Nintendo DMG-01 - Game Boy Console Manual page 54

2.13.1. I/O Registers
Mode 0:
000___000___000___000___000___000___000________________
Mode 1:
_______________________________________11111111111111__
Mode 2:
___2_____2_____2_____2_____2_____2___________________2_
Mode 3:
____33____33____33____33____33____33__________________3
The Mode Flag goes through the values 0, 2,
and 3 at a cycle of about 109uS. 0 is present
about 48.6uS, 2 about 19uS, and 3 about 41uS.
This is interrupted every 16.6ms by the VBlank
(1). The mode flag stays set at 1 for about 1.08
ms. (Mode 0 is present between 201-207 clks, 2
about 77-83 clks, and 3 about 169-175 clks. A
complete cycle through these states takes 456
clks. VBlank lasts 4560 clks. A complete screen
refresh occurs every 70224 clks.)
33. FF42 (SCY)
Name
Contents - Scroll Y
Page 54
- SCY
8 Bit value $00-$FF to scroll BG Y screen
position.
Game Boy
(R/W)
TM
CPU Manual
V 1.01