Atari 400 Basic Reference Manual page 118

DECIMAL
HEXADECIMAL
LABEL
LOCATION
LOCATION COMMENTS AND DESCRIPTIONS
PORTA 54016
D300
PIA Port A Controller jack I/0 ports.
PORTE 54017 D301 PIA Port B Initialized to hex 3C.
PACTL
54018
D302 Port A Control Register (on Program Recorder 52
= ON, 60 = OFF).
PBCTL 54019 D303 Port B control register.
SKCTL 53775 D20F
Serial Port control register. Bit 2=0 (last key still
pressed).
Appendix
1-3