Li Qing - Real-Time Concepts for Embedded Systems стр 11.

For many embedded applications, the run address is the same as the load address. These embedded applications are directly downloaded into the target system memory for immediate execution without the need for any code or data transfer from one memory type or location to another. This practice is common during the development phase. We revisit this topic in Chapter 3, which covers the topic of image transfer from the host system to the target system.

2.4 Mapping Executable Images into Target Embedded Systems

creating a linker command file is for the embedded developer to map the executable image into the target system accurately and efficiently.

2.4.1 Linker Command File

The MEMORY directive can be used to describe the target systems memory map . The memory map lists the different types of memory (such as RAM, ROM, and flash) that are present on the target system, along with the ranges of addresses that can be accessed for storing and running an executable image. An embedded developer needs to be familiar with the addressable physical memory on a target system before creating a linker command file. One of the best ways to do this process, other than having direct access to the hardware engineering team that built the target system, is to look at the target systems schematics , as shown in Figure 2.5, and the hardware documentation. Typically, the hardware documentation describes the target systems memory map.

Figure 2.5: Simplified schematic and memory map for a target system.

The linker combines input sections having the same name into a single output section with that name by default. The developer-created, custom-named sections appear in the object file as independent sections. Sometimes developers might want to change this default linker behavior of only coalescing sections with the same name. The embedded developer might also need to instruct the linker on where to map the sections, in other words, what addresses should the linker use when performing symbol resolutions. The embedded developer can use the SECTION directive to achieve these goals.

The MEMORY directive defines the types of physical memory present on the target system and the address range occupied by each physical memory block, as specified in the following generalized syntax

MEMORY {

area-name: org = start-address, len = number-of-bytes

}

In the example shown in Figure 2.5, three physical blocks of memory are present:

· a ROM chip mapped to address space location 0, with 32 bytes,

· some flash memory mapped to address space location 0x40, with 4,096 bytes, and

· a block of RAM that starts at origin 0x10000, with 65,536 bytes.

Translating this memory map into the MEMORY directive is shown in Listing 2.2. The named areas are ROM, FLASH, and RAM.

Listing 2.2: Memory map.

MEMORY {

ROM: origin = 0x0000h, length = 0x0020h

FLASH: origin = 0x0040h, length = 0x1000h

RAM: origin = 0x1000h, length = 0x10000h

}

The SECTION directive tells the linker which input sections are to be combined into which output section, which output sections are to be grouped together and allocated in contiguous memory, and where to place each section, as well as other information. A general notation of the SECTION command is shown in Listing 2.3.

Listing 2.3: SECTION command.

SECTION {

output-section-name: {contents} area-name

GROUP {

[ALIGN(expression)]

section-definition

} area-name

}

The example shown in Figure 2.6 contains three default sections (.text,.data, and.bss), as well as two developer-specified sections (loader and my_section), contained in two object files generated by a compiler or assembler (file1.o and file2.o). Translating this example into the MEMORY directive is shown in Listing 2.4.

Figure 2.6: Combining input sections into an executable image.

Listing 2.4: Example code.

SECTION {

.text:

{

my_section

*(.text)

}

loader: FLASH

GROUP ALIGN (4):

{

.text,

.data: {}

.bss: {}

} RAM

}

The SECTION command in the linker command file instructs the linker to combine the input section named my_section and the default.text sections from all object files into the final output.text section. The loader section is placed into flash memory. The sections.text,.data, and.bss are grouped together and allocated in contiguous physical RAM memory aligned on the 4-byte

boundary, as shown in Figure 2.7.

Figure 2.7: Mapping an executable image into the target system.

Tips on section allocation include the following:

· allocate sections according to size to fully use available memory, and

· examine the nature of the underlying physical memory, the attributes, and the purpose of a section to determine which physical memory is best suited for allocation.

2.4.2 Mapping Executable Images

2.4.3 Example in Practice

Table 2.3: Example embedded application with sections.