2.3.2. An example ARM assembly language module

Example 2.1 illustrates some of the core constituents of an assembly language module. The example is written in ARM assembly language. It is supplied as armex.s in the main examples directory install_directory\RVDS\Examples. See Code examples for instructions on how to assemble, link, and execute the example.

The constituent parts of this example are described in more detail in the following sections.

Example 2.1. 

        AREA     ARMex, CODE, READONLY
                                ; Name this block of code ARMex
        ENTRY                   ; Mark first instruction to execute
        MOV      r0, #10        ; Set up parameters
        MOV      r1, #3
        ADD      r0, r0, r1     ; r0 = r0 + r1
        MOV      r0, #0x18      ; angel_SWIreason_ReportException
        LDR      r1, =0x20026   ; ADP_Stopped_ApplicationExit
        SVC      #0x123456      ; ARM semihosting (formerly SWI)
        END                     ; Mark end of file

ELF sections and the AREA directive

ELF sections are independent, named, indivisible sequences of code or data. A single code section is the minimum required to produce an application.

The output of an assembly or compilation can include:

  • one or more code sections. These are usually read-only sections.

  • one or more data sections. These are usually read-write sections. They might be zero initialized (ZI).

The linker places each section in a program image according to section placement rules. Sections that are adjacent in source files are not necessarily adjacent in the application image. See Chapter 5 Using Scatter-loading Description Files in the Linker User Guide for more information on how the linker places sections.

In a source file, the AREA directive marks the start of a section. This directive names the section and sets its attributes. The attributes are placed after the name, separated by commas. See AREA for a detailed description of the syntax of the AREA directive.

You can choose any name for your sections. However, names starting with any nonalphabetic character must be enclosed in bars, or an AREA name missing error is generated. For example, |1_DataArea|.

Example 2.1 defines a single section called ARMex that contains code and is marked as being READONLY.

The ENTRY directive

The ENTRY directive marks the first instruction to be executed. In applications containing C code, an entry point is also contained within the C library initialization code. Initialization code and exception handlers also contain entry points.

Application execution

The application code in Example 2.1 begins executing at the label start, where it loads the decimal values 10 and 3 into registers r0 and r1. These registers are added together and the result placed in r0.

Application termination

After executing the main code, the application terminates by returning control to the debugger. This is done using the ARM semihosting SVC (0x123456 by default), with the following parameters:

  • r0 equal to angel_SWIreason_ReportException (0x18)

  • r1 equal to ADP_Stopped_ApplicationExit (0x20026).

See Chapter 8 Semihosting in the RVCT Developer Guide.

The END directive

This directive instructs the assembler to stop processing this source file. Every assembly language source module must finish with an END directive on a line by itself.

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