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S3C2500B
2.4 INSTRUCTION LENGTH
Instructions are either 32 bits long (in ARM state) or 16 bits long (in THUMB state).
2.5 DATA TYPES
ARM9TDMI supports byte (8-bit), half-word (16-bit) and word (32-bit) data types. Words must be aligned to four-
byte boundaries and half words to two-byte boundaries.
2.6 OPERATING MODES
ARM9TDMI supports seven modes of operation:
User (usr):
FIQ (fiq):
IRQ (irq):
Supervisor (svc):
Abort mode (abt):
System (sys):
Undefined (und):
Mode changes may be made under software control, or may be brought about by external interrupts or exception
processing. Most application programs will execute in User mode. The non-user modes known as privileged
modes-are entered in order to service interrupts or exceptions, or to access protected resources.
The normal ARM program execution state
Designed to support a data transfer or channel process
Used for general-purpose interrupt handling
Protected mode for the operating system
Entered after a data or instruction prefetch abort
A privileged user mode for the operating system
Entered when an undefined instruction is executed
PROGRAMMER' ' S MODEL
2-3
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