Scrolling Up the Screen in Assembly with Code

Following piece of code is taken from the program of scrolling up the screen write against each
instruction what it does?
Mov ax 80
Mu byte [bp+4]
Mov si, ax
Push si
Shl si1

mov ax, 80 ; load chars per row in ax
mul byte [bp+4] ; calculate source position
mov si, ax ; load source position in si
push si ; save position for later use
shl si, 1 ; convert to byte offset