EmBIOS Monitor Protocol

This article describes the USB communcation protocol of emBIOS monitor.

Endpoints

The emBIOS Monitor interface contains 4 bulk endpoints, in the following order:

• Command OUT Endpoint
• Command IN Endpoint
• Data OUT Endpoint
• Data IN Endpoint

If not stated otherwise, everything is little endian.

General Structure

Each packet send to the Command OUT Endpoint has a 16 byte header. The first 4 bytes, interpreted as a 32bit little endian word, contain the command ID. The meaning of the other bytes depends on the command. For commands that send data to the device, it will immediately follow that header.

After sending a packet to the Command OUT Endpoint, listen on the Command IN Endpoint for a response. The response also has a 16 byte header, followed by an optional data stage, depending on the command. The first 4 bytes of the header, interpreted as a 32bit word, is the status code, the meaning of the other bytes depends on the command.

Status Codes

Status Code	Description
0	Invalid response, you should bail out when receiving this
1	OK (everything went fine)
2	Command not supported
3	Device is busy, retry later (another asynchronous command is already running)

Commands

0: Invalid

Never issue this command. It will be rejected with status code 2.

1: Get device information

Use this command to figure out various device properties.

Get version information

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (1)
4	4	Requested information type (0)
8	8	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	4	SVN Revision Number
8	1	Major version
9	1	Minor version
10	1	Patch version
11	1	Software Type ID
12	4	Device Type ID

Software Types

Software Type ID	Description
0	invalid
1	emBIOS Debugger

Hardware Types

Software Type ID	Description
0	invalid
0x47324e49	iPod Nano 2G
0x47334e49	iPod Nano 3G
0x47344e49	iPod Nano 4G
0x4c435049	iPod Classic

Get packet size information

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (1)
4	4	Requested information type (1)
8	8	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	2	Maximum Command OUT Endpoint packet size
6	2	Maximum Command IN Endpoint packet size
8	4	Maximum Data OUT Endpoint packet size
12	4	Maximum Data IN Endpoint packet size

2: Reset

Reboot the device.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (2)
4	4	Type: Reboot forcibly (0) / Reboot gracefully (1)
8	8	Should be zero

Graceful reboots are asynchronous commands. Forced reboots won't send a response packet before rebooting.

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1 or 3)
4	12	Undefined

The response indicates that the request has been acknowledged, however there might be substantial delay before device actually reboots.

3: Power off

Power the device off.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (3)
4	4	Type: Power off forcibly (0) / Shut down gracefully (1)
8	8	Should be zero

Both variants are asynchronous commands.

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1 or 3)
4	12	Undefined

The response indicates that the request has been acknowledged, however there might be substantial delay before device actually powers off.

4: Read memory

Use this command to read small amouts of memory through the command pipe. You may not request a transfer that would exceed the maximum Command IN Endpoint packet size (including the header).

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (4)
4	4	Memory address to read from
8	4	Number of bytes to be read
12	4	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	12	Undefined
16	variable	The data read from memory

5: Write memory

Use this command to write small amouts of memory through the command pipe. You may not request a transfer that would exceed the maximum Command OUT Endpoint packet size (including the header).

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (5)
4	4	Memory address to write to
8	4	Number of bytes to write
12	4	Should be zero
16	variable	The data to be written

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	12	Undefined

6: Read memory using DMA

Use this command to read large amouts of memory through the data pipe. You may not request a transfer that would exceed the maximum Data IN Endpoint packet size.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (6)
4	4	Memory address to read from
8	4	Number of bytes to be read
12	4	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	12	Undefined

After receiving the response, read the requested data from the Data IN Endpoint.

7: Write memory using DMA

Use this command to read large amouts of memory through the data pipe. You may not request a transfer that would exceed the maximum Data OUT Endpoint packet size.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (7)
4	4	Memory address to write to
8	4	Number of bytes to be written
12	4	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	12	Undefined

After receiving the response, send the data to be written to the Data OUT Endpoint.

8: Read from I2C device

Use this command to read from an I2C slave. You may not request a transfer that would exceed the maximum Command IN Endpoint packet size (including the header) or 255 bytes (excluding the header), whichever is smaller.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (8)
4	1	I2C bus index
5	1	I2C slave address (in the upper 7 bits)
6	1	Start address on the I2C device
7	1	Number of bytes to be read
8	8	Should be zero

I2C transactions are asynchronous commands.

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1 or 3)
4	12	Undefined
16	variable	The data read from the I2C device (undefined if the status code is not 1)

9: Write to I2C device

Use this command to write to an I2C slave. You may not request a transfer that would exceed the maximum Command OUT Endpoint packet size (including the header) or 255 bytes (excluding the header), whichever is smaller.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (8)
4	1	I2C bus index
5	1	I2C slave address (in the upper 7 bits)
6	1	Start address on the I2C device
7	1	Number of bytes to be written
8	8	Should be zero
16	variable	The data to be written to the I2C device

I2C transactions are asynchronous commands.

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1 or 3)
4	12	Undefined

10: Read from the USB console

Use this command to get data written to the USB console. You may not request a transfer that would exceed the maximum Command IN Endpoint packet size (including the header).

As long as the console application is running, make sure to issue this request at least once a second. Otherwise the console might start dropping data and inserting an "\n\n[overflowed]\n\n" mark. If you can't receive any data but need to keep the console from dropping data, issue zero-length read requests.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (10)
4	4	Number of bytes requested
8	8	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	4	Actual number of valid response bytes
8	4	Size of the on-device USB console read buffer
12	4	Number of bytes still waiting in the on-device USB console read buffer
16	variable	Valid console data padded with undefined data to meet the requested size

11: Write to the USB console

Use this command to write data to the USB console. You may not request a transfer that would exceed the maximum Command OUT Endpoint packet size (including the header).

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (11)
4	4	Number of bytes to be written
8	8	Should be zero
16	variable	Data to be written

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	4	Actual number of bytes written (the remainder will have to be resent)
8	4	Size of the on-device USB console write buffer
12	4	Number of bytes still free in the on-device USB console write buffer

12: Write to device's consoles

Use this command to write data to one or more of the consoles. This is equivalent to the cwrite system call. You may not request a transfer that would exceed the maximum Command OUT Endpoint packet size (including the header).

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (12)
4	4	Bitmask of consoles to be written to
8	4	Number of bytes to be written
12	4	Should be zero
16	variable	Data to be written

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	12	Undefined

13: Read from device's consoles

Use this command to read data from one or more of the consoles. This is equivalent to the cread system call. You may not request a transfer that would exceed the maximum Command IN Endpoint packet size (including the header). This command will not block until there is data available.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (13)
4	4	Bitmask of consoles to be read from
8	4	Number of bytes to be read
12	4	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	4	Number of bytes actually read
8	8	Undefined
16	variable	The data read, padded with undefined data to meet the requested size

14: Flush device's console buffers

Use this command to flush one or more console's buffers. This is equivalent to the cflush system call.

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (14)
4	4	Bitmask of consoles to be flushed
8	8	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	12	Undefined

15: Get process information

Use this command to obtain the current state of the scheduler. You may not request a transfer that would exceed the maximum Command OUT Endpoint packet size (including the header).

Command Packet

Offset	Size (bytes)	Description
0	4	Command ID (15)
4	4	Offset of first byte requested
8	4	Number of bytes requested
12	4	Should be zero

Response Packet

Offset	Size (bytes)	Description
0	4	Status Code (1)
4	4	Process information struct version (incremented each time the format changes)
8	4	Total size of the process information table
12	4	Undefined
16	variable	The requested data, padded with undefined data to meet the requested size, if it exceeds bounds