Screen Memory

SCREEN Memory Segments

Screen 0 Text Segment &HB800

• The text video memory segment is located at hexadecimal memory address B800 or 47104 decimal.
• The normal SCREEN 0 WIDTH is 25 rows by 80 columns wide capable of up to 2000 text characters.
• Each text block memory offset consists of a text character byte followed by a color byte.
• Each unsigned byte can hold values from 0 to 255 only. PEEK can read and POKE can write positive values to those bytes.
• Text ASCII code values range from 0 to 127 with the extended codes ranging from 128 to 255.
• Color values from 0 to 127 are the normal 16 text colors with background color attributes ranging from 0 to 7.
• Color values from 128 to 255 are the high intensity blinking color values with background color attributes ranging from 0 to 7.

Example 1: Printing text with blinking colors in SCREEN 0 only.

DIM s AS STRING DIM i AS LONG DIM j AS LONG CLS s = "Hello, World!" DEF SEG = &HB800 FOR j = 1 TO 15 FOR i = 1 TO LEN(s) POKE (j * 80 + (i - 1)) * 2, ASC(MID$(s$, i, 1)) 'text characters POKE (j * 80 + (i - 1)) * 2 + 1, &H80 OR j 'blinking color NEXT NEXT DEF SEG 'restore to default segment END

Example 2: Displaying and coloring the 256 ASCII characters using POKE in SCREEN 0.

SCREEN 12 'set full screen in Qbasic only for flashing colors SCREEN 0 OUT &H3C8, 0: OUT &H3C9, 0: OUT &H3C9, 0: OUT &H3C9, 20 _FONT _LOADFONT("C:\Windows\Fonts\Cour.ttf", 20, "MONOSPACE") 'select monospace font. QB64 only! DEF SEG = &HB800 'SCREEN 0 text ONLY! FOR code = 0 TO 255 POKE 640 + code * 4, code 'poke the even text offsets with space between NEXT COLOR 11: LOCATE 20, 27: PRINT "Press a key to add color!" K$ = INPUT$(1) FOR colr = 0 TO 255 POKE 641 + colr * 4, colr 'poke the ODD color offsets(second byte) NEXT DEF SEG 'reset to default segment END

Code by Ted Weissgerber

Explanation: To POKE text characters to the screen in SCREEN 0, DEF SEG sets the memory segment to &HB800. Text values are poked at the even segment offsets starting 640 bytes(4 rows * 80 columns wide * 2 bytes) from the upper left corner 0 offset of the screen memory segment. To space the text it skips an even offset by multiplying by 4 instead of 2. The odd offsets can be written to to set the color. Using the same 4 byte offsets, the text and background are colored using values up to 128. Values over 128 cause the text to flash and the background colors 0 to 7 are repeated. The background color is incremented every 16 values.

4000 byte Video Memory Segment Text block #: 1 321 322 323 324 325 326 327 Text position: 1, 1 5, 1 5, 2 5, 3 5, 4 5, 5 5, 6 5, 7 Byte offset: 0, 1 640 642 644 646 648 650 652 Segment: (CHR$(0), COLOR 0),.......(0, 0), (0, 0), (1, 1), (0, 0), (2, 2), (0, 0), (3, 3),... Row% = Offset% \ 160 + 1 Column% = (Offset% MOD 160) \ 2 + 1 Offset% = (160 * (Row% - 1)) + (2 * (Column% - 1))

Graphic Screen Segment &HA000

• The graphic video memory segment is located at hexadecimal memory address A000 or 40960 decimal.
• Legacy graphic screen modes include 1, 2, 7, 8, 9, 10, 11, 12 and 13 with varying pixel widths, heights and color attributes.
• QB64 _NEWIMAGE or _LOADIMAGE screen modes can use the legacy modes above, 256 color or 32 bit color modes.

See also:

• PEEK, POKE
• DEF SEG = 0
• SCREEN
• SCREEN (function)
• _NEWIMAGE (screen pages)
• _LOADIMAGE (image files)

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