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

7.2.3. Image Complexity

7.2.3. Image Complexity

As application developers, we do not need to concern ourselves with the layout of a binary executable file when we develop applications for our favorite platform. The compiler and binary utilities are preconfigured to build a binary executable image containing the proper components needed for a given architecture. The linker places startup (prologue) and shutdown (epilogue) code into the image. These objects set up the proper execution context for your application, which typically starts at main() in your application.

This is absolutely not the case with a typical bootloader. When the bootloader gets control, there is no context or prior execution environment. In a typical system, there might not be any DRAM until the bootloader initializes the processor and related hardware. Consider what this means. In a typical C function, any local variables are stored on the stack, so a simple function like the one in Listing 7-1 is unusable.

Listing 7-1. Simple C function

int setup_memory_controller(board_info_t *p) {
 unsigned int *dram_controller_register = p->dc_reg;
 ... 

When a bootloader gains control on power-on, there is no stack and no stack pointer. Therefore, a simple C function similar to Listing 7-1 will likely crash the processor because the compiler will generate code to create and initialize the pointer dram_controller_register on the stack, which does not yet exist. The bootloader must create this execution context before any C functions are called.

When the bootloader is compiled and linked, the developer must exercise complete control over how the image is constructed and linked. This is especially true if the bootloader is to relocate itself from Flash to RAM. The compiler and linker must be passed a handful of parameters defining the characteristics and layout of the final executable image. Two primary characteristics conspire to add complexity to the final binary executable image.

The first characteristic that presents complexity is the need to organize the startup code in a format compatible with the processor's boot sequence. The first bytes of executable code must be at a predefined location in Flash, depending on the processor and hardware architecture. For example, the AMCC PowerPC 405GP processor seeks its first machine instructions from a hard-coded address of 0xFFFF_FFFC. Other processors use similar methods with different details. Some processors are configurable at power-on to seek code from one of several predefined locations, depending on hardware configuration signals.

How does a developer specify the layout of a binary image? The linker is passed a linker description file, also called a linker command script. This special file can be thought of as a recipe for constructing a binary executable image. Listing 7-2 contains a snippet from an existing linker description file in use in a popular bootloader, which we discuss shortly.

Listing 7-2. Linker Command ScriptReset Vector Placement

SECTIONS
{
 .resetvec 0xFFFFFFFC :
 {
  *(.resetvec)
 } = 0xffff
...

A complete description of linker command scripts syntax is beyond the scope of this book. The interested reader is directed to the GNU LD manual referenced at the end of this chapter. Looking at Listing 7-2, we see the beginning of the definition for the output section of the binary ELF image. It directs the linker to place the section of code called .resetvec at a fixed address in the output image, starting at location 0xFFFF_FFFC. Furthermore, it specifies that the rest of this section shall be filled with all ones (0xFFFF.) This is because an erased Flash memory array contains all ones. This technique not only saves wear and tear on the Flash memory, but it also significantly speeds up programming of that sector.

Listing 7-3 is the complete assembly language file from a recent U-Boot distribution that defines the .resetvec code section. It is contained in an assembly language file called .../cpu/ppc4xx/resetvec.S. Notice that this code section cannot exceed 4 bytes in length in a machine with only 32 address bits. This is because only a single instruction is defined in this section, no matter what configuration options are present.

Listing 7-3. Source Definition of .resetvec

/* Copyright MontaVista Software Incorporated, 2000 */
#include <config.h>
 .section .resetvec, "ax"
#if defined(CONFIG_440)
 b _start_440
#else
#if defined(CONFIG_BOOT_PCI) && defined(CONFIG_MIP405)
 b _start_pci
#else
 b _start
#endif
#endif 

This assembly language file is very easy to understand, even if you have no assembly language programming experience. Depending on the particular configuration (as specified by the CONFIG_* macros), an unconditional branch instruction (b in PowerPC assembler syntax) is generated to the appropriate start location in the main body of code. This branch location is a 4-byte PowerPC instruction, and as we saw in the snippet from the linker command script in Listing 7-2, this simple branch instruction is placed in the absolute Flash address of 0xFFFF_FFFC in the output image. As mentioned earlier, the PPC 405GP processor fetches its first instruction from this hard-coded address. This is how the first sequence of code is defined and provided by the developer for this particular architecture and processor combination.

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