Commodore 128 Programmer's Reference Manual page 341

PROGRAMMING THE 80-COLUMN (8563) CHIP 331
n
n
particular 8563 version in the system. The 8563 version can be easily ascertained
through software by reading the 8563 status register, located at $D600 in I/O memory.
Bits 0-2 contain the version number. Refer to the table below for the actual data to be
used.
7A
8
9
0
1
1
8563 rev version ($D600) register 25 value
$40
$47
$47
This situation does not directly affect software which utilizes the resident C128
operating system (Kernal). The Kernal correctly determines which version of the 8563 is
present and initializes it accordingly. Software which relies on the Kernal IOINIT
routine to accomplish this initialization will not have any problems. Software which
performs the initialization itself, or alters the contents of the affected register, may
exhibit the 80-column display anomalies described above, but will otherwise function
normally.
R25(5) PIXEL DOUBLE WIDTH
A change from an 80-column frame to a 40-column frame requires the registers of the
8563 to be initialized differently and requires a different pixel rate. To simplify this
change, a control bit on the 8563 causes the DCLK input signal to be divided by 2. If
R25(4) is a 0, then the pixel width is one DCLK period. If it is a 1, then the pixel width
is two DCLK periods and the internal character clock period is twice the normal period.
The AC timing is the same as normal. The 8563 RAM bus cycles occur at half the
normal rate, with the "idle" time spent with RAS and CAS "high." Note that the
contents of the horizontal initialization registers, and the contents of R22(7-4) and
R22(3-0) must be initialized with different values in this mode.
R25(5) SEMIGRAPHIC MODE
The displayed part of a character is limited to 8 pixels wide owing to the width of the
Display Data Bus (DD0-DD7). Low-resolution graphics regularly use a "block-graphics"
character set that may exceed 8 pixels in width. The 8563 allows this in a limited way
by allowing the last displayed pixel of a character to be repeated in the horizontal
intercharacter space, which is normally forced to the background color. If the control bit
R25(5) (semigraphic mode) is a 0, then normal character operation occurs. If it is a 1,
then semigraphics operation occurs, allowing characters to extend into the horizontal
intercharacter space, touching the next character.
R25(6) ATTRIBUTE ENABLE
In some applications, attributes are not necessary. In these cases, the amount of 8563
RAM memory used by the attributes may be excessive, so the 8563 has a control bit to