6.5.8.4.11 Get Stack Pointer

To transfer a data word from an Arm core register, special-purpose register, or Floating-point extension register, a debugger:

• Writes to the DCRSR, with the REGSEL value indicating the required register and the REGWnR bit set to ‘0’ to indicate a read access. This write clears the DHCSR.S_REGRDY bit to ‘0’.
• Polls DHCSR until DHCSR.S_REGRDY reads as ‘1’. This indicates that the processor has transferred the value of the selected register to DCRDR.
• Reads the required value from DCRDR

Reference: ARM DDI 0403E.b, C1.6.4 Debug Core Register Data Register, DCRDR