Programming Embedded Systems in C and C++

Building das Blinkenlights 27

text rom : /* The actual instructions. */

{

*(.text)

}

}

This script informs the GNU linker’s built-in locator about the memory on the tar-

get board and instructs it to locate the data and bss sections in RAM (starting at

address 0x00000) and the text section in ROM (starting at 0x80000). However,

the initial values of the variables in the data segment will be made a part of the

ROM image by the addition of >rom at the end of that section’s definition.

All of the names that begin with underscores (_TopOfStack, for example) are

variables that can be referenced from within your source code. The linker will use

these symbols to resolve references in the input object files. So, for example, there

might be a part of the embedded software (usually within the startup code) that

copies the initial values of the initialized variables from ROM to the data section

in RAM. The start and stop addresses for this operation can be established symbol-

ically, by referring to the integer variables _DataStart and _DataEnd.

The result of this final step of the build process is an absolutely located binary

image that can be downloaded to the embedded system or programmed into a

read-only memory device. In the previous example, this memory image would be

exactly 1 MB in size. However, because the initial values for the initialized data

section are stored in ROM, the lower 512 kilobytes ...