Used by the routine at F1BA.
|Set the instruction at E7EB to 'LD BC,$001F'
|Set the instruction at E7EE to 'LDIR'
|There are 8 columns to scroll on
|Save the column counter
|There are 20 rows to be scrolled left
|Point HL at the attribute byte for the cell at screen coordinates (1,0)
|Point DE at the attribute byte for the adjacent cell on the left
|Save the row counter
|Save the source attribute file address
|Shift this row of the screen left
|Restore the source attribute file address to HL
|Point HL at the next row down in the attribute file
|Restore the row counter to B
|Jump back until all 20 rows have been scrolled left
|7FFE holds the x-coordinate of the leftmost column of the play area on screen
|Increment this x-coordinate
|L=31 (rightmost screen column), H=19 (bottom row of the screen)
|Save the screen coordinates
|Print a tile at screen coordinates (31,H)
|Restore the screen coordinates to HL
|Have we printed every tile in the column yet?
|Jump back if not
|Restore the column counter to B
|Jump back until 8 columns have been scrolled on