Nintendo GAME BOY Programming Manual page 59

Changing the values of SCY and SCX scrolls the BG screen vertically and horizontally one bit at
a time.
NAME ADDRESS
LY FF44h
LY indicates which line of data is currently being transferred to the LCD driver. LY takes a value
of 0-153, with 144-153 representing the vertical blanking period.
When the value of bit 7 of the LCDC register is 1, writing 1 to this again does not change the
value of register LY.
Writing a value of 0 to bit 7 of the LCDC register when its value is 1 stops the LCD controller, and
the value of register LY immediately becomes 0. (Note: Values should not be written to the
register during screen display.)
NAME ADDRESS
FF45h
LYC
Register LYC is compared with register LY. If they match, the Matchflag of the STAT register is
set.
Note: The following 3 registers (BGP, OBP0, and OBP1) are valid in DMG and CGB
modes. For information of CGB color palette settings, see section 2, LCD Color
Display.
７ ６ ５ ４
BIT
BIT 7 6 5 4
３ ２ １ ０
3 2 1 0
59
Chapter 2: Display Functions
R LCDC y-coordinate
R/W LY Compare