Platform Abstraction Layer

The PAL, or Platform Abstraction Layer, enables the STRIDE Runtime to be platform-independent by providing a consistent interface for the STRIDE Runtime regardless of the operating system or data transport used. This interface layer is necessary given the broad variety of operating systems and data transports that exist within embedded systems today.

PAL Concepts

This section describes the basic concepts of the PAL and the system services it must provide.

Function Registration

Typically, the STRIDE Runtime calls PAL functions; however, there are some functions in the STRIDE Runtime that need to be called by the PAL. To eliminate PAL dependencies on the STRIDE Runtime, these functions are accessed through a function registration process initiated by the STRIDE Runtime at startup. You write the registration routine called by the Runtime. This registration routine passes in the address of the STRIDE Runtime function to be registered as the input parameter, and stores the address of the STRIDE Runtime function in your own function variable, which is actually a pointer to a function. You can then call the registered function using your function pointer variable.

The registered STRIDE Runtime functions allow you to complete such tasks as delivering a received I-block to the Runtime, checking the number of I-blocks the Runtime has ready to send out, or signalling the Runtime that your transport is ready for the next I-block. It is not necessary for the PAL to know the details of these Runtime functions.

Event Notification

Unique information is required by an operating system to notify a thread of a pending event. This information can be a simple index into a table, an address to a thread control block, the address of a semaphore or one of a number of other implementations. Although each implementation may be different, a unique notifier is necessary for each thread. The STRIDE Runtime calls this unique information a Notification Identifier (NID). A thread uses a NID when waiting for a STRIDE event, and the STRIDE Runtime uses the same NID to notify the thread of a pending STRIDE event.

An additional piece of information included with the notify routine is a box ID, which is the ID of the mailbox where the message is delivered.

Critical Data Protection

The STRIDE Runtime needs to protect critical data structures from multiple, simultaneous accesses. The specific method of protection is not dictated to the PAL. The PAL requires that when one thread calls a STRIDE function which in turn calls palProtect(), and another thread calls a STRIDE function which also calls palProtect(); the second thread is preempted until the first thread’s STRIDE function calls palUnprotect(). Once palUnprotect() is called, the second thread can be rescheduled and continue. In addition, the STRIDE Runtime guarantees that calls to palProtect() will not be nested.

Timer Administration

The STRIDE Runtime requires that at least one timer be available. When a timer is created, a callback is registered so that the STRIDE Runtime can be notified of timer expirations. A user parameter, provided when the timer is created, is passed to the callback when it gets called. If several timers share a callback routine, this user parameter can be used to identify which timer expired. The STRIDE Runtime can also stop, start and delete timers.

Because the STRIDE Runtime also has the need to timestamp trace log data, a routine that returns the system time, PalGetTime(), is also part of the PAL. This routine is not associated with timers.

Memory Allocation

The STRIDE Runtime dynamically allocates memory for messages and trace log storage. The Runtime uses palMemAlloc() and palMemFree() to allocate memory dynamically and then return it to the system.

Transport Services

The PAL transport routines are needed to transfer I-blocks (STRIDE data packets) into and out of the STRIDE Runtime on your target platform. These routines handle the buffering and transferring of data to and from your transport mechanism. The PAL also contains registration routines that allow for STRIDE Runtime routines to be called by the PAL.

The palOut() routine enables the STRIDE Runtime to transfer I-blocks to your transport. The STRIDE Runtime calls the palOut() routine whenever it needs to send out an Iblock. Your transport calls the routine registered with the palOutRdyReg() routine when the transport is ready for the next I-block to be transmitted. The STRIDE Runtime will not call the palOut() routine until you call the registered function. In this way, you can control the flow of transmitted I-blocks.

When a complete I-block has been received by your transport, the routine registered with the palInReg() routine should be calle to put the received I-block into the STRIDE Runtime.

In some cases it is useful to know when the transmit path is not being requested. Your transport mechanism can check the number of I-blocks the STRIDE Runtime has ready for transmission by calling the routine registered with the palOutPndReg() routine.

Host Services

The PAL also provides a way for the STRIDE Runtime to support your transport protocol on the desktop. A Windows® DLL linked to STRIDE Studio enables these host services. The DLL allows your data to be received and transmitted using your transport

PAL Organization

This section explains the two categories of PAL services that support the STRIDE Runtime, as well as the files used to implement these services on your target.

PAL Services

The PAL services include the following:

• Operating System (OS) Services
• Input/Output (IO) Services

PAL Operating System Services

The PAL Operating System (OS) Services are the routines that enable the STRIDE Runtime to work with the operating system on your target platform. In order to write your PAL OS services you must have detailed knowledge of how your operating system handles thread synchronization, timers, critical sections, dynamic memory, and notification. The PAL OS services make the features of your operating system available to the STRIDE Runtime. All of the functions described here must be fully implemented in your PAL.

