Trigger fetchaddress on Account (before insert , before update) {
    set<id> acid = new set<id>();
    for(Account ac: Trigger.new){
        if (ac.id != null)
            acid.add(ac.id);
    }
    Map<id,Address__c> fetch = new Map<id,Address__c>([select id,name,City__c,Country__c,Postal_Code__c,State__c,Billing_Street__c from Address__c where id in : acid]);

    for(Account a : Trigger.new){
        Address__c ad = fetch.get(a.id);

        if(A.id !=null)

        a.Billingstreet = ad.Billing_Street__c;
        a.Billingcity = ad.City__c;
        a.Billingstate = ad.State__c;
        a.BillingPostalcode = ad.Postal_Code__c;
        a.Billingcountry = ad.Country__c;
    }
}