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mateusz.vi |
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.8086
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STACK_SIZE = 2048
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dgroup group _TEXT,_DATA,CONST,CONST2,_STACK,_BSS
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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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_TEXT segment word public 'CODE'
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org 100h
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_small_code_ label near
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_cstart_:
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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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; 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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; 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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; step 1: if SP is higher than my top_of_stack, then set SP explicitely
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cmp sp, top_of_stack
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jle resizemem
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mov sp, top_of_stack
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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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mov ah, 4ah
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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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int 21h
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call main
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mov ah, 4ch
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int 21h
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; Stack overflow checking routine is absent. Remember to compile your
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; programs with the -s option to avoid referencing __STK
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;__STK:
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; ret
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_DATA segment word public 'DATA'
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_DATA ends
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CONST segment word public 'DATA'
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CONST ends
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CONST2 segment word public 'DATA'
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CONST2 ends
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_BSS segment word public 'BSS'
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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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; 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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; code instead (before calling main)
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top_of_stack:
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_STACK ends
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_TEXT ends
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end _cstart_
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