In RISC OS 2.00 it is possible to load modules which provide additional screen modes and additional monitor types. To provide this facility a number of new services are offered:-

Service_ModeExtension (&50)

        R3 = monitor type (or -1 for don't care)

        All registers preserved

        (If claimed)
        R1 = 0
        R2 preserved
        R3 -> VIDC list
        R4 -> workspace list

Format of VIDC list (all word values)

Offset          Value

0               0 (indicates format of list, to allow for new VIDCs at a
                later date)
4               VIDC base mode
8               VIDC parameter
12              VIDC parameter
.               .
.               .
n               -1

The VIDC base mode is the number of an existing operating system screen mode which is used to determine the values of VIDC registers not explicitly mentioned in the list. The VIDC parameters are in the form that would be written to the hardware ie the top 6 bits specify which register is programmed and the remainder specify the value to be programmed in that register. However, bits 6 and 7 of the control register should be set to 0 as these will be modified by the MOS to take the configured sync and the *TV interlace setting into account. Similarly the vertical parameters for border start, display start, display end and border end are modified by the MOS to take the *TV vertical offset into account.

VIDC parameters below &80000000 are ignored, since these correspond to palette registers (determined by the workspace base mode) and sound registers (not part of the display system).

Format of workspace list (all word values)

Offset          Value

0               0 (indicates format of list)
4               Workspace base mode
8               Mode variable index
12              Mode variable value
16              Mode variable index
20              Mode variable value
.               .
.               .
n               -1

The workspace base mode is the number of an existing operating system screen mode which is used to determine the values of mode variables not explicitly mentioned in the list. The mode variable indices are the same as for SWI OS_ReadModeVariable. Note: for the palette to be set properly, a workspace base mode should be
chosen which has the appropriate palette.

When the service is received, the module should check that R2 contains a mode that it knows about and that R3 holds a monitor type that is suitable for that mode. If not, the service should be passed on. If R3 holds -1 then the MOS is making a general enquiry about that mode (eg to determine the attributes of a sprite defined in that mode) so the module should only check R2.

Note that it is possible for a mode to have two or more different sets of VIDC parameters for different monitor types, but the workspace parameters MUST be the same, as the mode number is used as an identifier in sprites and in calls such as OS_ReadModeVariable.

Service_ModeTranslation (&51)

        R3 = monitor type

        All registers preserved

        (If claimed)
        R1 = 0
        R2 = substitute mode
        R3 preserved

This service is offered during a call to OS_CheckModeValid or a screen mode change, if the selected mode is not available with the current monitor type (this having been ascertained by offering Service_ModeExtension) and the monitor type is not one known to the MOS (ie not in the range 0..3). If the monitor type passed in R3 is known to the module, then the module should discover what the attributes of the mode in R2 are (by calling ReadModeVariable) and then choose a mode which is suitable for this monitor type and is closest in attributes to the selected mode. This mode number should be returned in R2.

Service_PreModeChange (&4D)

        R2 = selected mode (before possible translation)

        interfere.

        (Case 2)
        R1 = 0 (service claimed)
        R0 = 0
        This implies that the module does not want the mode change to take
        place, and has taken an alternative action.

        (Case 3)
        R1 = 0 (service claimed)
        R0 -> error block
        This implies that the module does not want the mode change to take
        place, and wishes to return the error pointed to by R0

        (Case 4 - possibly to be removed)
        R1 preserved
        R2 = new mode
        This implies that the module wants to substitute a mode for the
        specified mode. This is not a very good way of doing it, as other
        modules further down the chain will be offered the service with this
        new mode. The Service_ModeTranslation mechanism described above should
        be used by modules providing new monitor types.