1.3.1. Embedded software development

Many applications written for ARM architecture-based systems are embedded applications that are contained in ROM and execute on reset. There are a number of factors that you must consider when writing embedded operating systems, or embedded applications that execute from reset without an operating system, including:

The ARM core usually begins executing instructions from address 0x0000 at reset. For an embedded system, this means that there must be ROM at address 0x0000 when the system is reset. Typically, however, ROM is slow compared to RAM, and often only 8 or 16 bits wide. This affects the speed of exception handling. Having ROM at address 0x0000 means that the exception vectors cannot be modified. A common strategy is to remap ROM to RAM and copy the exception vectors from ROM to RAM after startup. See ROM/RAM remapping for more information.

After reset, an embedded application or operating system must initialize the system, including:

See Initialization sequence for more information.

Embedded systems often implement complex memory configurations. For example, an embedded system might use fast, 32-bit RAM for performance-critical code, such as interrupt handlers and the stack, slower 16-bit RAM for application RW data, and ROM for normal application code. You can use the linker scatter-loading mechanism to construct executable images suitable for complex systems. For example, a scatter-load description file can specify the load address and execution address of individual code and data regions. See Chapter 2 Embedded Software Development for a series of worked examples, and for information on other issues that affect embedded applications, such as semihosting.

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