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POKE

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Samuel Gomes edited this page Nov 8, 2022 · 1 revision

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

Syntax

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) 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(s)

Turning keyboard Lock and Insert modes on and off.

 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

Note: Use XOR instead of OR above to alternate between on and off modes.

 DEF SEG = 0
 oldsetting% = 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)
 DEF SEG

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!

A small PEEK and POKE fractal program.

SCREEN 13
DEF SEG = &HA000     'set to read screen buffer
DO
    FOR a& = 0 TO 65535
        POKE a&, PEEK((a& * 2) AND &HFFFF&) + 1
    NEXT
    _LIMIT 25
LOOP UNTIL INKEY$ <> ""
DEF SEG

Highlighting a row of text in Screen 0

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

SUB SelectRow (y, x1, x2, col)
DEF SEG = &HB800
addr& = (x1 - 1 + (y - 1) * _WIDTH) * 2 + 1
FOR x = x1 TO x2
  oldCol = PEEK(addr&) AND &B10001111   ' Mask foreground color and blink bit
  POKE addr&, oldCol OR ((col AND &B111) * &B10000) ' Apply background color
  addr& = addr& + 2
NEXT
END SUB

See Example(s)

See Also

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