15. Shared Memory Support Driver — RTEMS BSP and Driver Guide 5.4-rc2 (21st August 2026) documentation

# 15. Shared Memory Support Driver

The Shared Memory Support Driver is responsible for providing glue routines and configuration information required by the Shared Memory Multiprocessor Communications Interface (MPCI). The Shared Memory Support Driver tailors the portable Shared Memory Driver to a particular target platform.

This driver is only required in shared memory multiprocessing systems that use the RTEMS mulitprocessing support. For more information on RTEMS multiprocessing capabilities and the MPCI, refer to the Multiprocessing Manager chapter of the RTEMS Application C User’s Guide.

## 15.1. Shared Memory Configuration Table

The Shared Memory Configuration Table is defined in the following structure:

    typedef volatile uint32_t vol_u32;

    typedef struct {
      vol_u32 *address;        /* write here for interrupt    */
      vol_u32  value;          /* this value causes interrupt */
      vol_u32  length;         /* for this length (0,1,2,4)   */
    } Shm_Interrupt_information;

    struct shm_config_info {
      vol_u32           *base;                     /* base address of SHM         */
      vol_u32            length;                   /* length (in bytes) of SHM    */
      vol_u32            format;                   /* SHM is big or little endian */
      vol_u32          (*convert)();               /* neutral conversion routine  */
      vol_u32            poll_intr;                /* POLLED or INTR driven mode  */
      void             (*cause_intr)( uint32_t );
      Shm_Interrupt_information Intr;              /* cause intr information      */
    };

    typedef struct shm_config_info shm_config_table;

where the fields are defined as follows:

`base`

  is the base address of the shared memory buffer used to pass messages between the nodes in the system.

`length`

  is the length (in bytes) of the shared memory buffer used to pass messages between the nodes in the system.

`format`

  is either `SHM_BIG` or `SHM_LITTLE` to indicate that the neutral format of the shared memory area is big or little endian. The format of the memory should be chosen to match most of the inter-node traffic.

`convert`

  is the address of a routine which converts from native format to neutral format. Ideally, the neutral format is the same as the native format so this routine is quite simple.

`poll_intr`, `cause_intr`

  is either `INTR_MODE` or `POLLED_MODE` to indicate how the node will be informed of incoming messages.

`Intr`

  is the information required to cause an interrupt on a node. This structure contains the following fields:

  `address`

    is the address to write at to cause an interrupt on that node. For a polled node, this should be NULL.

  `value`

    is the value to write to cause an interrupt.

  `length`

    is the length of the entity to write on the node to cause an interrupt. This can be 0 to indicate polled operation, 1 to write a byte, 2 to write a sixteen-bit entity, and 4 to write a thirty-two bit entity.

## 15.2. Primitives

### 15.2.1. Convert Address

The `Shm_Convert_address` is responsible for converting an address of an entity in the shared memory area into the address that should be used from this node. Most targets will simply return the address passed to this routine. However, some target boards will have a special window onto the shared memory. For example, some VMEbus boards have special address windows to access addresses that are normally reserved in the CPU’s address space.

    void *Shm_Convert_address( void *address )
    {
      return the local address version of this bus address
    }

### 15.2.2. Get Configuration

The `Shm_Get_configuration` routine is responsible for filling in the Shared Memory Configuration Table passed to it.

    void Shm_Get_configuration(
      uint32_t           localnode,
      shm_config_table **shmcfg
    )
    {
      fill in the Shared Memory Configuration Table
    }

### 15.2.3. Locking Primitives

This is a collection of routines that are invoked by the portable part of the Shared Memory Driver to manage locks in the shared memory buffer area. Accesses to the shared memory must be atomic. Two nodes in a multiprocessor system must not be manipulating the shared data structures simultaneously. The locking primitives are used to insure this.

To avoid deadlock, local processor interrupts should be disabled the entire time the locked queue is locked.

The locking primitives operate on the lock `field` of the `Shm_Locked_queue_Control` data structure. This structure is defined as follows:

    typedef struct {
      vol_u32 lock;  /* lock field for this queue    */
      vol_u32 front; /* first envelope on queue      */
      vol_u32 rear;  /* last envelope on queue       */
      vol_u32 owner; /* receiving (i.e. owning) node */
    } Shm_Locked_queue_Control;

where each field is defined as follows:

`lock`

  is the lock field. Every node in the system must agree on how this field will be used. Many processor families provide an atomic “test and set” instruction that is used to manage this field.

`front`

  is the index of the first message on this locked queue.

`rear`

  is the index of the last message on this locked queue.

`owner`

  is the node number of the node that currently has this structure locked.

#### 15.2.3.1. Initializing a Shared Lock

The `Shm_Initialize_lock` routine is responsible for initializing the lock field. This routines usually is implemented as follows:

    void Shm_Initialize_lock(
      Shm_Locked_queue_Control *lq_cb
    )
    {
      lq_cb->lock = LQ_UNLOCKED;
    }

#### 15.2.3.2. Acquiring a Shared Lock

The `Shm_Lock` routine is responsible for acquiring the lock field. Interrupts should be disabled while that lock is acquired. If the lock is currently unavailble, then the locking routine should delay a few microseconds to allow the other node to release the lock. Doing this reduces bus contention for the lock. This routines usually is implemented as follows:

    void Shm_Lock(
      Shm_Locked_queue_Control *lq_cb
    )
    {
      disable processor interrupts
        set Shm_isrstat to previous interrupt disable level

      while ( TRUE ) {
        atomically attempt to acquire the lock
        if the lock was acquired
          return
        delay some small period of time
      }
    }

#### 15.2.3.3. Releasing a Shared Lock

The `Shm_Unlock` routine is responsible for releasing the lock field and reenabling processor interrupts. This routines usually is implemented as follows:

    void Shm_Unlock(
      Shm_Locked_queue_Control *lq_cb
    )
    {
      set the lock to the unlocked value
      reenable processor interrupts to their level prior
        to the lock being acquired.  This value was saved
        in the global variable Shm_isrstat
    }

## 15.3. Installing the MPCI ISR

The `Shm_setvec` is invoked by the portable portion of the shared memory to install the interrupt service routine that is invoked when an incoming message is announced. Some target boards support an interprocessor interrupt or mailbox scheme and this is where the ISR for that interrupt would be installed.

On an interrupt driven node, this routine would be implemented as follows:

    void Shm_setvec( void )
    {
      install the interprocessor communications ISR
    }

On a polled node, this routine would be empty.