DECLARE SUB paletteGet (array() AS INTEGER)
DECLARE SUB paletteSet (array() AS INTEGER)
DECLARE SUB toneGrayscale (array() AS INTEGER)
DECLARE SUB toneSepia (array() AS INTEGER)

CONST DAC.PELMask = &H3C6         ' DAC (Digital/Analog Converter) PEL Mask Register
CONST DAC.READ = &H3C7            ' DAC Read Port/DAC State Register
CONST DAC.WRITE = &H3C8           ' DAC Write Port/DAC State Register
CONST DAC.DATA = &H3C9            ' DAC Data Port

' Palette buffer
DIM clr(0 TO 767) AS INTEGER

' Enter Mode 13 (320x200, 256 colors)
SCREEN 13

' Display the whole color palette
FOR y% = 0 TO 15
  FOR x% = 0 TO 15
    LINE (x% * 10, y% * 10)-STEP(9, 9), x% + (y% * 16), BF
  NEXT x%
NEXT y%

' Wait for user input
COLOR 15: LOCATE 3, 25: PRINT "Press a key - 1": SLEEP
paletteGet clr()    ' get palette
toneSepia clr()     ' modify
paletteSet clr()    ' set palette

' Wait for user input
COLOR 15: LOCATE 3, 25: PRINT "Press a key - 2": SLEEP
paletteGet clr()    ' get palette
toneGrayscale clr() ' modify
paletteSet clr()    ' set palette

' Read original palette, {array} must be (0 to 767)
SUB paletteGet (array() AS INTEGER)
  OUT DAC.PELMask, &HFF                        ' Mask all registers
  OUT DAC.READ, 0                              ' Select first attribute
  FOR i% = 0 TO 767                            ' Read all 768 components
    array(i%) = INP(DAC.DATA)
  NEXT i%
END SUB

' Set palette, {array} must be (0 to 767)
SUB paletteSet (array() AS INTEGER)
  OUT DAC.PELMask, &HFF                        ' Mask all registers
  OUT DAC.WRITE, 0                             ' Select first attribute
  FOR i% = 0 TO 767                            ' Set all 768 components
    OUT DAC.DATA, array(i%)
  NEXT i%
END SUB

' Convert palette to Grayscale
SUB toneGrayscale (array() AS INTEGER)
  DIM grey AS INTEGER

  FOR i% = 0 TO 767 STEP 3
    grey = array(i%) * .299 + array(i% + 1) * .587 + array(i% + 2) * .114
    array(i%) = grey
    array(i% + 1) = grey
    array(i% + 2) = grey
  NEXT i%
END SUB

' Convert palette to sepia
SUB toneSepia (array() AS INTEGER)
  DIM r AS INTEGER, g AS INTEGER, b AS INTEGER

  FOR i% = 0 TO 767 STEP 3
    r = array(i%) * .393 + array(i% + 1) * .769 + array(i% + 2) * .189
    g = array(i%) * .349 + array(i% + 1) * .686 + array(i% + 2) * .168
    b = array(i%) * .272 + array(i% + 1) * .534 + array(i% + 2) * .131
    IF (r > 63) THEN array(i%) = 63 ELSE array(i%) = r
    IF (g > 63) THEN array(i% + 1) = 63 ELSE array(i% + 1) = g
    IF (b > 63) THEN array(i% + 2) = 63 ELSE array(i% + 2) = b
  NEXT i%
END SUB