We are using ARM CM0+ in our embedded SoC design and I noticed that boot takes a long time. Specifically, I saw that it took ~18K cycles after the unreset before we executed an instruction that I recognize (which is inside __rt_entry). We are of course using the CMSIS RTX RTOS on the CM0.
It appears from an RTL simulation that during 18000 cycles, the MCU is mostly inside a tight loop of 4 instructions. The PC keeps going looping across 0x178->0x17a->0x17c->0x17e->back. This is most likely some zero initialization (BSS?). The armlink generated .info suggests that the PC is inside !!handler_zi which is inside the object: c_p.l (__scatter_zi.o). This comes from ARM/sw/ARMCompiler5.05u1/lib/armlib/c_p.l.
Questions
- how do I find out what is being initialized?
- how do I not initialize (if not required in our design)?
The scatter text file is very simple and copy-pasted below.
Thanks in advance!!
LOAD 0x0000 SRAM_SIZE { EXEC_ROM_VECTOR_DATA +0 { startup_ARMCM0.s.o(RESET) } EXEC_ROM_VECTOR_HANDLERS +0 { startup_ARMCM0.s.o(.text) } EXEC_ROM +0 { *(+RO) } EXEC_RAM +0 { *(+RW) } EXEC_ZI +0 { *(+ZI) } HEAP +0 { startup_ARMCM0.s.o(HEAP) } STACK +0 { startup_ARMCM0.s.o(STACK) } }
Yes, this is likely zeroing the zero-initialised data pool (ZI/BSS). If you run armlink with --map it will provide a breakdown of how all of the input objects contributed to the final output image.
Simon.