SerialICE Serial Communications Protcol

On power up, the SerialICE Kernel initializes the SIO and the RAM, and then transfers control to the application (state0).

State0

In state0 the target is executing the application. Transition out of this state can only occur as a result of an exception.

All exceptions are first checked by the application. The application should tranfer control to the SerialICE Kernel's exception handler under the following two conditions:

• A byte arriving on the SIO from the SerialICE Controller
• A non-handled exception

The SerialICE Kernel's exception handler (located at 0xbfc00180) saves a small number of registers (currently 9) in the RAM save area. This operation requires the use of general registers k0 and k1. This means that the SerialICE Kernel cannot be used to debug code in which registers k0 and k1 hold useful values while interrupts are enabled, or to set breakpoints in a region of code where k0 and k1 are in use.

Once the registers have been saved, the SerialICE Kernel checks the exception type. If it is an SIO interrupt, the byte on the SIO is checked for value. If the byte has a value of 0x55 (ATTN) the SerialICE Kernel will respond with an 0xaa (ACK) and move to state1. For all other values the SerialICE Kernel will remain in state0 and return control to the application without sending any bytes back to the SerialICE Controller. If any other exception occurs, the the SerialICE Kernel will respond with an 0xaa (ACK) and move to state1.

State1

In state1 the SerialICE Kernel reads bytes from the SIO using polled mode. Groups of four bytes are assembled into words. Once a word has been assembled, there are four possibilties depending on the value. If the word has one of 3 special values, the action taken will be as follows:

0x12345678 (SENDA0)

Execute the code that is currently in the instruction buffer area. Transmit the 4-byte value of register a0 on completion.

0xdeadbeef (SENDSAP)

Transmit a 4-byte value containing the address of the register save area in the target memory.

0x87654321 (RUN_MODE)

Transfer control back to the application and switch to state0.

However, while the bytes are being assembled into words, the Kernel also checks the value of the first byte. If an ATTN byte (0x55) is seen, the kernel will send an ACK (0xaa), and go back to waiting for the first byte of the word.

For all other cases, the words are written to the instruction buffer area in RAM. In this case, the SerialICE Kernel does not transmit any bytes back to the SerialICE Controller. The instruction buffer has a capacity of 27 instructions.

Programming issues

The routines that are downloaded to the instruction buffer execute from kseg1 (non cacheable) and may use the following scratch registers: t0, t1, t2, a0, a1, k0, k1, and s0. If you choose to call a subroutine, you must save and restore register ra appropriately. The addresses of the get_word and put_word subroutines can be obtained from the register save area. These subroutines use registers k0, k1, and a1. get_word returns the word in register v0; put_word transmits the word in register a0.