ARM Cortex M0/M3/M4:Why PC is always Even number in Thumb State

Question

As far as I understand it, ARM Cortex-M CPUs are always in Thumb state, which means:

Thumb state indicated by program counter being odd (LSB = 1). Branching to an even address will cause an exception, since switching back to ARM state is not allowed.

However, while I am using CortexM0 and M4 CPU, the PC is always even . Each time I branch, the LR records PC+1 and each time I return, the PC gives LR-1.

For example, if lr = 0x0000_01D5,

Execute

BX lr

Then PC should be 0x0000_01D5, whereas it gives 0x0000_01D4.

Isn't this impossible?

Any comment will be appreciated.

Answer 1

From Cortex-M4 Technical Reference Manual :

2.3.1 Program counter

Register R15 is the Program Counter (PC).

Bit [0] is always 0, so instructions are always aligned to word or halfword boundaries.

Reading from PC shouldn't return an odd address. However when you write to PC , LSB of value is loaded into the EPSR T-bit. From Cortex-M3 Devices Generic User Guide - 2.1.3. Core registers

Thumb state

The Cortex-M3 processor only supports execution of instructions in Thumb state. The following can clear the T bit to 0:

 instructions BLX, BX and POP{PC} restoration from the stacked xPSR value on an exception return bit[0] of the vector value on an exception entry or reset. 

Attempting to execute instructions when the T bit is 0 results in a fault or lockup. See Lockup for more information.

In other words, you can read even values from PC but can't write such values under normal circumstances.

Answer 2

I had that confusion as well. The lsbit is set for situations where the address is going to be used by a BX. The lsbit is stripped when it goes into the pc itself. If you disassemble some simple pc relative addressing that will demonstrate what is going on.

Answer 3

While there are 4,294,967,296 [2^32] memory locations, the actual data bus is only 8 bits wide.

For 16-bit instructions, you need 2 memory accesses per instruction load: that's where arguments about "Endian" start. Whichever way you order it, 2 bytes get read in with the 1st byte at an address ending in 0.

[sigh] If the ARM data bus was 16 [or 32] bits wide, you could forget about bits in the PC - and have double [or quadruple] the instruction space.