Routines
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AA4B: Move Up A Line
Used by the routines at ScrollAreaLeft, PrintCharacter, DrawPicture and DrawFrameLeft.
Moves a screen address up by one pixel line.
Input
HL The screen address
Output
HL The screen address of the pixel line above
MoveUpALine AA4B LD A,H Fetch the high byte of the address.
AA4C DEC H Move up a pixel line.
AA4D AND %00000111 Keep the number of the pixel line which it was in.
AA4F RET NZ Return unless that was the top pixel line of a character row.
AA50 LD A,L Fetch the low byte of the address.
AA51 SUB $20 Move up a character row.
AA53 LD L,A Store it.
AA54 RET C Return if the row was at the top of a section of the screen.
AA55 LD A,H Fetch the high byte of the address.
AA56 ADD A,$08 Move to the bottom pixel line of the character row.
AA58 LD H,A Store it.
AA59 RET Return.
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