In your code the restart will happen as follows:

An external interrupt needs to be enabled to be able to happen, i.e. the external interrupt enable flag needs to be set and the EA flag needs to be set. Then if the INT0 pin is pulled low it will casue an interrupt to occur which will vector the program to the Intr0: label. At this point the code you show causes the address of SDIReset to be pushed into the stack. The next RETI instruction then casues the program to return not to the normal place but instead to the SDIReset: label.

Michael Karas