Nintendo DMG-01 Manual - Page 35
Special Registers
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Game BoyTM CPU Manual 2.12.2. Interrupt Descriptions 5. High-to-Low of P10-P13 This interrupt occurs on a transition of any of the keypad input lines from high to low. Due to the fact that keypad "bounce"* is virtually always present, software should expect this interrupt to occur one or more times for every button press and one or more times for every button release. * - Bounce tends to be a side effect of any button making or breaking a connection. During these periods, it is very common for a small amount of oscillation between high & low states to take place. 2.13. Special Registers 2.13.1. I/O Registers 1. FF00 (P1) Name - P1 Contents - Register for reading joy pad info and determining system type. (R/W) Bit 7 - Not used Bit 6 - Not used Bit 5 - P15 out port Bit 4 - P14 out port Bit 3 - P13 in port Bit 2 - P12 in port Bit 1 - P11 in port Bit 0 - P10 in port by DP Page 35