SV Part-6 communication

Inter process communication

Inter-Process Communication (IPC) means:

Two or more processes (different blocks of code running in parallel) communicating or synchronizing with each other.

• All three are IPC mechanisms in SystemVerilog testbenches.

• They differ in what problem they solve:

• Events - sync,

• Semaphores - resource control,

• Mailboxes - data transfer.

you choose the right IPC primitive depending on whether you need synchronization, resource sharing, or message passing

Events

Verilog events

• In Verilog , event is just a flag.

• It does not carry data.

• It is not persistent : if you miss the trigger at that moment, you never catch it later.

System Verilog Events

• System Verilog event is more powerful.

• Supports persistent triggering : if the event was triggered before you wait, you still catch it.

Persistence means:

Even if the event is triggered before someone waits for it, the waiter will still see it.

Semaphores

What is a semaphore : A semaphore is a System Verilog built-in synchronization object that controls access to a shared resource using keys.

why we use semaphore : To prevent race conditions when multiple processes/threads want to use the same resource.

How it works :

• semaphore is creatced with some number of keys

• A thread gets() keys - enters critical section.

• A thread puts() - leaves critical section.

• if keys are not available - thread waits.

Methods in Semaphore

These 4 are the main methods. (Semaphore is simple only these are used in real testbenches.)

mailbox

mailbox is used to connect the classes

a mailbox does not explicitly use indices like an array or queue to access its contents. Instead, a mailbox operates as a FIFO (First-In-First-Out) queue, where messages are added to the end (via put()) and retrieved from the front (via get() or peek()).

• mailbox does not provide a mechanism to access messages by an index (e.g., mbox[0], mbox[1]). Unlike arrays or queues, mailboxes are designed for inter-process communication with a strict FIFO order, and you can only interact with the head of the queue (for retrieval) or the tail (for insertion).

• The mailbox methods (put(), get(), peek(), etc.) do not expose or allow direct access to specific positions in the mailbox’s internal storage.

Types: Based on the size declaration

1.Bounded mailbox syntax: mailbox <mailbox_name> = new(size);

2.Un-Bounded mailbox syntax: mailbox <mailbox_name> = new();

Based on the type

1.generic mailbx --> it allow different datatype syntax: mailbox <mailbox_name> = new();

2.parameterized mailbox syntax: mailbox #(parameter type)<mailbox_name> = new();

Table: SystemVerilog Mailbox Methods and Their Details