How to update multiple outputs simultaneously I need to update simultaneously all the outputs of a specific port.  

I am familiar with using micro-processors assembly language. After initializing a port (Data Direction Register), I can write to that port and all outputs will update simultaneously.  

In the Arduino commands  : I am aware of how to configure pins, such as pinMode(pin, mode), to make a pin an output or input. 

I know of the digitalWrite(pin, value) , but this only updates one pin per command.  

For timing reasons, I need to update several (same port) outputs without any time delay.  

Normally (using assembly language) , I would keep a register with the data I desire on the output port, and write that register's data to the port.  

Is there a command I can use in Arduino to accomplish the same thing?
If not a single command, what other choices do I have?  

I use Arduino  micro,  UNO,  but I think the question might be valid for most Arduino
Yes, the ports of an Uno are addressed in C/C++ as PORTB, PORTC, and PORTD respectively, each 8 bits wide. You can read them or assign to them. Other processors' ports have similar names. Of course the pins' port assignments and functions differ from one to the other, which is one reason for providing the single-pin pinMode() and digitalRead(), digitalWrite() functions. But where you need to deal directly with the hardware (and probably don't care about chip-to-chip portability), the registers are exposed with the above symbols.

Also note that most other CPU registers are exposed the same way - the ports' data-direction registers as DDRA, DDRB, DDRC. You may not care about setting them up simultaneously, but if you wanted to make all of your hardware accesses similarly (and save the flash space for the portable pin functions), you could.

Update: 


  Is there a specific command I can use ? Such as Write PortD ?


Yes, you can use the PORTx names like any other C/C++ variable:
PORTD = 0xFF;
PORTD ^= 0x02;
if( (PORTD &amp; 0x40) != 0 ){, for just a couple of examples - just like real C! :-) 

PS: Pighixxx makes absolutely top-notch pinout diagrams for boards &amp; processors. Here is his diagram for the 28-pin DIP-packaged AtMega328p for instance. They're extremely useful when you're working with a chip at the port or board level.