Currently, there are two exception models in the ARM architecture (reset is considered a kind of exception):
The classic model, used in pre-Cortex chip and current Cortex-A/R chips. In it, the memory at 0 contains several exception handlers:
Offset Handler
===============
00 Reset
04 Undefined Instruction
08 Supervisor Call (SVC)
0C Prefetch Abort
10 Data Abort
14 (Reserved)
18 Interrupt (IRQ)
1C Fast Interrupt (FIQ)
When the exception happens, the processor just starts execution from a specific offset, so usually this table contains single-instruction branches to the complete handlers further in the code. A typical classic vector table looks like following:
00000000 LDR PC, =Reset
00000004 LDR PC, =Undef
00000008 LDR PC, =SVC
0000000C LDR PC, =PrefAbort
00000010 LDR PC, =DataAbort
00000014 NOP
00000018 LDR PC, =IRQ
0000001C LDR PC, =FIQ
At runtime, the vector table can be relocated to 0xFFFF0000, which is often implemented as a tightly-coupled memory range for the fastest exception handling. However, the power-on reset usually begins at 0x00000000 (but in some chips can be set to 0xFFFF0000 by a processor pin).
The new microcontroller model is used in the Cortex-M line of chips. There, the vector table at 0 is actually a table of vectors (pointers), not instructions. The first entry contains the start-up value for the SP register, the second is the reset vector. This allows writing the reset handler directly in C, since the processor sets up the stack. Again, the table can be relocated at runtime. The typical vector table for Cortex-M begins like this:
__Vectors DCD __initial_sp ; Top of Stack
DCD Reset_Handler ; Reset Handler
DCD NMI_Handler ; NMI Handler
DCD HardFault_Handler ; Hard Fault Handler
DCD MemManage_Handler ; MPU Fault Handler
DCD BusFault_Handler ; Bus Fault Handler
DCD UsageFault_Handler ; Usage Fault Handler
[...more vectors...]
Note that in the modern complex chips such as OMAP3 or Apple’s A4 the first piece of code which is executed is usually not user code but the on-chip Boot ROM. It might check various conditions to determine where to load the user code from and whether to load it at all (e.g. it could require a valid digital signature). In such cases, the user code might have to conform to different start-up conventions.