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; minimalist Watcom C startup routine - TINY memory model ONLY - (.COM files)
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; kindly contributed by Bernd Boeckmann
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.8086
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.8086
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STACK_SIZE = 2048
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STACK_SIZE = 2048
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      dgroup group _TEXT,_DATA,CONST,CONST2,_STACK,_BSS
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      DGROUP group _TEXT,_DATA,CONST,CONST2,_BSS,_EOF
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      extrn   "C",main : near
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      extrn   "C",main : near
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;      public _cstart_, _small_code_, __STK
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;      public _cstart_, _small_code_, __STK
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      public _cstart_, _small_code_
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      public _cstart_, _small_code_
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      ; the entire memory (not only the process' block) to the program, which
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      ; the entire memory (not only the process' block) to the program, which
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      ; makes it impossible to allocate memory or run child processes.
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      ; makes it impossible to allocate memory or run child processes.
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      ; for this reasons it is beneficial to resize the memory block we occupy
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      ; for this reasons it is beneficial to resize the memory block we occupy
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      ; into a more reasonable value
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      ; into a more reasonable value
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      ; step 1: if SP is higher than my top_of_stack, then set SP explicitely
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      ; step 1: if SP > COM size + stack size, then set SP explicitely
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      ; POTENTIAL error: offset DGROUP:_EOF + STACK_SIZE may overflow!
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      cmp sp, top_of_stack
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      ; Has to be detected at linking stage, but WLINK does not catch it.
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      jle resizemem
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    adjustsp:
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      cmp sp, offset DGROUP:_EOF + STACK_SIZE
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      jbe resizemem   ; JBE instead of JLE (unsigned comparison!)
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      mov sp, top_of_stack
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      mov sp, offset DGROUP:_EOF + STACK_SIZE
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      ; step 2: resize our memory block to sp bytes (ie. sp/16 paragraphs)
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      ; step 2: resize our memory block to sp bytes (ie. sp/16 paragraphs)
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    resizemem:
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    resizemem:
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      mov ah, 4ah
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      mov ah, 4ah
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      mov bx, sp
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      mov bx, sp
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      shr bx, 1
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      shr bx, 1
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      shr bx, 1
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      shr bx, 1
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      inc bx
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      inc bx
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      int 21h
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      int 21h
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      ; clear _BSS to be ANSI C conformant
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      mov di, offset DGROUP:_BSS
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      mov cx, offset DGROUP:_EOF
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      sub cx, di
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      xor al, al
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      cld
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      rep stosb
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      call  main
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      call  main
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      mov   ah, 4ch
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      mov   ah, 4ch
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      int   21h
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      int   21h
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; Stack overflow checking routine is absent. Remember to compile your
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; Stack overflow checking routine is absent. Remember to compile your
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CONST2  ends
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CONST2 ends
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_BSS    segment word public 'BSS'
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_BSS  segment word public 'BSS'
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_BSS    ends
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_BSS  ends
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_STACK  segment para public 'BSS'
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        db      (STACK_SIZE) dup(0) ; set this explicitely to zero, otherwise
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                                    ; static variables are not properly
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                                    ; initialized. this makes the COM file
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; _EOF should be the last segment in .COM (see linker memory map)
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                                    ; much bigger, but it is irrelevant if it
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                                    ; is UPXed afterwards anyway. If you care,
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                                    ; then you may zero out this area in
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; if not, someone introduced other segments, or startup.obj is not the first
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                                    ; code instead (before calling main)
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; linker object file
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        top_of_stack:
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_EOF  segment word public
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_STACK  ends
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_EOF  ends
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_TEXT ends
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_TEXT ends
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      end _cstart_
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      end _cstart_
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