Nintendo DMG-01 Manual - Page 47

FF20 NR 41, FF21 NR 42

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Game BoyTM CPU Manual 2.13.1. I/O Registers 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 = 65536/(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. 23. FF20 (NR 41) Name - NR 41 Contents - Sound Mode 4 register, sound length (R/W) Bit 5-0 - Sound length data (t1: 0-63) Sound Length = (64-t1)*(1/256) seconds 24. FF21 (NR 42) Name - NR 42 Contents - Sound Mode 4 register, envelope (R/W) Bit 7-4 - Initial volume of envelope Bit 3 - Envelope UP/DOWN 0: Attenuate 1: Amplify by DP Page 47

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Game Boy
TM
CPU Manual
2.13.1. I/O Registers
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
= 65536/(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.
23.
FF20 (NR 41)
Name
- NR 41
Contents - Sound Mode 4 register, sound length (R/W)
Bit 5-0 - Sound length data (t1: 0-63)
Sound Length = (64-t1)*(1/256) seconds
24.
FF21 (NR 42)
Name
- NR 42
Contents - Sound Mode 4 register, envelope (R/W)
Bit 7-4 - Initial volume of envelope
Bit 3 -
Envelope UP/DOWN
0: Attenuate
1: Amplify
by DP
Page 47