SystemVerilog Mailbox
Table of Contents
A mailbox is a communication mechanism that allows messages to be exchanged between processes. The process which wants to talk to another process posts the message to a mailbox, which stores the messages temporarily in a system defined memory object, to pass it to the desired process.
Based on the sizes mailboxes are categorized as,
- bounded mailbox
- unbounded mailbox
A bounded mailbox is with the size defined. mailbox becomes full when on storing a bounded number of messages. A process that attempts to place a message into a full mailbox shall be suspended until enough space becomes available in the mailbox queue.
Unbounded mailboxes are with unlimited size.
Mailbox types
There are two types of mailboxes,
- Generic Mailbox
- Parameterized mailbox
Generic Mailbox (type-less mailbox)
The default mailbox is type-less. that is, a single mailbox can send and receive data of any type.
mailbox mailbox_name;
Parameterized mailbox (mailbox with particular type)
Parameterized mailbox is used to transfer a data of particular type.
mailbox#(type) mailbox_name;
Mailbox Methods
SystemVerilog Mailbox is a built-in class that provides the following methods. these are applicable for both Generic and Parameterized mailboxes
new(); - Create a mailbox put(); - Place a message in a mailbox try_put(); - Try to place a message in a mailbox without blocking get(); or peek();- Retrieve a message from a mailbox num(); - Returns the number of messages in the mailbox try_get(); or try_peek(); - Try to retrieve a message from a mailbox without blocking
new( );
Mailboxes are created with the new() method.
mailbox_name = new(); // Creates unbounded mailbox and returns mailbox handle mailbox_name = new(m_size); //Creates bounded mailbox with size m_size and returns mailbox handle ,where m_size is integer variable
SystemVerilog Mailbox example
In the example below,
Mailbox is used for communication between generator and driver.
- Process-1(Generator class) will generate (created and randomize) the packet and put into the mailbox mb_box
- Process-2(Driver class) gets the generated packet from the mailbox and display the fields
//-------------------------------------------------------------------------
// Packet
//-------------------------------------------------------------------------
class packet;
rand bit [7:0] addr;
rand bit [7:0] data;
//Displaying randomized values
function void post_randomize();
$display("Packet::Packet Generated");
$display("Packet::Addr=%0d,Data=%0d",addr,data);
endfunction
endclass
//-------------------------------------------------------------------------
//Generator - Generates the transaction packet and send to driver
//-------------------------------------------------------------------------
class generator;
packet pkt;
mailbox m_box;
//constructor, getting mailbox handle
function new(mailbox m_box);
this.m_box = m_box;
endfunction
task run;
repeat(2) begin
pkt = new();
pkt.randomize(); //generating packet
m_box.put(pkt); //putting packet into mailbox
$display("Generator::Packet Put into Mailbox");
#5;
end
endtask
endclass
//-------------------------------------------------------------------------
// Driver - Gets the packet from generator and display's the packet items
//-------------------------------------------------------------------------
class driver;
packet pkt;
mailbox m_box;
//constructor, getting mailbox handle
function new(mailbox m_box);
this.m_box = m_box;
endfunction
task run;
repeat(2) begin
m_box.get(pkt); //getting packet from mailbox
$display("Driver::Packet Recived");
$display("Driver::Addr=%0d,Data=%0d\n",pkt.addr,pkt.data);
end
endtask
endclass
//-------------------------------------------------------------------------
// tbench_top
//-------------------------------------------------------------------------
module mailbox_ex;
generator gen;
driver dri;
mailbox m_box; //declaring mailbox m_box
initial begin
//Creating the mailbox, Passing the same handle to generator and driver,
//because same mailbox should be shared in-order to communicate.
m_box = new(); //creating mailbox
gen = new(m_box); //creating generator and passing mailbox handle
dri = new(m_box); //creating driver and passing mailbox handle
$display("------------------------------------------");
fork
gen.run(); //Process-1
dri.run(); //Process-2
join
$display("------------------------------------------");
end
endmodule
Simulator Output
------------------------------------------ Packet::Packet Generated Packet::Addr=3,Data=38 Generator::Packet Put into Mailbox Driver::Packet Recived Driver::Addr=3,Data=38 Packet::Packet Generated Packet::Addr=118,Data=92 Generator::Packet Put into Mailbox Driver::Packet Recived Driver::Addr=118,Data=92 ------------------------------------------
