ARM PROCESSOR ARCHITECTURE with reference to ARM state and Thumb state. CPSR register and shift from one state to another ,applications of Thumb and limitations of Thumb. https://www.youtube.com/watch?v=MW2M6hvAuis

1. ARM PROCESSOR ARCHITECTUR-4
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2. • ARM7TDMI (ARM v4) Processor works in two
operating states .One is ARM state and the other is
Thumb state.
• In ARM state instructions are 32 bit wide, and in
Thumb state instructions are 16 bit wide.
• Thumb mode allows for code to be smaller, and can
potentially be faster if the target has slow memory.
INTRODUCTION

3. • The Nintendo Game Boy Advance (GBA) is a
portable games console which first used this thumb
instruction of ARM7TDMI CPU.Later it was used in
PDAs,music players and other gaming applications
where memory is very critical.
Contd

4. Why Thumb ?
• Except in the high speed embedded devices, the cost
of memory is much more critical than the execution
speed of the processor.
• But all the embedded applications do not need these
high speed and memory requirements.
• So, to reduce memory requirements and cost, the
ARM company has created the Thumb instruction
set as an option for their RISC processor cores.
• The most well-known chip that includes the Thumb
instruction set is the ARM7TDMI. The “T” in the
core's full name specifies Thumb.
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5. contd
• The Thumb instruction set consists of 16-bit
instructions that act as a compact shorthand for a
subset of the 32-bit instructions of the standard
ARM.
• Every Thumb instruction could instead be executed
via the equivalent 32-bit ARM instruction.
• But the limitation is not all ARM instructions are
available in the Thumb subset , for example, there's
no way to access status or coprocessor registers.
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6. contd
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7. contd
• The difference between two equivalent instructions
lies in how the instructions are fetched and
interpreted prior to execution, not in how they
function.
• Since the expansion from 16-bit to 32-bit instruction
is accomplished via dedicated hardware within the
chip, it doesn't slow execution even a bit.
• But the narrower 16-bit instructions do offer memory
advantages.
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8. Block Diagram of ARM7ARM & THUMB states
• Upon power on reset , ARM processors always start
in ARM state.
• To enter to Thumb state the user must explicitly
change to Thumb state using a BX (Branch and
exchange instruction set) instruction.
• The Thumb instruction set is a subset of the 32bit
ARM instruction set.
• Some of the of 32 bit ARM instructions are not
available when operating in THUMB state.

9. CPSR Register
• The T bit of the CPSR Decides whether the
processor is 16 bit Thumb state or 32 bit ARM
state.
• When the T bit is 1, the processor is in Thumb state.
• There are also other methods to switch between
these state.

10. contd
• A comparison between the two states is given
below.
• From the table it is clear that no of core
instructions is less in Thumb mode.

11. Thumb State Registers
The Thumb state provides a subset of the ARM state,
registers: •Eight general purpose registers r0 to r7
(also known as the low registers)
• SP register (banked in all modes)
• LR register (banked in all modes)
• PC register & • CPSR register
• SPSR (banked for the privileged modes) .
The high registers r8 to r15 are generally not
accessible when in the Thumb state (except for the
three instructions MOV , ADD, and CMP).

12. Stack in Thumb State
• The biggest register difference involves the SP
register. The Thumb state has unique stack mnemonics
(PUSH, POP) that don't exist in the
ARM state.These instructions
assume the existence of a stack
pointer, for which R13 is used.
They translate into load and
store instructions in the ARM
state.

13. Methods of Switching the states.
• On power on reset the ARM processor starts up in
ARM state .
• When an exception occurs, the processor
automatically begins executing in ARM state at the
address of the exception vector.
• So another way to change state is to placing the 32-
bit code in an exception handler.
• In addition, a simple execution of the branch and
Exchange (BX ) instruction can also do this.

14. Simple program
mov R0,#5 ;Argument to
function is in R0
add R1,PC,#1 ;Load address of
SUB_BRANCH, Set
for THUMB by
adding 1
BX R1
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This example first switches to Thumb state, then calls
a subroutine that was written in Thumb code.
• Upon return from the subroutine, the system again
switches back to ARM state.

15. Thumb Code Density
• Here the comparison between ARM and Thumb
code is shown for the Division program.It proves
that the Thumb code density is higher than ARM
code

16. contd
• Another important point is that there are no MSR
(ARM register to system coprocessor register) and
MRS(general-purpose register to PSR) equivalent
Thumb instructions.
• To alter the CPSR or SPSR , one must switch into
ARM state to use MSR and MRS. Similarly, there
are no coprocessor instructions in Thumb state.
• You need to be in ARM state to access the
coprocessor for configuring cache and memory
management.

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