Dear Sylvain,

I think you have to creat the code such as,

for horizontal scroll left to right:

initialize the LCD with cursor off, display shift from left to right when receive any character.
send cursor to the last character location in first row, i.e. 0Fh.

load a character counter register (CCR) with no. of char. i.e. 24character.
load a character location pointer (CLP) for example org100h.
load a defecto 8bit register, which will be used to hold character which is going to be sent to LCD. lets say name it LCDD

DIS_UPDATE:
mov LCDD, @CLP ; mov character from chr. loc. pointer to LCDD
ACALL LCDSENT routine ; call routine which will send data to display
inc CLP ; increment character location pointer
DJNZ CCR,DIS_UPDATE ; see if all char. are not displayed so goback.

if you want to again scroll the same string to display, so reload CLP and CCR counters and call DIS_UPDATE routine.

For vertical row scroll will be lenghthy as you have to scroll all rows data and update them again and again. you will be requiring more ram
as well.

try this out, and if you need further info. please email me.