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However, what you actually have is something like this:

The programmable characters which make up the bit map are arranged in 25 rows of 40 columns each. While this is a good method of organization for text, it makes bit mapping somewhat difficult. (There is a good reason for this method. See the section on MIXED MODES.)

The following formula will make it easier to control a dot on the bit map screen:

The start of the display memory area is known as the BASE, The row number (from 0 to 24) of your dot is:

   ROW = INT(Y/8) (There are 320 bytes per line.)
The character position on that line (from 0 to 39) is:
   CHAR = INT(X/8) (There are 8 bytes per character.)
The line of that character position (from 0 to 7) is:
   LINE = Y AND 7

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This page has been created by Sami Rautiainen.
Read the small print. Last updated May 12, 2002.