Programming Model Rules (2)

Read-only bits

• Attempts to write to read-only bits must leave the existing values unchanged!

Unimplemented bits

• Actions on unimplemented bits should have a defined behaviour: eg ignored on write, zero on read.

Power up status

• On power up, all registers should be set to zero - unless otherwise stated in the (full and complete!) documentation.

Volatile information

• Data integrity cannot be guaranteed if the computer tries to read or write data which the module can also update at the same time. However the Status and Control registers should always be accessible to determine the status of the module.

Address space

• When the address space occupied by the module (including unimplemented addresses) is addressed it will always respond with a handshake to avoid a bus error.