Nintendo DMG-01 - Game Boy Console Manual page 47

TM
Game Boy CPU Manual
23. FF20 (NR 41)
Name
Contents - Sound Mode 4 register, sound length (R/W)
24. FF21 (NR 42)
Name
Contents - Sound Mode 4 register, envelope (R/W)
by DP
Bit 7 - Initial (when set,sound restarts)
Bit 6 - Counter/consecutive flag
Bit 2-0 - Frequency's higher 3 bits (x).
Frequency = 4194304/(64*(2048-x)) Hz
Counter/consecutive Selection
0 = Regardless of the length data in
NR31 sound can be produced
consecutively.
1 = Sound is generated during the time
period set by the length data in
NR31. After this period the sound 3
ON flag (bit 2 of NR52) is reset.
- NR 41
Bit 5-0 - Sound length data (t1: 0-63)
Sound Length = (64-t1)*(1/256) seconds
- NR 42
Bit 7-4 - Initial volume of envelope
Bit 3 -
= 65536/(2048-x) Hz
Envelope UP/DOWN
0: Attenuate
1: Amplify
2.13.1. I/O Registers
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