The contents below is appliable in STM32F1xx.
What SystemInit() I mean here?
It is a standard CMSIS Function name located at system_stm32f1xx.c
What SYSCLK means?
SYSCLK stands for System clock. //main peripherals clock source
System clock selection is performed on startup, however, the internal RC 8 MHz oscillator is selected as default CPU clock on reset.
The values on the BOOT pins are latched on the 4th rising edge of SYSCLK after a reset. It is up to the user to set the BOOT1 and BOOT0 pins after Reset to select the required boot mode.
The CPU fetches the top-of-stack value from address 0x0000 0000, then starts code execution from the boot memory starting from 0x0000 0004.
I found what it has done are
1. enable the HSI clock
2. Select HSI as system clock
3. All AHB, APB1, APB2 prescaler set to 1
4. ADC prescaler set to default PCLK2 divided by 2
5. No MCO output
6. HSE oscillator OFF
7. Clock security system Disable
8. PLL OFF
9. external oscillator not bypassed
10. HSI oscillator clock / 2 selected as PLL input clock
11. HSE clock not divided
12. PLL input clock x 2
13. USB prescaler PLL clock is divided by 1.5