ARM CPU

• The ARM7TDMI is a 32bit RISC (Reduced Instruction Set Computer) CPU, designed by ARM (Advanced RISC Machines) for both high performance and low power consumption.

Detailed infomation

1. ARM CPU Register Set
2. ARM CPU Flags
3. ARM CPU Exceptions
4. ARM 32bit Instruction Set (ARM Code)
5. ARM 16bit Instruction Set (THUMB Code)

General information

Data formats

• All opcodes are sized 32 ou 16 bits, with pipelining allowing the simultaneous execution of one instruction, decodification of the next one and fetching of yet another one.
• The CPU deals with 8, 16 and 32 bit data, called:
  • 8 bit: byte
  • 16 bit: halfword
  • 32 bit: word

The Two CPU States

• Two CPU states exist:
  • ARM State: Uses the full 32bit instruction set (32bit opcodes)
  • THUMB State: Uses the full 32bit instruction set (32bit opcodes)

Regardless of the opcode-width, both states are using 32bit registers, allowing 32bit memory addressing as well as 32bit arithmetic/logical operations.

ARM State

The good stuff

• Each single opcode provides more functionality, resulting in faster execution when using a 32bit bus memory system (such like opcodes stored in GBA Work RAM).
• All registers R0-R15 can be accessed directly.

The bad stuff

• Not so fast when using 16bit memory system (but it still works though).
• Program code occupies more memory space.

THUMB State

The happy stuff

• Faster execution up to approx 160% when using a 16bit bus memory system (such like opcodes stored in GBA GamePak ROM).
• Reduces code size, decreases memory overload down to approx 65%.

The sad stuff

• Not as multi-functional opcodes as in ARM state, so it will be sometimes required use more than one opcode to gain a similar result as for a single opcode in ARM state.
• Most opcodes allow only registers R0-R7 to be used directly.

Combining ARM with THUMB state

• Switching between ARM and THUMB states is done by a normal branch (BX) instruction, which takes some cycles to execute, allowing the shift to occur whith almost no overload.
• As both ARM and THUMB are using the same register set, it’s possible to pass data between ARM and THUMB modes very easily.
• The best possible can be achieved by combining both states: THUMB for normal program code, and ARM for time critical subroutines (like interruption handlers).

ARM and THUMB code cannot be executed simultaneously

Automatic mode change

• Beside for the above manual state switching by using BX instructions, the following situations involve automatic state changes:
  • CPU switches to ARM state when executing an exception
  • User switches back to old state when leaving an exception

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