It has a 32 bit register, of which 13 are general purpose registers (r0-r12), SP (r13), link register (r14), program counter (r15), program status register (xPSR).

i) SP- stack pointer - There are two stack pointers. They are banked only one is visible at a time.

• MSP- main stack pointer: It is a default stack pointer.
• PSP- process stack pointer: It is used by user application code.

ii) LR- link register - When a subroutine is called, the return address is stored in link register.

iii) PC- program counter - It stores the current program address.

Special Registers of Cortex M3 -

a) xPSR- Program Status Register : It has three different categories, Application PSR, Interrupt PSR and Execution PSR.

• APSR- Application Program Status Register: contains condition flags

• IPSR- Interrupt Program Status Register: contains ISR number of current process
• EPSR- Execution Program Status Register: for execution.

b) Interrupt mask registers:

• PRIMASK- disables all interrupts except NMI and hard faults.

• FAULT MASK- disables all interrupt except NMI

• BASEPRI- disables all interrupts of specific priority or low priority.

c) Control registers:

• CONTROL- defines privilege status and SP selection.