Introduction
The RISC OS system uses memory which is preserved whilst the power is off to store configuration information. This allows the system to start with the correct settings such as hardware configuration and user preferences. Examples of hardware configuration are settings such as the filing system to boot from and which drive should be used. User preferences include the type and volume of system beep.
Technical details
Under earlier versions of the operating system the non-volatile RAM ('NVRAM', also referred to as CMOS RAM, or battery backed RAM) was handled entirely by the Kernel. From Kernel 9.48, the handling of NVRAM is delegated to hardware support modules. The Kernel communicates with these modules through the vector NVRAMV.
Driver modules which provide the NVRAMV vector should be initialised with the early initialisation flag (module flags bit 1) set. This allows the modules to be started before the configuration for unplugged modules is required.
Terminology
The configuration data handled by NVRAMV has generally been termed 'CMOS' or 'CMOS data'. Historically, the configuration information was used CMOS technology to store the contents of the memory, but this is not required. The name 'non-volative RAM' is a more general term which does not imply the use of a particular technology, so is used to describe the mecanishm for storing the configuration data.
Software vectors
| R0 | = | reason code:
|
| R0 | = | -1 if handled, preserved if not handled |
| R1 - R9 | = | dependant on reason code |
This vector is called by the Kernel to control the configuration data stored in the NVRAM.
| R0 | = | 0 (reason code) |
| R1 | = | pointer to cache block to fill |
| R2 | = | number of bytes to fill |
| R0 | = | -1 (operation complete) |
| R1 | preserved | |
| R2 | = | number of bytes populated |
This vector is called by the Kernel to fill in its cache of NVRAM values. A cache is provided in order to reduce the impact of repeated reading of configuration data by clients. Clients should write 0 to the cache for unsupported values. The number of bytes to fill may take any value. The total amount of NVRAM should be returned, not the amount of NVRAM filled. Only the first 240 bytes of NVRAM will be used by the Kernel initially.
| R0 | = | 1 (reason code) |
| R1 | = | byte to read |
| R0 | = | -1 (operation complete) |
| R1 | = | value read, or 0 if byte is out of range |
This vector is called by the Kernel to read a single value. It will usually only be used before the NVRAM cache has been populated during system initialisation.
| R0 | = | 2 (reason code) |
| R1 | = | byte to read |
| R0 | = | -1 (operation complete) |
| R1 | = | value read, or 0 if byte is out of range |
This vector is called by the Kernel to write a single value to the NVRAM.