Editing CTIA and GTIA (section)

===Other CTIA/GTIA Functions===

====Joystick Triggers====

=====TRIG0 $D010 Read=====
SHADOW: STRIG0 $0284

Joystick 0 trigger

=====TRIG1 $D011 Read=====
SHADOW: STRIG1 $0285

Joystick 1 trigger.

=====TRIG2 $D012 Read=====
SHADOW: STRIG2 $0286

Joystick 2 trigger.

=====TRIG3 $D013 Read=====
SHADOW: STRIG3 $0287

Joystick 3 trigger

{| class="wikitable"
|-
! Bit 7 !! Bit 6 !! Bit 5 !! Bit 4 !! Bit 3 !! Bit 2 !! Bit 1 !! Trigger
|-
| 0 || 0 || 0 || 0 || 0 || 0 || 0 || ? 
|}

Bits 7 through 1 are always 0.  Bit 0 reports the state of the joystick trigger.  Value 1 indicates the trigger is not pressed.   Value 0 indicates the trigger is pressed.

The trigger registers report button presses in real-time. The button pressed state will instantly clear when the button is released.

The triggers may be configured to latch, that is, lock, in the pressed state and remain that way until specifically cleared.  [[CTIA and GTIA#GRACTL $D01D Write|GRACTL]] bit 2 enables the latch behavior for all triggers.   Clearing GRACTL bit 2 returns all triggers to real-time behavior.

====PAL $D014 Read====
PAL flags.

{| class="wikitable"
|-
! Bit 7 !! Bit 6 !! Bit 5 !! Bit 4 !! Bit 3 !! Bit 2 !! Bit 1 !! Bit 0
|-
| - || - || - || - || Video 3 || Video 2 || Video 1 || Video 0 
|}

This register reports the display standard for the system.  When Bits 3 to 0 are set to 1 (value $f<sub>hex</sub>/15<sub>dec</sub>) the system is operating in NTSC.  When the bits are zero the system is operating in PAL mode.

====CONSPK $D01F Write====
Console Speaker

{| class="wikitable"
|-
! Bit 7 !! Bit 6 !! Bit 5 !! Bit 4 !! Bit 3 !! Bit 2 !! Bit 1 !! Bit 0
|-
| 0 || 0 || 0 || 0 || Speaker || - || - || - 
|}

Bit3 controls the internal speaker of the Atari 800/400.  In later models the console speaker is removed and the sound is mixed with the regular [[POKEY]] audio signals for output to the monitor port and RF adapter.   The Atari OS uses the console speaker to output the keyboard click and the bell/buzzer sound.

The Operating System sets the speaker bit during the vertical blank routine.   Repeatedly writing 0 to the bit will produce a 60&nbsp;Hz buzzing sound as the vertical blank resets the value.  Useful tones can be generated using [[6502]] code effectively adding a fifth audio channel, albeit a channel requiring CPU time to maintain the audio tones.

====CONSOL $D01F Read====
Console Keys

{| class="wikitable"
|-
! Bit 7 !! Bit 6 !! Bit 5 !! Bit 4 !! Bit 3 !! Bit 2 !! Bit 1 !! Bit 0
|-
| - || - || - || - || - || Option || Select || Start 
|}

A bit is assigned to report the state of each of the special console keys,  Start, Select, and Option.  Bit value 0 indicates a key is pressed and 1 indicates the key is not pressed.  Key/Bit values:

* Start Key = Bit value $1
* Select Key = Bit value $2
* Option Key = Bit value $4