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4 Mixing C, C++, and Assembly Language
This chapter describes how to write mixed C, C++, and ARM assembly language code. It also describes how to use
the ARM inline assemblers from C and C++. It contains the following sections:
• Using the inline assemblers
• Accessing C global variables from assembly code
• Using C header files from C++
• Calling between C, C++, and ARM assembly language.
4.1 Using the inline assemblers
The inline assembler built into the C and C++ compilers enables you to features of the target processor that cannot
be accessed directly from C. For example:
• saturating arithmetic (see Assembler Guide)
• custom coprocessors
• the PSR.
The inline assembler supports very flexible interworking with C and C++. Any register operand can be an arbitrary C
or C++ expression. The inline assembler also expands complex instructions and optimizes the assembly language
code.
Note
Inline assembly language is subject to optimization by the compiler if optimization is enabled either by default or with
the -O1 or -O2 compiler options.
The armcc and armcpp inline assemblers implement most of the ARM instruction set including generic coprocessor
instructions, halfword instructions and long multiply. The tcc and tcpp inline assemblers implement, again with two
exceptions, the full Thumb instruction set.
Note
The tcc and tcpp inline assemblers are deprecated and will not be supported in future releases of the tools.
See Differences between the inline assemblers and armasm for information on restrictions.
The inline assembler is a high-level assembler. The code it generates is not always exactly what you write. Do not
use it to generate more efficient code than the compiler generates. Use the ARM assembler armasm for this
purpose.
Some low-level features that are available to the ARM assembler armasm, such as branching by writing to pc, are
not supported.
4.1.1 Invoking the inline assembler
The ARM C compilers support inline assembly language with the __asm specifier.
The ARM C++ compilers support the asm syntax proposed in the ANSI C++ Standard, with the restriction that the
string literal must be a single string. For example:
asm("instruction[;instruction]");
The asm syntax is supported by the C++ compilers when compiling both C and C++. The asm statement must be
inside a C or C++ function. Do not include comments in the string literal. An asm statement can be used anywhere a
C or C++ statement is expected.
In addition to the asm syntax, ARM C++ supports the C compiler __asm syntax.
The inline assembler is invoked with the assembler specifier. The specifier is followed by a list of assembler
instructions inside braces. For example:
__asm
{
instruction [; instruction]
...
[instruction]
}
If two instructions are on the same line, you must separate them with a semicolon. If an instruction is on multiple
lines, line continuation must be specified with the backslash character (\). C or C++ comments can be used
anywhere within an inline assembly language block.
Mixing C, C++, and Assembly Language
Copyright ?1999 2001 ARM Limited 4-1
- ARM® Developer Suite 1
- Version 1.2 1
- Developer Guide 1
- About this book 2
- ARM publications 3
- Other publications 3
- Feedback 4
- 1 Introduction 5
- Introduction 6
- 1.3 Developing for the ARM 7
- 1.3.5 Writing Code for ROM 8
- 2.1.1 ATPCS variants 9
- 2.1.2 ARM C libraries 9
- • Read-only memory 10
- 2.2 Register roles and names 11
- 2.3 The stack 12
- 2.4 Parameter passing 14
- 2.5 Stack limit checking 15
- 2.6.2 Writing code for ROPI 17
- 2.7.1 Reentrant routines 18
- 2.9 Floating-point options 20
- 3 Interworking ARM and Thumb 22
- 3.1 About interworking 22
- Interworking ARM and Thumb 23
- 3.2.3 Example ARM header 24
- Building the example 25
- 3.2.4 ARM architecture v5T 25
- 3.2.5 Labels in Thumb code 26
- C interworking example 27
- 5. Type go to run the code 31
- String copying example 33
- Operand expressions 33
- Physical registers 33
- 4.1.4 Usage 35
- 4.1.5 Examples 36
- Dot product 37
- Long multiplies 37
- • LDRB/STRB for char 39
- • LDR/STR for int 39
- C++ calling conventions 42
- C++ data types 42
- Symbol name mangling 43
- 4.4.3 Examples 43
- Calling C from C++ 44
- Calling C++ from C 45
- Handling Processor Exceptions 48
- • the address of the handler 53
- 5.4 SWI handlers 55
- Nested SWIs in C and C++ 57
- 5.5 Interrupt handlers 61
- • Single-channel DMA transfer 63
- • Dual-channel DMA transfer 63
- • Interrupt prioritization 63
- • Context switch 63
- Interrupt prioritization 64
- Context switch 65
- 5.6 Reset handlers 67
- 5.8 Prefetch Abort handler 69
- 5.9 Data Abort handler 70
- • A single extended handler 71
- • Several chained handlers 71
- Handling the exception 72
- 5.11.2 The return address 72
- FIQ and IRQ handlers 73
- Prefetch Abort handlers 73
- Data Abort handlers 73
- 5.12 System mode 75
- 6 Writing Code for ROM 76
- 6.2.1 ROM at 0x0 77
- 6.2.2 RAM at 0x0 77
- Implementing RAM at 0x0 77
- System decoder 78
- 6.3 Initializing the system 79
- 6.4.1 Memory map 81
- 6.4.2 Sample code 82
- • five execution regions: 83
- 6.5.3 Sample code 84
- 6.5.4 Building the example 88
- Using the CodeWarrior IDE 89
- Using the command line 89
- 6.6.1 Memory map 90
- 6.6.3 Initialization code 91
- 6.6.4 Building the example 91
- 6.7.1 Memory map 93
- 6.7.2 Building the example 93
- 6.7.3 Sample code 93
- 6.8.1 Memory map 97
- 6.8.2 Building the example 97
- 6.8.4 Sample code 98
- 6.9.2 Using unions 100
- Using an array of shorts 100
- Using a struct 100
- 6.9.4 Using scatter loading 101
- 6.9.5 Code efficiency 102
- 6.10 Troubleshooting 103
- Writing Code for ROM 104
- Code (or read-only) segments 105
- Data (or read-write) segments 105
- Debug data 105
- 7.1.1 About caches 106
- ARMulator Pagetable model 106
- 7.1.4 Cache performance 107
- 7.3 Memory protection units 109
- 7.4 Configuring a PU 110
- 7.5 Memory management units 113
- 7.6 Configuring an MMU 115
- 7.7 Tightly coupled memory 117
- 7.7.3 ARM966E-S warm reset 118
- Debug Communications Channel 120
- 8.4 Target transfer of data 123
- • Using armsd 125
- • Using AXD 125
- Issuing commands 126
- Using AXD 126
- 8.7 Access from Thumb state 129
- 8.8 Semihosting 130
- Glossary 131
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