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114 lines
3.7 KiB
Markdown
114 lines
3.7 KiB
Markdown
The **POKE** statement sets the value of a specified memory address offset. **QB64 currently has limited access!**
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## Syntax
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> POKE *segment_offset*, *offset_value*
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* Writes a value to the *segment_offset* address in memory.
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* POKE can only be used to set a value from 0 to 255 (one byte).
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* A segment should be defined using [DEF SEG](DEF-SEG), if you don't define a segment qbasics ordinary segment will be used.
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* POKE sends byte values to memory areas. It does not directly access registers.
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* Important [SCREEN (statement)](SCREEN-(statement)) segments using [PEEK](PEEK) and [POKE](POKE) include &HB800 (text segment) and &HA000 (graphics segment).
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* [DEF SEG](DEF-SEG) should always be used to reset the default segment when access to other memory is no longer necessary.
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* POKE is safer to use than [OUT](OUT) which could damage a PC register.
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* **Warning: DEF SEG, VARSEG , VARPTR, PEEK or POKE access QB64's emulated 16 bit conventional memory block!**
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> **It is highly recommended that QB64's [_MEM](_MEM) memory system be used to avoid running out of memory.**
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## Example(s)
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> Turning keyboard Lock and Insert modes on and off.
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```vb
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DEF SEG = 0
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oldsetting% = PEEK(1047)
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POKE 1047,PEEK(1047) OR 16 ' ENABLES SCROLL LOCK
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POKE 1047,PEEK(1047) OR 32 ' ENABLES NUMBER LOCK
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POKE 1047,PEEK(1047) OR 64 ' ENABLES CAPS LOCK
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POKE 1047,PEEK(1047) OR 128 ' ENABLES INSERT MODE
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DEF SEG
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```
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> *Note: Use [XOR](XOR) instead of [OR](OR) above to alternate between on and off modes.*
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```vb
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DEF SEG = 0
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oldsetting% = PEEK(1047)
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POKE 1047,PEEK(1047) AND 239 ' TURNS OFF SCROLL LOCK (239 = 255 - 16)
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POKE 1047,PEEK(1047) AND 223 ' TURNS OFF NUMBER LOCK (223 = 255 - 32)
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POKE 1047,PEEK(1047) AND 191 ' TURNS OFF CAPS LOCK (191 = 255 - 64)
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POKE 1047,PEEK(1047) AND 127 ' TURNS OFF INSERT MODE (127 = 255 - 128)
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DEF SEG
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```
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> *Note: Using [AND](AND) requires that the bit value is subtracted from 255 to turn off a bit.* The above examples won't work in NT.
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> **Warning: The keyboard lights may NOT change so it is a good idea to restore the original settings!**
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A small PEEK and POKE fractal program.
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```vb
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SCREEN 13
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DEF SEG = &HA000 'set to read screen buffer
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DO
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FOR a& = 0 TO 65535
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POKE a&, PEEK((a& * 2) AND &HFFFF&) + 1
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NEXT
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_LIMIT 25
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LOOP UNTIL INKEY$ <> ""
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DEF SEG
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```
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Highlighting a row of text in Screen 0
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```vb
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minX = 20: maxX = 60: minY = 10: maxY = 24
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selection = 0 'the screen Y coordinate of the previously highlighted item
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FOR i% = 1 TO 25: LOCATE i%, 40: PRINT i%;: NEXT
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DO: _LIMIT 100
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IF _MOUSEINPUT THEN
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'Un-highlight any selected row
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IF selection THEN selectRow selection, minX, maxX, 0
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x = CINT(_MOUSEX)
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y = CINT(_MOUSEY)
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IF x >= minX AND x <= maxX AND y >= minY AND y <= maxY THEN
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selection = y
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ELSE
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selection = 0
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END IF
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'Highlight any selected row
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IF selection THEN SelectRow selection, minX, maxX, 2
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IF _MOUSEBUTTON(1) THEN LOCATE 1, 2: PRINT x, y, selection
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END IF
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LOOP UNTIL INKEY$ <> ""
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SUB SelectRow (y, x1, x2, col)
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DEF SEG = &HB800
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addr& = (x1 - 1 + (y - 1) * _WIDTH) * 2 + 1
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FOR x = x1 TO x2
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oldCol = PEEK(addr&) AND &B10001111 ' Mask foreground color and blink bit
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POKE addr&, oldCol OR ((col AND &B111) * &B10000) ' Apply background color
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addr& = addr& + 2
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NEXT
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END SUB
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```
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## See Example(s)
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* [SelectScreen](SelectScreen) (Screen mode selection)
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## See Also
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* [DEF SEG](DEF-SEG), [DEF SEG = 0](DEF-SEG-=-0) (reference)
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* [PEEK](PEEK) (read memory), [OUT](OUT) (write to register)
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* [VARSEG](VARSEG), [VARPTR](VARPTR)
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* [_MEMGET (function)](_MEMGET-(function)), [_MEMPUT](_MEMPUT)
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* [Scancodes](Scancodes) (demo), [Screen Memory](Screen-Memory)
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* [PEEK and POKE Library](PEEK-and-POKE-Library)
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