A flash microcontroller has a Static Random-Access-Memory (SRAM) buffer that stores several blocks of boot code read from a flash memory. The boot code includes an initial boot loader, boot code and a control program that are executed by the flash microcontroller, and an operating system OS image and an external-host control program that are executed by an external host. Both the external host and the microcontroller are booted from boot code buffered in the SRAM buffer. A first-reset-read address from the external host is captured by the microcontroller during its boot sequence and stored in a mapping table along with a physical address of the block in the SRAM buffer with the operating system OS image and the external-host control program. A boot-loader state machine reads the flash ID and programs flash parameter registers with timing parameters for the flash memory.