QB64-PE/internal/help/POKE.txt

The '''POKE''' statement sets the value of a specified memory address offset. '''QB64 currently has limited access!''' 


{{PageSyntax}}
:: POKE ''segment_offset'', ''offset_value''


* Writes a value to the ''segment_offset'' address in memory.
* POKE can only be used to set a value from 0 to 255 (one byte).
* A segment should be defined using [[DEF SEG]], if you don't define a segment qbasics ordinary segment will be used.
* POKE sends byte values to memory areas. It does not directly access registers.
* Important [[SCREEN (statement)|SCREEN]] segments using [[PEEK]] and [[POKE]] include &HB800 (text segment) and &HA000 (graphics segment).
* [[DEF SEG]] should always be used to reset the default segment when access to other memory is no longer necessary.
* POKE is safer to use than [[OUT]] which could damage a PC register.
* '''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 1:'' Turning keyboard Lock and Insert modes on and off.
{{CodeStart}} '' ''
 DEF SEG = 0
 oldsetting% = PEEK(1047)
 POKE 1047,PEEK(1047) OR 16 ' ENABLES SCROLL LOCK
 POKE 1047,PEEK(1047) OR 32 ' ENABLES NUMBER LOCK
 POKE 1047,PEEK(1047) OR 64 ' ENABLES CAPS LOCK
 POKE 1047,PEEK(1047) OR 128 ' ENABLES INSERT MODE
 DEF SEG

{{CodeEnd}}
:''Note: Use [[XOR]] instead of [[OR]] above to alternate between on and off modes.''
{{CodeStart}} '' ''
 {{Cl|DEF SEG}} = 0
 oldsetting% = {{Cl|PEEK}}(1047)
 POKE 1047,PEEK(1047) AND 239 ' TURNS OFF SCROLL LOCK (239 = 255 - 16)
 POKE 1047,PEEK(1047) AND 223 ' TURNS OFF NUMBER LOCK (223 = 255 - 32)
 POKE 1047,PEEK(1047) AND 191 ' TURNS OFF CAPS LOCK (191 = 255 - 64)
 POKE 1047,PEEK(1047) AND 127 ' TURNS OFF INSERT MODE (127 = 255 - 128)
 {{Cl|DEF SEG}} '' ''
{{CodeEnd}}
:''Note: Using [[AND]] requires that the bit value is subtracted from 255 to turn off a bit.'' The above examples won't work in NT.

:'''Warning: The keyboard lights may NOT change so it is a good idea to restore the original settings!'''


''Example 2:'' A small PEEK and POKE fractal program.
{{CodeStart}} '' ''
{{Cl|SCREEN (statement)|SCREEN}} 13
{{Cl|DEF SEG}} = {{Cl|&H}}A000     'set to read screen buffer
{{Cl|DO}}
    {{Cl|FOR...NEXT|FOR}} a& = 0 {{Cl|TO}} 65535
        {{Cl|POKE}} a&, {{Cl|PEEK}}((a& * 2) {{Cl|AND (boolean)|AND}} {{Cl|&H}}FFFF&) + 1
    {{Cl|NEXT}}
    {{Cl|_LIMIT}} 25
{{Cl|LOOP}} {{Cl|UNTIL}} {{Cl|INKEY$}} <> ""
{{Cl|DEF SEG}} '' ''
{{CodeEnd}} 


''Example 3:'' Highlighting a row of text in Screen 0
{{CodeStart}}
minX = 20: maxX = 60: minY = 10: maxY = 24
selection = 0 'the screen Y coordinate of the previously highlighted item
{{Cl|FOR}} i% = 1 {{Cl|TO}} 25: {{Cl|LOCATE}} i%, 40: {{Cl|PRINT}} i%;: {{Cl|NEXT}}
{{Cl|DO}}: {{Cl|_LIMIT}} 100
  {{Cl|IF}} {{Cl|_MOUSEINPUT}} {{Cl|THEN}}
    'Un-highlight any selected row
    {{Cl|IF}} selection {{Cl|THEN}} selectRow selection, minX, maxX, 0
    x = {{Cl|CINT}}({{Cl|_MOUSEX}})
    y = {{Cl|CINT}}({{Cl|_MOUSEY}})    
    {{Cl|IF}} x >= minX {{Cl|AND (boolean)|AND}} x <= maxX {{Cl|AND (boolean)|AND}} y >= minY {{Cl|AND (boolean)|AND}} y <= maxY {{Cl|THEN}}
      selection = y
    {{Cl|ELSE}}
      selection = 0
    {{Cl|END IF}}
    'Highlight any selected row
    {{Cl|IF}} selection {{Cl|THEN}} SelectRow selection, minX, maxX, 2 
    {{Cl|IF}} {{Cl|_MOUSEBUTTON}}(1) {{Cl|THEN}} {{Cl|LOCATE}} 1, 2: {{Cl|PRINT}} x, y, selection 
  {{Cl|END IF}}
{{Cl|LOOP}} {{Cl|UNTIL}} {{Cl|INKEY$}} <> ""

{{Cl|SUB}} SelectRow (y, x1, x2, col)
{{Cl|DEF SEG}} = {{Cl|&H}}B800
addr& = (x1 - 1 + (y - 1) * {{Cl|_WIDTH (function)|_WIDTH}}) * 2 + 1
{{Cl|FOR}} x = x1 {{Cl|TO}} x2
  oldCol = {{Cl|PEEK}}(addr&) {{Cl|AND (boolean)|AND}} {{Cl|&B}}10001111   ' Mask foreground color and blink bit
  {{Cl|POKE}} addr&, oldCol {{Cl|OR}} ((col {{Cl|AND (boolean)|AND}} {{Cl|&B}}111) * {{Cl|&B}}10000) ' Apply background color
  addr& = addr& + 2
{{Cl|NEXT}}
{{Cl|END SUB}} '' ''
{{CodeEnd}}


''See Example:'' [[SelectScreen]] (Screen mode selection) 


''See also:'' 
* [[DEF SEG]], [[DEF SEG = 0]] {{text|(reference)}}
* [[PEEK]] {{text|(read memory)}}, [[OUT]] {{text|(write to register)}}
* [[VARSEG]], [[VARPTR]]
* [[_MEMGET (function)]], [[_MEMPUT]]
* [[Scancodes]] {{text|(demo)}}, [[Screen Memory]] 
* [[PEEK and POKE Library]]


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