Understanding LDS File Creation for TAE32G5800

Figure 2.1 TAE32G5800 Memory Space Division

The ENTRY keyword is used to define the entry point of the application. The ld link tool has several ways to set the code entry address:

1. ld -e option specified;

2. ENTRY(Symbol) declaration in the link script;

3. start symbol defined in the link script;

4. If there is a .text section, use the first address of the .text section as the entry;

5. Use address 0 as the entry;

The priority of these five methods is from high to low, and once a high-priority condition is met, lower priority conditions are ignored. By default, it is recommended to use the ENTRY method for explicit declaration, as shown below.

Using Reset_Handler as the program entry, which includes actions such as setting up the environment.

SECTIONS{} wraps the entire code area content. It is necessary to understand the following points in advance:

1. VMA (Virtual Memory Address) and LMA (Load Memory Address);

2. Basic section concepts, including: code section (.text), data section (.data), heap area, stack area, BSS section (.bss);

3. Variable relationships required for setting up the Cortex-M4 environment;

The concept of address is divided into two layers.

First, VMA is the running address, the address during code execution; LMA is the download address, the physical address stored in Flash, and VMA and LMA are classified as “code layer addresses”. Then, the other two concepts introduced by dual Bank are mapped address and actual physical address, where the mapped address refers to the LMA address seen in the code, while the actual physical address refers to the Flash address within the chip. Due to the introduction of the dual Bank mapping switch function, the actual physical address and the mapped address have a mapping relationship, and these two address concepts are classified as “space layer addresses”.

The basic sections include code section (.text), data section (.data), stack area, heap area, BSS section (.bss), etc., with the concepts shown below.

Table 2.1 Section Concept

The data area mainly stores variables with initialized values, thus occupying Flash space and needing to be copied from Flash to RAM during startup; the bss area mainly stores uninitialized variables, so it only needs to obtain the starting address of the area and the size, and clear this area during startup without occupying Flash space.

The stack area does not need to occupy Flash space, the stack area is mainly for push and pop operations of sub-functions and interrupts, storing temporary variables, and local variables. The heap area is mainly used for dynamic allocation with malloc() and free().

The ram area means that LMA is Flash address, VMA is SRAM address, and it needs to execute frequently.

During the startup phase, RAM space code needs to be set up, generally using the GCC cross-compilation tool to reference ARM’s official setup function, as shown in program listing 2.2.

Program Listing 2.2 GCC Calls ARM Setup Space Code

File path: cmsis_gcc.h

VSCode uses the gcc compilation toolchain, so it uses the cmsis_gcc.h file. The __PROGRAM_START macro is called by Reset_Handler(). First, declare the imported external variables __copy_table_start, __copy_table_end, __zero_table_start, and __zero_table_end, which are defined from the ld file. As shown below.

Program Listing 2.3 Copy Address Declaration of lds File