Message structure
The common message structure is shown in the following diagram:
| Field | Size | Description |
|---|---|---|
| LEN | 8 bits | Length of the payload in 32-bit words. |
| TXN | 8 bits | Transaction number. Values link response to the request. (Response is sent with TXN value of the request) |
| Reserved | 2 bits | These bits are reserved for future use. Must be 0. |
| RESP | 1 bit | Response flag. 0 = Request, 1 = Response |
| RPC_ID | 12 bits | Each RPC[^1] has a unique RPC_ID assigned to it. MSB[^2] first. |
| PAYLOAD | 4 × LEN bytes | Meaning is defined by the RPC_ID and the RESP bit. All data is laid out with no gaps, there is no padding for alignment. |
[^1]: Remote Procedure Call [^2]: Most Significant Byte
Important
All primitive types in data are always transmitted as big-endian.
Note
We assume that byte is always 8 bits long. Byte is equal to octet in Compost.
Request and response messages for reading a byte from 32bit address.
In your protocol definition file you would define the RPC call using Python syntax:
@rpc(0x001)
def read8(address: U32) -> U8:
"""Reads 8 bits of data from the specified address"""The corresponding signature in C that Compost generates for you:
uint8_t read8(uint32_t address);Request to call
read8(0x60000000)corresponds to the following Compost message
[0x01, 0x02, 0x00, 0x01, 0x60, 0x00, 0x00, 0x00]
Response corresponding to result 0xFF:
[0x01, 0x02, 0x10, 0x01, 0xFF, 0xXX, 0xXX, 0xXX]
