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. | I
2. Check the value of bit 7 until it equals 1 in order for the data to be LJ
valid.
3. Once bit 7 becomes high (1), perform the read or write operation. j i
Later in this chapter, you'll see example programs that illustrate the behavior of
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 j j
and the 8502 during read and write operations respectively. Each bit in this register [J
is a data bit. All data that passes between 8563 and the 8502 must pass through this
register. { ,
FUNDAMENTAL BACKGROUND
ABOUT 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.
u
u
u