QB64-PE/internal/help/DEF_SEG.txt

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


==Legacy support==
* '''QB64 implements memory access using [[_MEM]] and related functions. For that reason, [[DEF SEG]] isn't recommended practice anymore and is supported to maintain compatibility with legacy code.'''


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


{{PageDescription}}
* 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, in order to reset the segment to QBasic's default value.
* [[DEF SEG]], [[VARSEG]], [[VARPTR]], [[PEEK]] and [[POKE]] access QB64's emulated 16 bit conventional memory block. '''It is highly recommended to use QB64's [[_MEM]] memory system to avoid running out of memory.'''

<!--
{{PageExamples}}
''Example:'' In a Qbasic(ONLY) file delete, '''SEG''' forces the parameter to be passed as a far pointer.
{{CodeStart}} '' ''
{{Cl|CONST}} file = "trashme.tmp"  'example temporary file name to delete
{{Cl|DEFINT}} A-Z
{{Cl|DIM}} filename {{Cl|AS}} {{Cl|STRING}}
{{Cl|DIM}} result {{Cl|AS}} {{Cl|LONG}}
{{Cl|DIM}} t {{Cl|AS}} {{Cl|STRING}}
{{Cl|DIM}} i {{Cl|AS}} {{Cl|INTEGER}}
{{Cl|CONST}} codelen = 48
{{Cl|DIM}} code {{Cl|AS}} {{Cl|STRING}} * codelen

{{Cl|CLS}}

t = "5589E51E8B560C8EDA8B5E0A8B5702B441CD218B56088EDA8B5E06720B6631C0"
t = t + "6689071F5DCA0800660D0000FFFFEBF0"

{{Cl|FOR...NEXT|FOR}} i = 0 {{Cl|TO}} codelen - 1
{{Cl|MID$}}(code, i + 1, 1) = {{Cl|CHR$}}({{Cl|VAL}}("&h" + {{Cl|MID$}}(t, i + i + 1, 2)))
{{Cl|NEXT}}

{{Cl|OPEN}} file {{Cl|FOR...NEXT|FOR}} {{Cl|APPEND}} {{Cl|AS}} 1  'create temporary file
{{Cl|PRINT (file statement)|PRINT}} #1, "I am doomed! :-("
{{Cl|CLOSE}}

{{Cl|PRINT}} "now you see it:"
{{Cl|SHELL}} "dir " + file
K$ = {{Cl|INPUT$}}(1)

filename = file + {{Cl|CHR$}}(0)  'create zero string name for DOS
{{Cl|DEF SEG}} = {{Cl|VARSEG}}(code)
{{Cl|CALL}} absolute('''SEG''' filename, '''SEG''' result, {{Cl|VARPTR}}(code))

{{Cl|IF...THEN|IF}} result {{Cl|THEN}}  'check results
{{Cl|PRINT}} "oops. error: 0x"; {{Cl|HEX$}}(result {{Cl|AND (boolean)|AND}} {{Cl|&H}}FFFF&)
{{Cl|ELSE}}
{{Cl|PRINT}} "now you don't:"
{{Cl|END IF}}
{{Cl|SHELL}} "dir " + file
{{Cl|END}} '' ''
{{CodeEnd}}
{{small|Code by Michael Calkins as Public Domain(2011)}}
-->

''See also:'' 
* [[DEF SEG = 0]]
* [[VARPTR]], [[VARSEG]] 
* [[PEEK]], [[POKE]]
* [[BSAVE]], [[BLOAD]]


{{PageNavigation}}
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