ORG instruction

Hello,
I got some question about the ORG XXXXH instruction...isit there is a must for us to start coding from ORG 0000H before LJMP, why can't we just start our coding from ORG 0001H?

8051's reset vector location is 00H, so the first instruction execution starts from 0H location. If you start your code from 01H or any other location, then probably your code will not run at all.

8051's reset vector location is 00H, so the first instruction execution starts from 0H location. If you start your code from 01H or any other location, then probably your code will not run at all.

ajay_bhargav

Itis still got other instruction can let us to start the coding from 01H or other except LJMP ?

It is still got other instruction can let us to start the coding from 01H or other except LJMP ?

tanyp1997

When the 8051 starts/resets it starts reading code from 0000.
If you don't put valid code there it will not run.

The first 38 memory bytes are used by interrupts.
You could use this area for your code if you don't enable
any interrupts, but normally you don't.

Why don't you want to use a jump ?

Your second interrupt vector address is at 0003H (for INT0) which leaves a room of 3 bytes where only an LJMP (3 byte instruction) can fit properly. Is there a special requirement you want to start from 01H?