ARM Cortex-A53 Baremetal C Programming for SoC Testing
When transitioning from assembly to C for testing ARM Cortex-A53 based SoCs, it is essential to understand the foundational concepts of baremetal programming. Baremetal programming refers to writing code that runs directly on the hardware without an operating system. This is particularly relevant for SoC testing, where low-level control over the hardware is necessary to validate functionality, performance, and integration.
Baremetal C programming for ARM Cortex-A53 involves setting up the execution environment, initializing the processor, and managing memory and peripherals directly. The ARM Cortex-A53 processor, being part of the ARMv8-A architecture, supports both AArch64 (64-bit) and AArch32 (32-bit) execution states. For simplicity, we will focus on the AArch64 state, which is more commonly used in modern SoCs.
The first step in baremetal C programming is to define the entry point of the program. In assembly, this is typically the reset vector, which is the first instruction executed by the processor after a reset. In C, the entry point is usually the main() function. However, before reaching main(), the processor must be initialized, and the C runtime environment must be set up. This is typically done in a startup file written in assembly, such as startup.s.
The startup file performs several critical tasks, including setting up the stack pointer, initializing the vector table, and configuring the processor’s execution state. Once these tasks are completed, the startup file branches to the __main function, which is part of the C runtime library. The __main function initializes the C runtime environment, including setting up the heap and initializing global variables, before calling the main() function.
To illustrate this process, consider the following example of a simple baremetal C program that reads from a memory location:
#include <stdint.h>
#define MEMORY_ADDRESS 0x0
int main() {
uint32_t value = *(volatile uint32_t *)MEMORY_ADDRESS;
return 0;
}
In this example, the program reads a 32-bit value from memory location 0x0. The volatile keyword is used to indicate that the value at this memory location may change unexpectedly, so the compiler should not optimize away the read operation.
The corresponding startup file, startup.s, might look like this:
.global _start
_start:
LDR SP, =stack_top // Set up stack pointer
BL __main // Branch to C runtime initialization
B . // Infinite loop to prevent falling off
.section .stack
stack_bottom:
.space 1024 // Reserve 1KB for the stack
stack_top:
In this assembly code, the _start label is the entry point of the program. The stack pointer (SP) is set to the top of the stack, which is defined in the .stack section. The program then branches to __main, which initializes the C runtime environment and calls main(). Finally, the program enters an infinite loop to prevent it from executing unintended code.
Linker Script Configuration for ARM Cortex-A53 Baremetal Programs
The linker script plays a crucial role in baremetal programming, as it defines how the program’s code and data are laid out in memory. For ARM Cortex-A53 based SoCs, the linker script must ensure that the startup code is placed at the correct memory address, typically the reset vector address specified in the SoC’s memory map.
The linker script is responsible for specifying the memory regions and sections of the program. For example, the .text section contains the executable code, the .data section contains initialized global variables, and the .bss section contains uninitialized global variables. The linker script also defines the stack and heap regions.
A typical linker script for an ARM Cortex-A53 baremetal program might look like this:
ENTRY(_start)
MEMORY
{
ROM (rx) : ORIGIN = 0x80000000, LENGTH = 256K
RAM (rwx) : ORIGIN = 0x80040000, LENGTH = 512K
}
SECTIONS
{
.text : {
*(.text)
*(.rodata)
} > ROM
.data : {
*(.data)
} > RAM
.bss : {
*(.bss)
} > RAM
.stack : {
. = ALIGN(8);
. = . + 1024;
stack_top = .;
} > RAM
}
In this linker script, the ENTRY directive specifies _start as the entry point of the program. The MEMORY directive defines two memory regions: ROM for read-only memory (typically flash memory) and RAM for read-write memory. The SECTIONS directive defines how the different sections of the program are placed in memory.
The .text section, which contains the executable code and read-only data, is placed in the ROM region. The .data section, which contains initialized global variables, is placed in the RAM region. The .bss section, which contains uninitialized global variables, is also placed in the RAM region. The .stack section is defined at the end of the RAM region, with a size of 1KB.
The linker script ensures that the startup code is placed at the beginning of the .text section, which is located at the start of the ROM region. This ensures that the processor executes the startup code first after a reset.
Debugging and Verification Strategies for ARM Cortex-A53 Baremetal C Programs
Debugging and verifying baremetal C programs for ARM Cortex-A53 based SoCs requires a systematic approach to ensure that the program behaves as expected. This involves using a combination of simulation, emulation, and hardware debugging tools.
Simulation is often the first step in the verification process. ARM provides a variety of simulation tools, such as ARM Fast Models and ARM Fixed Virtual Platforms (FVPs), which allow developers to simulate the behavior of ARM Cortex-A53 based SoCs. These tools can be used to run and debug baremetal C programs in a controlled environment before deploying them on actual hardware.
Emulation is another valuable tool for debugging and verification. ARM Cortex-A53 based SoCs often include an Embedded Trace Macrocell (ETM), which can be used to trace the execution of the program in real-time. This allows developers to capture the program’s execution flow and analyze it for potential issues.
Hardware debugging tools, such as JTAG probes, are essential for debugging baremetal C programs on actual hardware. These tools allow developers to set breakpoints, inspect registers, and step through the program’s execution. ARM DS-5 Development Studio is a popular tool for debugging ARM Cortex-A53 based SoCs, as it provides a comprehensive set of debugging features.
In addition to using debugging tools, it is important to implement a robust verification strategy. This includes writing test cases that cover all possible execution paths, as well as corner cases that may not be immediately obvious. For example, a test case might involve writing to and reading from different memory locations to verify that the memory controller is functioning correctly.
Another important aspect of verification is ensuring that the program handles exceptions and interrupts correctly. The ARM Cortex-A53 processor supports a variety of exceptions, such as undefined instructions, data aborts, and prefetch aborts. The program must be able to handle these exceptions gracefully, either by recovering from the error or by entering a safe state.
To illustrate this, consider the following example of a simple exception handler in C:
#include <stdint.h>
void undefined_instruction_handler() {
// Handle undefined instruction exception
}
void data_abort_handler() {
// Handle data abort exception
}
void prefetch_abort_handler() {
// Handle prefetch abort exception
}
int main() {
// Set up exception handlers
*(volatile uint64_t *)0x800 = (uint64_t)undefined_instruction_handler;
*(volatile uint64_t *)0x808 = (uint64_t)data_abort_handler;
*(volatile uint64_t *)0x810 = (uint64_t)prefetch_abort_handler;
// Main program logic
return 0;
}
In this example, the program sets up exception handlers for undefined instructions, data aborts, and prefetch aborts. The exception handlers are placed at specific memory addresses, which correspond to the exception vectors defined in the ARM Cortex-A53 processor’s vector table. When an exception occurs, the processor jumps to the corresponding exception handler, allowing the program to handle the exception appropriately.
In conclusion, transitioning from assembly to C for ARM Cortex-A53 based SoC testing involves understanding the fundamentals of baremetal programming, configuring the linker script, and implementing a robust debugging and verification strategy. By following these steps, developers can create effective and reliable test programs that validate the functionality and performance of their SoCs.
