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6 Writing Code for ROM
This chapter describes how to build images for embedded applications. These images are typically programmed into
ROM or flash memory. There are also suggestions on how to avoid the most common errors in writing code for
ROM.
This chapter contains the following sections:
• About writing code for ROM
• Memory map considerations
• Initializing the system
• The reference C example using semihosting
• Loading the ROM image at address 0
• Loading the ROM image at address 0
• Using both scatter loading and remapping
• A semihosted application with interrupt handling
• An embeddable application with interrupt handling
• Using scatter loading with memory-mapped I/O
• Troubleshooting.
• Measuring code and data size.
6.1 About writing code for ROM
This chapter describes how to write code for ROM, and shows different methods for simple and complex images.
Sample initialization code is given, as well as information on initializing data, stack pointers, interrupts, and so on.
This chapter contains examples of using scatter loading to build complex images. For detailed reference information
on the linker and scatter loading, refer to Linker and Utilities Guide
.
Note
The examples used in this chapter target the ARM Integrator board and are located in
install_directory\Examples\embedded. You can use these examples as the basis for the initialization code
for your own system.
The reference examples (embed, embed_cpp, ledflash, and rps_irq) can be built in different configurations in
increasing levels of complexity:
• As a simple semihosted application that links with the C libraries. This example uses the semihosting SWI
functions of the C libraries for I/O. See The reference C example using semihosting.
• As an application that links with the C libraries and can be embedded into ROM to execute at address 0x0
using scatter loading. This example does not use the semihosting SWI functions, but instead uses a retargeting
layer for I/O. See Loading the ROM image at address 0.
• As an application that uses scatter loading and memory remapping to move RAM to 0x0 after initialization. See
Using both scatter loading and remapping.
Additional files are provided to demonstrate function retargeting and processor initialization:
• retarget.c implements a retargetting layer for low level input/output
• stack.s, heap.s, uart.c, and scat_c.scf demonstrate placement of stack and heap and
memory-mapped peripherals using the scatter file
• init.s demonstrates the use of lengths and offsets to initialize stack pointers for each mode
• the cache subdirectory contains cache and clock initialization code for a variety of ARM cores.
CodeWarrior IDE projects are available for the examples as embed.mcp, embed_cpp.mcp, ledflash.mcp, and
rps_irq.mcp.
Additional code examples and information about writing code for ROM is in the ARM Firmware Suite (AFS) example
code and documentation. Refer to the following ARM publications for more details on AFS:
• ARM Firmware Suite User Guide
• ARM Firmware Suite Reference Guide.
Writing Code for ROM
Copyright ?1999 2001 ARM Limited 6-1
- 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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