Documentation for C/CPP compiler and for Assembler
ARM Architecture Reference Manuals: (You must register)
For Cortex-M3/M4
ARMv7-M Architecture Reference Manual - https://silver.arm.com/download/download.tm?pv=1874087
For Cortex M0/M0+/M1
ARMv6-M Architecture Reference Manual - https://silver.arm.com/download/download.tm?pv=1102513
Also, the programming manual by STM32:
PM0214: STM32F3 and STM32F4 Series Cortex®-M4 programming manual - http://www.st.com/st-web-ui/static/active/en/resource/technical/document/programming_manual/DM00046982.pdf
PM0056: STM32F10xxx/20xxx/21xxx/L1xxxx Cortex-M3 programming manual - http://www.st.com/st-web-ui/static/active/en/resource/technical/document/programming_manual/CD00228163.pdf
PM0223: STM32L0 Series Cortex®-M0+ programming manual - http://www.st.com/st-web-ui/static/active/en/resource/technical/document/programming_manual/DM00104451.pdf
PM0215: STM32F0xxx Cortex-M0 programming manual - http://www.st.com/st-web-ui/static/active/en/resource/technical/document/programming_manual/DM00051352.pdf