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RealView Assembler User's Guide
Preface Introduction Writing ARM Assembly Language
Introduction Overview of the ARM architecture Architecture versions ARM, Thumb, Thumb‑2, and Thumb‑2EE instruction ARM, Thumb, and ThumbEE state Processor mode Registers Instruction set overview Instruction capabilities Structure of assembly language modules Layout of assembly language source files An example ARM assembly language module Calling subroutines Conditional execution The ALU status flags Conditional execution Using conditional execution Example of the use of conditional execution The Q flag Loading constants into registers Direct loading with MOV and MVN Loading with MOV32 Loading with LDR Rd, =const Loading addresses into registers Direct loading with ADR and ADRL Loading addresses with LDR Rd, =label Load and store multiple register instructions Load and store multiple instructions available in Implementing stacks with LDM and STM Block copy with LDM and STM Using macros Test‑and‑branch macro example Unsigned integer division macro example Using frame directives Assembly language changes Assembler Reference ARM and Thumb Instructions Directives ReferenceAn example ARM assembly language module
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| Home > Writing ARM Assembly Language > Structure of assembly language modules > 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.
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
start
MOV r0, #10 ; Set up parameters
MOV r1, #3
ADD r0, r0, r1 ; r0 = r0 + r1
stop
MOV r0, #0x18 ; angel_SWIreason_ReportException
LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit
SVC #0x123456 ; ARM semihosting (formerly SWI)
END ; Mark end of file
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 3 Using the Basic Linker Functionality in the RealView Compilation Tools Linker and Utilities 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 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.
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.
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).
| Copyright © 2007 ARM Limited. All rights reserved. | ARM DUI 0379A |
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