Introduction to Microcontrollers

ARM Cortex M4 Core Registers

• The core registers are not memory mapped and are accessed by register name
• 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 all 16-bit instructions
• Stack Pointer
• Register r13 is used as the stack ponter(SP)
• The processor implements two stacks with a pointer for each held in independent register "Main SP" and "Process SP"
• Link Register
• Register r14 is the subroutine LinkRegister(LR). The LR recieves the return address from PC when a Branch and Link
• The LR is also used for exception return
• At all other times, you can treat r14 as a general-purpose register
• PRIMASK Register
• prevents activation of all exceptions with programmable priority
• FAULT MASK
• Register prevents activation of all exceptions except for the Non-Maskable Interrupt(NMI)
• Both PRIMASK and FAULTMASK
• This register is only accessible in privileged mode.
• The MSR and MRS instructions are used to access the PRIMASK register, and
• The CPS instruction may be used to change the value of the PRIMASK register
• The Control Register Controls
• The stack used
• the privilege level for software execution when the processor is in Thread mode
• indicates whether the FPU state is active.
• This register is only accessible in privileged mode

How to switch from\to privileged mode?

• In thread mode, the control register controls whether the software execution is privileged or unprivileged
• In Handler mode, software execution is always privileged
• Only privileged software can write to the control register

Cortex-M bus Interfaces