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Source: Mapping the Atari, by Ian Chadwick
D300: PORTA D301: PORTB D302: PACTL D303: PBCTL
Reads or writes data from or to the game controller jacks one and two if bit 2 of PACTL is set. Writes directional control bits if bit 2 of PACTL is clear. Clearing a directional control bit will set that line for input (the default for using joysticks.) Setting a directional control bit will set that line for output.
The arrangement of the bits in relation to the controllers are as follows:
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Jack | Player 2 | Player 1 | ||||||
| Joystick | Right | Left | Back | Forth | Right | Left | Back | Forth |
| Paddle Trigger | 3 | 2 | 1 | 0 | ||||
| Keypad Row | 4 | 3 | 2 | 1 | 4 | 3 | 2 | 1 |
On the Atari 400 and Atari 800, PORTB is exactly the same as PORTA, controlling players 3 and 4.
On the XL series of computers, PORTB has been changed to a memory and LED control register (read/write):
| Self Test ROM | Reserved | LED #2 | LED #1 | BASIC ROM | OS ROM |
Self Test ROM: If set, RAM is enabled for the address range $5000-$57FF. If clear, the self-test ROM (which is physically located at $D000-$D7FF, under the hardware registers) is remapped to this memory area.
LED #n: If set, the corresponding LED is turned off. If clear, the LED will be on.
BASIC ROM: If set, RAM is enabled for the address range $A000-$BFFF. If clear, the built-in BASIC ROM is enabled at this address. And if there is a cartridge installed in the computer, it makes no difference.
OS ROM: If set, the built-in OS is enabled, and occupies the address range $C000-$FFFF (except that the area $D000-$D7FF will only access the hardware registers.) If clear, RAM is enabled in this area (again, save for the hole.)
| IRQ Status | 0 | 1 | 1 | Motor Control | Data / Direction | 0 | IRQ Enable |
IRQ Status: This bit is set when peripheral A generates an interrupt. It is cleared by reading PORTA.
Motor Control: Used by the Program Recorder (cassette drive). If set, the motor is turned off. If clear, the motor is turned on.
Data / Direction: If set, PORTA is used as a data I/O register. If clear, PORTA is used as a direction control register.
IRQ Enable: If set, enables interrupts from peripheral A. If clear, interrupts are disabled.
| IRQ Status | 0 | 1 | 1 | SIO Command | Data / Direction | 0 | IRQ Enable |
IRQ Status: This bit is set when peripheral Bgenerates an interrupt. It is cleared by reading PORTB.
SIO Command: Peripheral command identification.When cleared, serial devices are alerted that the following data will be a command.
Data / Direction: If set, PORTB is used as a data I/O register. If clear, PORTB is used as a direction control register. (Only effective on the Atari 400 or Atari 800.)
IRQ Enable: If set, enables interrupts from peripheral B. If clear, interrupts are disabled.
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