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authorChris Johns <chrisj@rtems.org>2018-05-20 08:32:42 +1200
committerChris Johns <chrisj@rtems.org>2018-11-03 16:36:32 +1100
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downloadrtems-docs-8b67c9135cb78abc394d43e449ccc6387c73f014.tar.bz2
user: Add RTEMS executable and test documentation.
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+.. comment SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. comment: Copyright (c) 2018 Chris Johns <chrisj@rtems.org>
+.. comment: All rights reserved.
+
+Target Execution
+================
+.. index:: Target Execution
+
+Fixed position statically linked executables have a fixed address in a target's
+address space. The location in the address space for code, data and read-only
+data is fixed. The BSP defines the memory map and it is set by the BSP
+developer based on the target's hardware requirements and it's bootloader.
+
+Targets typically contains a bootloader that is executed after the target's
+processor exits reset. A bootloader is specific to a target's processor and
+hardware configuration and is responsible for the low level initialization of
+the hardware resources needed to load and execute an operating system's
+kernel. In the case of RTEMS this is the RTEMS executable.
+
+Bootloaders vary in size, complexity and functionality. Some architectures have
+a number of bootloader stages and others have only minimal support. An example
+of a high end system is Xilinx's Zynq processor with three stages. First a mask
+ROM in the System On Chip (SOC) executes after reset loading a first stage
+bootloader (FSBL) from an SD card, QSPI flash or NAND flash depending on
+signals connected to the device. The FSBL loads a second stage bootloader
+(SSBL) such as U-Boot and this loads the kernel. U-Boot can be configured to
+load a kernel from a range of media and file system formats as well as over a
+network using a number of protocols. This structure provides flexibility at the
+system level to support development environments such as a workshop or
+laboratory through to tightly control production configurations.
+
+Bootloaders often have custom formats for the executable image they load. The
+formats can be simple to keep the bootloader simple or complex to support
+check-sums, encryption or redundancy in case an image becomes corrupted. A
+bootloader often provides a host tool that creates the required file from the
+RTEMS executable's ELF file.
+
+If RTEMS is to run from RAM the bootloader reads the image and loads the code,
+initialized data and read-only data into the RAM and then jumps to a known
+entry point. If the code is executed from non-volatile storage the process to
+write the image into that storage will have extracted the various binary parts
+and written those to the correct location.
+
+The important point to note is the binary parts of the executable are somehow
+loaded into the target's address space ready to execute. The way this done may
+vary but the out come is always the same, the binary code, data and read-only
+data is resident in the processor's address space at the BSP defined
+addresses.