The ANTIC chip has a vertical line counter at $0D4B (54283). This counter shows the high 8 bits of a 9 bit counter. This gives two line resolution. The value of this counter is placed into PENV [$D40D (54285)] when a joystick trigger is pressed.
The POKEY chip has some registers which control the serial port.
The serial port control register, SKCTL [$D20F (53775)], controls the serial port configuration and the game paddle scan mode. and some keyboard circuitry.
The serial port control register
7 6 5 4 3 2 1 0
-----------------
SKCTL | | | | | | | | |
-----------------
1 6 3 1 8 4 2 1
2 4 2 6
8
bits
0 1 = enable keyboard debounce
1 1 = enable keyboard scan
both 0 = set initialization mode.
2 1 = fast pot scan
3 1 = serial output is two tone (for cassette)
instead of logical true/false
4\
5 >- serial port mode control
6/
7 1 = forced logical 0 on output
If the serial port control register is read from it gives the serial port status. The register is then called SKSTAT
Serial port status register
7 6 5 4 3 2 1 0
-----------------
| | | | | | | |1|
-----------------
1 6 3 1 8 4 2 1
2 4 2 6
8
bits
0 not used, reads 1
1 0 = serial input shift register busy
2 0 = last key is still pressed
3 0 = shift key pressed
4 0 = direct from serial input port
5 0 = keyboard over-run
6 0 = serial data input over-run
7 1 = serial data input frame error
The serial port status is latched and must be reset by writing any number to its' reset register, SKRES [$D20A (53770)].
When a full byte of serial input data has been received, it is read from the serial input data register, SERIN [$D20D (53773). Serial output data is written to the same register, which is then called the serial output data register, SEROUT. This register is usually written to in response to a serial output data interrupt (bit 4 of IRQST).
The addresses for the hardware chips are not completely decoded. For example, the PIA needs only four bytes of memory but is active from $D300 - D3FF. Enough room for 64 PIA chips. A second pair of parallel ports could be added by accessing the address bus and further decoding the address for a second PIA. (This would also require a small modification of the computer's circuit board to disable the original PIA when the new one is active.) Similarly, there is room for 15 more POKEY or ANTIC chips and 7 gtia chips, should you ever need them. (GTIA uses $D000 - D0FF, POKEY uses $D200 - $D2FF and ANTIC uses $D400 - $D4FF.)
Useful data base variables and OS equates
SKRES $D20A (53770): serial port status reset
SEROUT $D20D (53773): serial output data
SERIN $D20D (53773): serial input data
SKCTL $D20F (53775): serial port control
SKSTAT $D20F (53775): serial port status
VCOUNT $D40B (54283): vertical line counter
Os shadow registers
SSKCTL $0232 (562): SKCTL