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END
The code in Example 6-4 performs ROM/RAM remapping (if required), initializes stack pointers and interrupts for
each mode, and finally branches to __main in the C library (__main eventually calls main()). On reset, the ARM
core starts up in Supervisor (SVC) mode, in ARM state, with IRQ and FIQ disabled.
Example 6-4 init.s
AREA Init, CODE, READONLY
; - Set up if ROM/RAM remapping required
; GBLL ROM_RAM_REMAP
;ROM_RAM_REMAP SETL {TRUE} ; change to {FALSE} if remapping not required
; - ensure no functions that use semihosting SWIs are linked from the C library
IMPORT __use_no_semihosting_swi
; - Standard definitions of mode bits and interrupt (I & F) flags in PSRs
Mode_USR EQU 0x10
Mode_FIQ EQU 0x11
Mode_IRQ EQU 0x12
Mode_SVC EQU 0x13
Mode_ABT EQU 0x17
Mode_UNDEF EQU 0x1B
Mode_SYS EQU 0x1F ; available on ARM Arch v4 and later
I_Bit EQU 0x80 ; when I bit is set, IRQ is disabled
F_Bit EQU 0x40 ; when F bit is set, FIQ is disabled
; --- System memory locations
CM_ctl_reg EQU 0x1000000C ; Address of Core Module Control Register
Remap_bit EQU 0x04 ; Bit 2 is remap bit of CM_ctl
; --- Amount of memory (in bytes) allocated for stacks
Len_FIQ_Stack EQU 0
Len_IRQ_Stack EQU 256
Len_ABT_Stack EQU 0
Len_UND_Stack EQU 0
Len_SVC_Stack EQU 1024
; Len_USR_Stack EQU 1024
; Add lengths >0 for FIQ_Stack, ABT_Stack, UNDEF_Stack if you need them
; offsets will be loaded as immediate values
; Offsets must be 8 byte aligned
Offset_FIQ_Stack EQU 0
Offset_IRQ_Stack EQU Offset_FIQ_Stack + Len_FIQ_Stack
Offset_ABT_Stack EQU Offset_IRQ_Stack + Len_IRQ_Stack
Offset_UND_Stack EQU Offset_ABT_Stack + Len_ABT_Stack
Offset_SVC_Stack EQU Offset_UND_Stack + Len_UND_Stack
; Offset_USR_Stack EQU Offset_SVC_Stack + Len_SVC_Stack
ENTRY
; --- Perform ROM/RAM remapping, if required
IF :DEF: ROM_RAM_REMAP
; On reset, an aliased copy of ROM is at 0x0.
; Continue execution from 'real' ROM rather than aliased copy
LDR pc, =Instruct_2
Instruct_2
; Remap by setting Remap bit of the CM_ctl register
LDR r1, =CM_ctl_reg
LDR r0, [r1]
ORR r0, r0, #Remap_bit
STR r0, [r1]
; RAM is now at 0x0.
; The exception vectors (in vectors.s) must be copied from ROM to the RAM
; The copying is done later by the C library code inside __main
ENDIF
Writing Code for ROM
Copyright ?1999 2001 ARM Limited 6-10
- 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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