after debugging.

     mov  si,ax     ; Save a copy
     sub  dx,dx     ; Clear DX for multiply
     dec  ax        ; Get paragraph before environment
     mov  es,ax     ; Load into DS
     mov  ax,es:03  ; Size in paragraphs is at byte 4
     mov  cx, 16    ; Multiply by 16
     mul  cx
     mov  es,si     ; Restore environment address
     sub  di,di     ; Point to start
     mov  cx,ax     ; Load maximum count (size of 
     mov  bx,ax     : environment) and save a copy
     sub  ax,ax     : Search for double null
null2:    repne     scasb     ; Look for null
     jz   noerr     ; If not out of space, continue
     sub  ax,ax     ; else error (return 0)


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     jmp  error2
noerr:    cmp  BYTE PTR es: [di],0 ; Is it double null?
     jne  null2     ; No? Look again
     mov  cx,di     ; Yes? Save length in CX
     mov  ax,bx     ; Reload size to AX

     ret
GetEnv    ENDP