SPI Flash Layout
Overall, the SPI Flash consists of a Header, Checksum and an Image Payload (which includes the image information and the image binary).
The specific images of the flash consists of the Caliptra FW, MCU RT, SoC Manifest, and other SoC images, if any.
Layout
Note: All fields are little-endian byte-ordered unless specified otherwise.
A typical overall flash layout is:
| Flash Layout |
|---|
| Header |
| Payload |
The Payload contains the following fields:
| Payload |
|---|
| Image Info (Caliptra FMC + RT) |
| Image Info (SoC Manifest) |
| Image Info (MCU RT) |
| Image Info (SoC Image 1) |
| ... |
| Image Info (SoC Image N) |
| Caliptra FMC + RT Package |
| SoC Manifest |
| MCU RT |
| SoC Image 1 |
| ... |
| SoC Image N |
- Caliptra FMC and RT (refer to the Caliptra Firmware Image Bundle Format)
- SoC Manifest (refer to the description of the SoC Manifest)
- MCU RT: This is the image binary of the MCU Runtime firmware
- Other SoC images (if any)
Header
The Header section contains the metadata for the images.
| Field | Size (bytes) | Description |
|---|---|---|
| Magic Number | 4 | A unique identifier to mark the start of the header. The value must be 0x464C5348 ("FLSH" in ASCII) for flash or 0x54465450 ("TFTP" in ASCII) for Network Boot |
| Header Version | 2 | The header version format, allowing for backward compatibility if the package format changes over time. (Current version is 0x0002) |
| Image Count | 2 | The number of images contained in the Payload.Each image will have its own image information section. |
| Payload Offset | 4 | Offset in bytes of the header to where the first byte of the Payload is located. |
| Header Checksum | 4 | Checksum calculated for the header excluding this field |
Image Information
The Image Information section is repeated for each image and provides detailed manifest data specific to that image.
| Field | Size (bytes) | Descr |
|---|---|---|
| Identifier | 4 | Vendor selected unique value to distinguish between images. |
0x00000000: Caliptra FMC+RT | ||
0x00000001: SoC Manifest | ||
0x00000002: MCU RT0x00001000-0xFFFFFFFF - Reserved for other Vendor-defined SoC images | ||
| ImageLocationOffset | 4 | Offset in bytes from byte 0 of the header to where the image content begins. Used in flash-based boot. |
| Size | 4 | Size in bytes of the image. This is the actual size of the image without padding. |
| The image itself as written to the flash should be 4-byte aligned and additional | ||
| padding will be required to guarantee alignment. | ||
| Filename | 64 | Used in network boot to specify the TFTP path relative to the TFTP server root. Field is not used in flash-based boot |
| Image Checksum | 4 | Checksum calculated for the binary image. |
| Image Info Checksum | 4 | Checksum calculated for the header excluding this field |
Image
The images (raw binary data) are appended after the Image Information section, and should be in the same order as their corresponding Image Information.
| Field | Size (bytes) | Description |
|---|---|---|
| Data | N | Image content. |
| Note: The image should be 4-byte aligned. | ||
| If the size of a firmware image is not a multiple of 4 bytes, | ||
0x00 padding bytes will be added to meet the alignment requirement. | ||
| Only used in flash-based boot |