Programmer’s Model
ARM DDI 0210C Copyright © 2001, 2004 ARM Limited. All rights reserved. 2-3
2.2 Processor operating states
The ARM7TDMI processor has two operating states:
ARM 32-bit, word-aligned ARM instructions are executed in this state.
Thumb 16-bit, halfword-aligned Thumb instructions are executed in this state.
In Thumb state, the Program Counter (PC) uses bit 1 to select between alternate
halfwords.
Note
Transition between ARM and Thumb states does not affect the processor mode or the
register contents.
2.2.1 Switching state
The operating state of the ARM7TDMI core can be switched between ARM state and
Thumb state using the BX instruction. This is described in the ARM Architecture
Reference Manual.
All exception handling is entered in ARM state. If an exception occurs in Thumb state,
the processor reverts to ARM state. The transition back to Thumb state occurs
automatically on return. An exception handler can change to Thumb state but it must
return to ARM state to enable the exception handler to terminate correctly.
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