Commodore 128 Programmer's Reference Manual page 306

296 COMMODORE 128
This is the sequence of steps to perform a read or write operation:
1. Address the registers (by number) or internal RAM location.
2. Check the value of bit 7 until
valid.
3. Once bit 7 becomes high (1), perform the read or write operation.
Later in this chapter, you'll see example
these registers.
The rest of the bits in $D600 except bit 6, which involves a light pen are used to
represent a register number for read or write access. Six bits are necessary since the
8563 has 37 registers; therefore, enough bits are needed to represent numbers up to 37.
Location $D601 is strictly a data register that passes values between the 8563
and the 8502 during read and write operations respectively. Each bit in this register
is a data bit. All data that passes between 8563 and the 8502 must pass through this
register.
FUNDAM ENTAL BACKGROUND
A B O U T THE 8563 CHIP
CHARACTERS
As you know, the 8563 chip is primarily designed for displaying characters within an 80
by 25 character matrix. The visible character is an 8 by 8 pixel matrix. The character
can be defined by 16 or 32 bytes and can be displayed less than 8 by 8 pixels.
it equals 1 in order for the data
programs that illustrate the behavior of
to be