Ch 11: Real-Time Interrupts & Timer Programming

cld
std

scrollup:
    push bp
    mov bp, sp
    push ax
    push cx
    push si
    push di
    push es
    push ds

    mov ax, 80            ; Load characters per row in AX
    mul byte [bp+4]       ; Calculate source word position (N * 80)
    mov si, ax            ; SI = source word position
    push si               ; Save position for clearing bottom lines later
    shl si, 1             ; Convert word index to byte offset

    mov cx, 2000          ; Total screen word locations (80x25)
    sub cx, ax            ; Count of words to move (2000 - N*80)

    mov ax, 0xb800
    mov es, ax            ; ES points to Video RAM base
    mov ds, ax            ; DS points to Video RAM base
    xor di, di            ; DI = 0 (top-left column)
    cld                   ; Set autoincrement mode
    rep movsw             ; Copy screen content up

    mov ax, 0x0720        ; Space character with normal attribute
    pop cx                ; Restore count of positions to clear (N * 80)
    rep stosw             ; Clear bottom lines with spaces

    pop ds
    pop es
    pop di
    pop si
    pop cx
    pop ax
    pop bp
    ret 2

strlen:
    push bp
    mov bp, sp
    ...
    repne scasb