Function Name	Description
Synchronization
palCreateNID	Create a Notification Identifier (NID)
palDeleteNID	Delete a Notification Identifier (NID)
palCreateRFCProxyNID	Create the RFC Proxy Notification Identifier (NID)
palDeleteRFCProxyNID	Delete the RFC Proxy Notification Identifier (NID)
palWait	Wait for an event
palNotify	Signal a thread that an event is pending
Timers
palTimerCreate	Create a timer
palTimerDelete	Delete a timer
palTimerStart	Start a timer
palTimerStop	Stop a timer
palGetTime	Return system time (e.g., tick count)
Critical Section
palProtect	Begin critical section
palUnprotect	End critical section
Memory
palMemAlloc	Allocate a block of dynamic memory
palMemFree	Free a block of dynamic memory

PAL Input/Output Services

The PAL Input/Output (IO) Services are the routines that enable the STRIDE Runtime to work with different data transport mechanisms. These routines enable your transport to send and receive data between the host and the target.

The IO services have also been defined to allow the target platform to control how memory is managed and the rate of data exchange. See Appendix A beginning on page 121 for definitions of the header file functions

Function Name	Description
Transmit
PalOutPndReg	Query the current output queue of the Runtime
PalOutRdyReg	Identify the transport as ready to receive data
PalOut	Send data to the host platform
Receive
PalInReg	Data extracted from the transport and identified for the STRIDE Runtime

Integrating the PAL

PAL Operating System (OS) Services

The PAL OS Services provide the glue between the STRIDE Runtime and the Target Operating System. The services it must provide are:

• Task protection and synchronization (e.g. semaphores)
• Periodic Timers
• Get Time Stamp
• Memory Management

By convention, these services are implemented in a file called palOS.c.

PAL Transport (I/O) Services

The Transport provides the I/O services between the Host and Target. Most of the pre-packaged PALs below provide both a serial and a TCP/IP transport.

On the Target side, the Transport is usually implemented in file palIO.c.

On the Host side, the Panel program that manages the link between the Host and Target, must be loaded with the appropriate Transport protocol. STRIDE includes Host side Transports for serial and TCP/IP connections.

What about USB? S2 does not yet provide a native USB transport. If you wish to use one of our pre-packaged transports, then the USB can either be used to emulate a serial (COM) port, or can be used with TCP/IP by installing and using USBNet. USBNet is open software for Linux and Windows that permits TCP/IP connections over USB. Google "USBNet" to locate a driver for your systems.

What if I need a custom transport? A custom transport can be readily written to your specific needs. When STRIDE is installed on the Host, a Host Transport Wizard is installed in Visual Studio C++ which will create a new project (solution) which can be used to implement the Host side Transport. It is recommended that custom Target side transport be constructed by using one of the pre-packaged PALs as a guide. More information on the Transport Wizard can be found in the STRIDE Developer's Guide in Online Help.

Using a Pre-Packaged PAL

S2 can provide a ready made PAL for many common operating systems. Most contain support for both a serial and a TCP/IP transport. Select the optional runtime section when installing STRIDE to get the currently supported PALs installed on your machine, or contact us for help in obtaining a PAL for other platforms.

Creating your own PAL

The PAL enables STRIDE to be completely agnostic to the underlying operating system and hardware. In fact, an operating system is not really required, so long as the PAL can provide the needed services.

If there is no pre-packaged PAL for your system, you can use an existing one as a starting point, or even write a new one from scratch. Past experience has shown that a PAL can usually be implemented in about two days.

More information on creating a custom PAL, including a custom Transport can be found in the Online Help and in the Platform Abstraction Layer (PAL) Specification which can be found in C:\STRIDE\doc\s2sPAL.pdf. S2's marvelously talented System's Engineers are also available to assist you with this task.

PAL Resource Requirements

Most PALs require a reader task to monitor the Transport input data stream.

PAL memory usage is limited to a few dozen bytes for table and control block storage. The size of I/O buffers used by the reader task are Transport dependent but are usually in the 8KB range.

PAL Initialization

There is no "formal" mechanism required for initializing the PAL. That is, an initialization function is not required by the STRIDE Runtime or IM. However, over time the convention of providing a palInit() function has developed. All of the Pre-Packaged PALS use this convention and their palInit() function must be called before any other STRIDE Runtime function.

If you roll your own PAL, it is recommended that you maintain this convention.