JSR - Jump to Subroutine ;flags()
The return address is the address of the next instruction after the JSR. The
return address if first pushed on the stack, LOW byte then HIGH byte.
;push PCL ;SP <= SP - 1
;push PCH ;SP <= SP - 1
The following then happens depending upon the addressing mode:
JSR opr ;PC <= opr ;(DIR) 8 bit direct
JSR opr ;PC <= opr ;(EXT) 16 bit extended
JSR ,X ;PC <= X ;(IX) indexed no offset
JSR opr,X ;PC <= X + opr ;(IX1) indexed w/8 bit offset
JSR opr,X ;PC <= X + opr ;(IX2) indexed w/16 bit offset