DEF SEG - QB64 Wiki

DEF SEG

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DEF SEG is used to define the area in memory to access QB64's emulated conventional memory.


Syntax:

DEF SEG [=][{segment|VARSEG(variable}]


  • Used to set the pointer to a memory area of a variable/array or register.
  • PEEK and POKE require a segment memory address (often just 0) without using VARSEG.
  • Important segments using PEEK and POKE include &HB800 (text segment) and &HA000 (graphics segment).
  • BSAVE and BLOAD require a VARSEG reference to the grahic array(0 index) used.
  • ALWAYS use DEF SEG when the procedure is completed to reset the segment to Qbasic's default value!
  • Warning: DEF SEG, VARSEG , VARPTR, PEEK or POKE access QB64's emulated 16 bit conventional memory block!
It is highly recommended that QB64's _MEM memory system be used to avoid running out of memory.


Example: In a Qbasic(ONLY) file delete, SEG forces the parameter to be passed as a far pointer.

CONST file = "trashme.tmp" 'example temporary file name to delete DEFINT A-Z DIM filename AS STRING DIM result AS LONG DIM t AS STRING DIM i AS INTEGER CONST codelen = 48 DIM code AS STRING * codelen CLS t = "5589E51E8B560C8EDA8B5E0A8B5702B441CD218B56088EDA8B5E06720B6631C0" t = t + "6689071F5DCA0800660D0000FFFFEBF0" FOR i = 0 TO codelen - 1 MID$(code, i + 1, 1) = CHR$(VAL("&h" + MID$(t, i + i + 1, 2))) NEXT OPEN file FOR APPEND AS 1 'create temporary file PRINT #1, "I am doomed! :-(" CLOSE PRINT "now you see it:" SHELL "dir " + file K$ = INPUT$(1) filename = file + CHR$(0) 'create zero string name for DOS DEF SEG = VARSEG(code) CALL absolute(SEG filename, SEG result, VARPTR(code)) IF result THEN 'check results PRINT "oops. error: 0x"; HEX$(result AND &HFFFF&) ELSE PRINT "now you don't:" END IF SHELL "dir " + file END

Code by Michael Calkins as Public Domain(2011)


See also:



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