Datasheet

Programmer’s Model

ARM7TDMI Data Sheet

ARM DDI 0029E

3-2

Open Access

3.1 Processor Operating States

From the programmer’s point of view, the ARM7TDMI can be in one of two states:

ARM state

which executes 32-bit, word-aligned ARM instructions.

THUMB state

which operates with 16-bit, halfword-aligned THUMB

instructions. In this state, the PC uses bit 1 to select between

alternate halfwords.

Note

Transition between these two states does not affect the processor mode or the

contents of the registers.

3.2 Switching State

Entering THUMB state

Entry into THUMB state can be achieved by executing a BX instruction with the state

bit (bit 0) set in the operand register.

Transition to THUMB state will also occur automatically on return from an exception

(IRQ, FIQ, UNDEF, ABORT, SWI etc.), if the exception was entered with the processor

in THUMB state.

Entering ARM state

Entry into ARM state happens:

1 On execution of the BX instruction with the state bit clear in the operand

2 On the processor taking an exception (IRQ, FIQ, RESET, UNDEF, ABORT,

SWI etc.).

In this case, the PC is placed in the exception mode’s link register, and

execution commences at the exception’s vector address.

3.3 Memory Formats

ARM7TDMI views memory as a linear collection of bytes numbered upwards from

zero. Bytes 0 to 3 hold the ﬁrst stored word, bytes 4 to 7 the second and so on.

ARM7TDMI can treat words in memory as being stored either in

Big Endian

Little

Endian

format.