Description: Apple II Hardware: Character Generator ROM
Header: Apple II Hardware: Character Generator ROM

 To allow you to write programs involving special characters or lower case
 characters, you can get a 2716 PROM to replace the the character generator ROM
 in revision 7 and later Apples. This note describes how the characters are
 mapped in the ROM.

 Characters storage uses a scheme of eight bytes per character, arranged in the
 ROM in the order shown in Table 7 on page 15 of the Apple II Reference Manual.
 The starting address for each character is the address from Table 7 multiplied
 by eight. Lower case characters, if desired, should be mapped in place of the
 numbers and punctuation in columns $E0 and $F0.

 Each character is made up of eight bytes.  Each byte represents one row of
 dots.  The most significant bit of each byte is ignored by the hardware.  The
 lowest-addressed byte of each character is the topmost row of dots of the
 displayed character.  The first and last bits of each row of dots are usually
 set to zero to supply a two dot space between characters.  The bottom row of
 dots is usually left set to zero to allow a one row space between lines. Some
 lower case character sets use the bottom row for descenders (the letter "g",
 for example).  This causes overlap when the descender is directly above an
 upper case letter like "B". The following diagram shows how the characters are
 built.  The three digit hexadecimal number is the hexadecimal ROM address for
 each byte and the two digit hexadecimal number is the pattern of on and off
 bits.

 $208  $08  ...*...    $708  $00  .......    $730  $08  ...*...
 $209  $14  ..*.*..    $709  $00  .......    $731  $14  ..*.*..
 $20A  $22  .*...*.    $70A  $1C  ..***..    $732  $10  ..*....
 $20B  $22  .*   *.    $70B  $02  .....*.    $733  $10  ..*....
 $20C  $3E  .*****.    $70C  $1E  ..****.    $734  $3E  .*****.
 $20D  $22  .*...*.    $70D  $22  .*...*.    $735  $10  ..*....
 $20E  $22  .*...*.    $70E  $1E  ..****.    $736  $10  ..*....
 $20F  $00  .......    $70F  $00  .......    $737  $00  .......
 $210  $3C  .****..    $710  $20  .*.....    $738  $00  .......
 $211  $22  .*...*.    $711  $20  .*.....    $739  $00  .......
 $212  $22  .*...*.    $712  $3C  .****..    $73A  $1C  ..***..
 $213  $3C  .****..    $713  $22  .*...*.    $73B  $22  .*...*.
 $214  $22  .*...*.    $714  $22  .*...*.    $73C  $22  .*...*.
 $215  $22  .*...*.    $715  $22  .*...*.    $73D  $1E  ..****.
 $216  $3C  .****..    $716  $3C  .****..    $73E  $02  .....*.
 $217  $00  .......    $717  $00  .......    $73F  $1C  ..***..
 Apple Tech Notes


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