STM32F091启动文件中.size伪指令作用及语法解析

Great question—digging into startup scripts and assembler directives is a key part of mastering embedded systems, so let’s break this down step by step.

What’s the .size Assembler Directive? #

The .size directive is part of the GNU Assembler (GAS), the standard toolchain for most ARM Cortex-M targets including your STM32F091. Its core purpose is to record the memory size associated with a symbol in the object file.

Linkers, debuggers, and memory map tools use this information to:

• Correctly calculate how much RAM/Flash a function, variable, or section uses
• Generate accurate debug symbols (so your debugger knows not to treat a marker as a block of memory)
• Validate that your code and data fit within the MCU’s memory limits

Breaking Down .size __HeapBase, . - __HeapBase #

Let’s unpack the syntax and context here, since this line is tied to how STM32 defines its heap region:

First, the key symbols:

• __HeapBase: This is a linker-defined marker pointing to the start address of the heap (the RAM region used for dynamic allocation like malloc()). It’s not a variable that takes up memory—it’s just a reference point.
• . (the dot): In assembler/linker scripts, this represents the current location counter—think of it as the address the assembler is currently "working at".

The .size directive follows this format:

.size <symbol>, <size_value>

In your specific line:

1. The size_value is calculated as . - __HeapBase. Since this line is placed immediately after __HeapBase is defined, the current location counter (.) matches the address of __HeapBase exactly. This means the size evaluates to 0.
2. We’re telling the assembler to mark __HeapBase as a zero-sized symbol. This is critical because __HeapBase isn’t a block of memory—it’s just an address marker. Setting its size to 0 ensures tools like debuggers and linkers don’t misinterpret it as a memory-occupying variable, keeping your memory map and debug information accurate.

Quick Summary #

• .size is a GAS directive that records a symbol’s memory size for toolchain tools.
• Your specific line marks __HeapBase (the heap start marker) as a zero-sized symbol, correctly identifying it as an address reference rather than a variable that takes up space.

内容的提问来源于stack exchange，提问作者Gaurav Sharma