foboot/doc/BOOT-SEQUENCE.md

Fomu Boot Sequence

When Foboot starts, it must decide which mode to go into. The following modes are defined:

Mode Name	Short Name	Description
Foboot Rescue	FBU	Initial state -- rescue mode uses DFU
Foboot Main	FBM	Presents a disk drive that makes it easy to load programs
User Python Program	UPY	Uses a locally-stored Python interpreter to run Python code
User RISC-V Program	URV	Uses the built-in RISC-V softcore to execute user programs
User ICE40 Bitstream	UBS	Exits the Foboot RISC-V core and switches to the provided bitstream

At start, FBU is loaded by the hardware. FBU will look for a special FBM signature at a given offset. If this signature exists, it will transfer execution to that program. Otherwise it will enter DFU Mode and await further communication.

Boot Flags

A series of Boot Fags determine how FBU and FBM hand over execution to the subsequent program.

Boot Flags are indicated by the magic number 0xb469075a, followed by a 32-bit field, similar to the following C struct:

struct {
    uint32_t magic;    // 0xb469075a
    uint32_t bitfield;
}

Bit Flag	Short Name	Description
00000001	QPI_EN	Enable QPI mode on the SPI flash
00000002	DDR_EN	Enable DDR mode on the SPI flash
00000004	CFM_EN	Enable CFM mode on the SPI flash
00000008	FLASH_UNLOCK	Don't lock flash prior to running program
00000010	FLUSH_CACHE	Issue a "fence.i" prior to running user program
00000020	NO_USB_RESET	Don't reset the USB -- useful for debugging

RAM boot

During development, it may be inconvenient to load a program onto SPI flash. Or maybe you want to run something that doesn't modify the contents of SPI. This is possible with RAM boot.

If the DFU bootloader encounters the magic number 0x17ab0f23 within the first 56 bytes, then it will enable RAM boot mode. In this mode, the SPI flash won't be erased, and the program will be loaded to RAM.

Note that the value following the magic number indicates the offset where the program will be loaded to. This should be somewhere in RAM. 0x10001000 is a good value, and is guaranteed to not interfere with Foboot itself.

Magic Numbers

Value	Description
0xb469075a	Denotes the start of boot flags bitfield
0x17ab0f23	Loads the program into RAM instead of flash
0x032bd37d	Indicates a valid FBM image