1.4 Process  (Page 3/9)

To see what a typical activation record looks like, consider the following HLA procedure declaration:

Whenever an HLA program calls this ARDemo procedure, it begins by pushing the data for the parameters onto the stack. The calling code will push the parameters onto the stack in the order they appear in the parameter list, from left to right. Therefore, the calling code first pushes the value for the i parameter, then it pushes the value for the j parameter, and it finally pushes the data for the k parameter. After pushing the parameters, the program calls the ARDemo procedure. Immediately upon entry into the ARDemo procedure, the stack contains these four items arranged as shown in Figure 3.1

Figure 3.1 Stack Organization Immediately Upon Entry into ARDemo

The first few instructions in ARDemo (note that it does not have the @NOFRAME option) will push the current value of EBP onto the stack and then copy the value of ESP into EBP. Next, the code drops the stack pointer down in memory to make room for the local variables. This produces the stack organization shown in Figure 3.2

Figure 3.2 Activation Record for ARDemo

To access objects in the activation record you must use offsets from the EBP register to the desired object. The two items of immediate interest to you are the parameters and the local variables. You can access the parameters at positive offsets from the EBP register, you can access the local variables at negative offsets from the EBP register as Figure 3.3 shows:

Figure 3.3 Offsets of Objects in the ARDemo Activation Record

Intel specifically reserves the EBP (extended base pointer) for use as a pointer to the base of the activation record. This is why you should never use the EBP register for general calculations. If you arbitrarily change the value in the EBP register you will lose access to the current procedure's parameters and local variables.

Threads

In modern operating systems, each process can have several threads. Multiple threads share the same program code, operating system resources (such as memory and file access) and operating system permissions (for file access as the process they belong to). A process that has only one thread is referred to as a single-threaded process, while a process with multiple threads is referred to as a multi-threaded process. Multi-threaded processes have the advantage that they can perform several tasks concurrently without the extra overhead needed to create a new process and handle synchronized communication between these processes. For example a word processor could perform a spell check as the user types, without freezing the application - one thread could handle user input, while another runs the spell checking utility

Process control block (pcb)

A Process Control Block (PCB) is a data structure in the operating system kernel containing the information needed to manage a particular process. The PCB is "the manifestation of a process in an operating system".