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; minimalist Watcom C startup routine - TINY memory model ONLY - (.COM files)
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; WMINICRT - minimal runtime for (Open) Watcom C to generate DOS .COM files
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;
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; The default behaviour of the runtime is:
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;   - sets up a stack of 400H bytes
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;   - releases additional memory beyond stack to DOS
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;   - panics on initialization, if not enough memory for stack size
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;   - panics if running out of stack during execution
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;
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; WASM definitions:
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;   STACKSIZE=<value>     define stack size other than 400h
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;   NOSTACKCHECK          do not assemble __STK function
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;   STACKSTAT             remember the minimum SP, maintained by __STK__
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;                         and exported via _stack_low_.
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;
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; To build an executable without build- and linker scripts run:
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;   wasm startup.asm
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;   wcc -0 -bt=dos -ms -os -zl your_c_file.c
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;   wlink system dos com file startup,your_c_file
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; To change the stack size, add -DSTACKSIZE=<value> to the wasm call
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;
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; kindly contributed by Bernd Boeckmann
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; To compile without stack checking:
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;   a) compile this startup file with wasm -DNOSTACKCHECK startup.asm
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;   b) call the C compiler with -s flag
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;
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; To build a debug version of your program:
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;   wasm -d1 startup.asm
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;   wcc -bt=dos -ms -d2 -zl your_c_file.c
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;   wlink system dos com debug all option map file startup,your_c_file
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;
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; TODO:
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;   - display stack statistics on program termination if STACKSTAT is enabled
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;   - proper Makefiles
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;   - many more (optional) things while keeping it small :-)
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.8086
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.8086
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IFNDEF STACKSIZE
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STACK_SIZE = 2048
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STACKSIZE = 400h
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ENDIF
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      DGROUP group _TEXT,_DATA,CONST,CONST2,_BSS,_EOF
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      DGROUP group _TEXT,_DATA,CONST,CONST2,_BSS,_STACK
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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_
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      public _cstart_, _small_code_
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_TEXT segment word public 'CODE'
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_TEXT segment word public 'CODE'
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      org   100h
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      org   100h
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_small_code_ label near
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_small_code_ label near
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_cstart_:
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_cstart_ proc
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      ; DOS puts the COM program in the largest memory block it can find
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      ; DOS puts the COM program in the largest memory block it can find
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      ; and sets SP to the end of this block. On top of that, it reserves
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      ; and sets SP to the end of this block. On top of that, it reserves
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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 > COM size + stack size, then set SP explicitely
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    @verify_stack_size:
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      ; POTENTIAL error: offset DGROUP:_EOF + STACK_SIZE may overflow!
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      cmp sp,offset DGROUP:_stack_end_
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      ; Has to be detected at linking stage, but WLINK does not catch it.
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    adjustsp:
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      ja @resize_mem
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      cmp sp, offset DGROUP:_EOF + STACK_SIZE
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      mov dx,offset @memerr
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      jbe resizemem   ; JBE instead of JLE (unsigned comparison!)
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      jmp _panic_
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      mov sp, offset DGROUP:_EOF + STACK_SIZE
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      @memerr db 'MEMERR$'
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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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    @resize_mem:
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      mov sp,offset DGROUP:_stack_end_
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      IFDEF STACKSTAT
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        mov [_stack_low_],sp
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      ENDIF STACKSTAT
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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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      add bx,0fh
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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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      shr bx, 1
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      shr bx,1
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      inc bx
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      shr bx,1
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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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      ; clear _BSS to be ANSI C conformant
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      mov di, offset DGROUP:_BSS
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      mov di,offset DGROUP:_BSS
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      mov cx, offset DGROUP:_EOF
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      mov cx,offset DGROUP:_STACK
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      sub cx, di
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      sub cx,di
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      shr cx,1
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      xor al, al
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      xor ax,ax
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      cld
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      cld
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      rep stosb
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      rep stosw
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      call main
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      mov ah,4ch
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      int 21h
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_cstart_ endp
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_panic_ proc
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      ; output error message in DX, then terminate with FF
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      call  main
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      mov ah,9
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      mov   ah, 4ch
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      int 21h
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      mov ax,4cffh      ; terminate if not enough stack space
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      int   21h
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      int 21h
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_panic_ endp
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      IFNDEF NOSTACKCHECK
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public __STK
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__STK proc
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; Stack overflow checking routine is absent. Remember to compile your
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      ; ensures that we have enough stack space left. the needed bytes are
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      ; given in AX. panics if stack low.
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      sub ax,sp         ; subtract needed bytes from SP
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      neg ax            ; SP-AX = -(AX-SP)
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      cmp ax,offset DGROUP:_STACK - 2 ; -2 is to compensate for __STK ret addr
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; programs with the -s option to avoid referencing __STK
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      jae @l1           ; enough stack => return, else panic
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      int 3             ; trap into debugger
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      mov dx,offset @stkerr
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      add sp,200h       ; make sure we have enough stack to call DOS
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      jmp _panic_
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      @stkerr db 'STKERR$'
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;__STK:
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@l1:
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      IFDEF STACKSTAT   ; update lowest stack pointer if statistics enabled
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        cmp [_stack_low_],ax
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        jbe @r
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        mov [_stack_low_],ax
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      ENDIF STACKSTAT
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@r:
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;      ret
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      ret
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__STK endp
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      ENDIF
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_DATA segment word public 'DATA'
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_DATA segment word public 'DATA'
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_DATA ends
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_DATA ends
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CONST segment word public 'DATA'
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CONST segment word public 'DATA'
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CONST2 segment word public 'DATA'
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CONST2 segment word public 'DATA'
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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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      IFDEF STACKSTAT
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        public _stack_low_
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        _stack_low_:  dw 1 dup(?)
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      ENDIF STACKSTAT
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_BSS  ends
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_BSS  ends
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; _EOF should be the last segment in .COM (see linker memory map)
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; stack definition, available memory is checked at runtime
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; if not, someone introduced other segments, or startup.obj is not the first
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; defined as segment so that linker may detect .COM size overflow
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_STACK segment para public 'TINY_STACK'
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; linker object file
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public _stack_end_
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_EOF  segment word public
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      db STACKSIZE dup(?)
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_stack_end_:
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_EOF  ends
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_STACK 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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 -