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Fastboot Commands

Command / Instruction	Description
fastboot devices	Result should be a serial number, similar to the adb devices command. If entering this command shows your device name with a serial number, it implies that your phone is good to go for fastboot operations. However, if the result is empty then you might want to check the fastboot installation and drivers.
Flash	external flash memory, size depending on the board variant between 512 KB and 4 MB.
IRAM	Instruction RAM, static RAM, total size 64 KB. 32KB contains persistent code that needs to execute quickly or compete with other Flash read routines (such as Interrupt Service Routines (ISR)), this area is populated from Flash at boot time. 32 KB, divided into two 18 KB blocks, serves as an instruction cache which is loaded from the Flash at runtime with the respective current code.
DRAM	Data RAM, static RAM, size 96 KB, contains the program data. 16 KB are already consumed by the Sytems RAM block, so that about 80 KB are still available for the application. In the Arduino environment, the SDK already consumes about 32 KB. For application data (static data, stack and heap) thus remain about 48 KB.