Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

This chapter is from the book

7.2 Bootloader Challenges

Even a simple “Hello World” program written in C requires significant hardware and software resources. The application developer does not need to know or care much about these details because the C runtime environment transparently provides this infrastructure. A bootloader developer has no such luxury. Every resource that a bootloader requires must be carefully initialized and allocated before it is used. One of the most visible examples of this is Dynamic Random Access Memory (DRAM).

7.2.1 DRAM Controller

DRAM chips cannot be directly read from or written to like other microprocessor bus resources. They require specialized hardware controllers to enable read and write cycles. To further complicate matters, DRAM must be constantly refreshed or the data contained within will be lost. Refresh is accomplished by sequentially reading each location in DRAM in a systematic manner and within the timing specifications set forth by the DRAM manufacturer. Modern DRAM chips support many modes of operation, such as burst mode and dual data rate for high-performance applications. It is the DRAM controller’s responsibility to configure DRAM, keep it refreshed within the manufacturer’s timing specifications, and respond to the various read and write commands from the processor.

Setting up a DRAM controller is the source of much frustration for the newcomer to embedded development. It requires detailed knowledge of DRAM architecture, the controller itself, the specific DRAM chips being used, and the overall hardware design. Though this is beyond the scope of this book, the interested reader can learn more about this important concept by referring to the references at the end of this chapter. Appendix D, “SDRAM Interface Considerations,” provides more background on this important topic.

Very little can happen in an embedded system until the DRAM controller and DRAM itself have been properly initialized. One of the first things a bootloader must do is to enable the memory subsystem. After it is initialized, memory can be used as a resource. In fact, one of the first actions many bootloaders perform after memory initialization is to copy themselves into DRAM for faster execution.

7.2.2 Flash Versus RAM

Another complexity inherent in bootloaders is that they are required to be stored in nonvolatile storage but are usually loaded into RAM for execution. Again, the complexity arises from the level of resources available for the bootloader to rely on. In a fully operational computer system running an operating system such as Linux, it is relatively easy to compile a program and invoke it from nonvolatile storage. The runtime libraries, operating system, and compiler work together to create the infrastructure necessary to load a program from nonvolatile storage into memory and pass control to it. The aforementioned “Hello World” program is a perfect example. When compiled, it can be loaded into memory and executed simply by typing the name of the executable (hello) on the command line (assuming, of course, that the executable exists somewhere on your PATH).

This infrastructure does not exist when a bootloader gains control upon power-on. Instead, the bootloader must create its own operational context and move itself, if required, to a suitable location in RAM. Furthermore, additional complexity is introduced by the requirement to execute from a read-only medium.

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 Script—Reset Vector Placement

  .resetvec 0xFFFFFFFC :
  } = 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
#if defined(CONFIG_BOOT_PCI) && defined(CONFIG_MIP405)
      b _start_pci
      b _start

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.

7.2.4 Execution Context

The other primary reason for bootloader image complexity is the lack of execution context. When the sequence of instructions from Listing 7-3 starts executing (recall that these are the first machine instructions after power-on), the resources available to the running program are nearly zero. Default values designed into the hardware ensure that fetches from Flash memory work properly and that the system clock has some default values, but little else can be assumed.2 The reset state of each processor is usually well defined by the manufacturer, but the reset state of a board is defined by the hardware designers.

Indeed, most processors have no DRAM available at startup for temporary storage of variables or, worse, for a stack that is required to use C program calling conventions. If you were forced to write a “Hello World” program with no DRAM and, therefore, no stack, it would be quite different from the traditional “Hello World” example.

This limitation places significant challenges on the initial body of code designed to initialize the hardware. As a result, one of the first tasks the bootloader performs on startup is to configure enough of the hardware to enable at least some minimal amount of RAM. Some processors designed for embedded use have small amounts of on-chip static RAM available. This is the case with the PPC 405GP we’ve been discussing. When RAM is available, a stack can be allocated using part of that RAM, and a proper context can be constructed to run higher-level languages such as C. This allows the rest of the processor and platform initialization to be written in something other than assembly language.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020