Registers

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Processor Register Set – Cortex-M4

is the processor core register.

The processor has the following 32-bit registers:

• 13 general-purpose registers, R0-R12.
• Stack Pointer (SP), R13 alias of banked registers, SP_process and SP_main.
• Link Register (LR), R14.
• Program Counter (PC), R15.
• Special-purpose Program Status Registers, (xPSR).

The general-purpose registers R0-R12 have no special architecturally-defined uses.

Most instructions that can specify a general-purpose register can specify R0-R12.

Low registers

Registers R0-R7 are accessible by all instructions that specify a
general-purpose register.

High registers

Registers R8-R12 are accessible by all 32-bit instructions that specify a general-purpose register.
Registers R8-R12 are not accessible by most 16-bit instructions.
Registers R13, R14, and R15 have the following special functions:

Stack pointer

Register R13 is used as the Stack Pointer (SP). Because the SP ignores writes to bits [1:0], it is autoaligned to a word, four-byte boundary.
Handler mode always uses SP_main, but you can configure Thread mode to use either SP_main or SP_process.

Link register

Register R14 is the subroutine Link Register (LR).
The LR receives the return address from PC when a Branch and Link (BL) or Branch and Link with Exchange (BLX) instruction is executed.
The LR is also used for exception return.
At all other times, you can treat R14 as a general-purpose register.

Program counter

Register R15 is the Program Counter (PC).
Bit [0] is always 0, so instructions are always aligned to word or halfword boundaries.

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