in a,($7e)         ; Check line number
    add a,a            ; Double it. This will make numbers below $80 alias to those above it; the game uses line interrupts from line $7f to $bf.
    ld ($de87),a       ; Self-modify the following instruction to index the ath byte in a buffer at $c800.
    ld  a,($c800)      ; The previous line modifies the middle byte of this opcode: the LSB of the address.
    out ($bf),a        ; Set the HScroll to that value
    ld a,$88
    out ($bf),a
    pop af             ; Clean up and return
    ei
    ret