Книга: Embedded Linux Primer: A Practical, Real-World Approach

2.3.7. Process Virtual Memory

2.3.7. Process Virtual Memory

When a process is spawnedfor example, when the user types ls at the Linux command promptthe kernel allocates memory for the process and assigns a range of virtual-memory addresses to the process. The resulting address values bear no fixed relationship to those in the kernel, nor to any other running process. Furthermore, there is no direct correlation between the physical memory addresses on the board and the virtual memory as seen by the process. In fact, it is not uncommon for a process to occupy multiple different physical addresses in main memory during its lifetime as a result of paging and swapping.

Listing 2-4 is the venerable "Hello World," as modified to illustrate the previous concepts. The goal with this example is to illustrate the address space that the kernel assigns to the process. This code was compiled and run on the AMCC Yosemite board, described earlier in this chapter. The board contains 256MB of DRAM memory.

Listing 2-4. Hello World, Embedded Style

#include <stdio.h>
int bss_var; /* Uninitialized global variable */
int data_var = 1; /* Initialized global variable */
int main(int argc, char **argv) {
 void *stack_var; /* Local variable on the stack */
 stack_var = (void *)main; /* Don't let the compiler optimize it out */
 printf("Hello, World! Main is executing at %pn", stack_var);
 printf("This address (%p) is in our stack framen", &stack_var);
 /* bss section contains uninitialized data */
 printf("This address (%p) is in our bss sectionn", &bss_var);
 /* data section contains initializated data */
 printf("This address (%p) is in our data sectionn", &data_var);
 return 0;
}

Listing 2-5 shows the console output that this program produces. Notice that the process called hello thinks it is executing somewhere in high RAM just above the 256MB boundary (0x10000418). Notice also that the stack address is roughly halfway into a 32-bit address space, well beyond our 256MB of RAM (0x7ff8ebb0). How can this be? DRAM is usually contiguous in systems like these. To the casual observer, it appears that we have nearly 2GB of DRAM available for our use. These virtual addresses were assigned by the kernel and are backed by physical RAM somewhere within the 256MB range of available memory on the Yosemite board.

Listing 2-5. Hello Output

root@amcc:~# ./hello
Hello, World! Main is executing at 0x10000418
This address (0x7ff8ebb0) is in our stack frame
This address (0x10010a1c) is in our bss section
This address (0x10010a18) is in our data section
root@amcc:~#

One of the characteristics of a virtual memory system is that when available physical RAM goes below a designated threshold, the kernel can swap memory pages out to a bulk storage medium, usually a hard disk drive. The kernel examines its active memory regions, determines which areas in memory have been least recently used, and swaps these memory regions out to disk, to free them up for the current process. Developers of embedded systems often disable swapping on embedded systems because of performance or resource constraints. For example, it would be ridiculous in most cases to use a relatively slow Flash memory device with limited write life cycles as a swap device. Without a swap device, you must carefully design your applications to exist within the limitations of your available physical memory.

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