This document describes extensions to the RISC OS mode extension system for machines which have programmable VIDC clock speeds and sync polarities, such as the A540. Familiarity with the RISC OS 2.00 mode extension system is assumed (this is described in the existing Programmer's Reference Manual).
The A540 has extra hardware to allow the selection of different VIDC clocks and to determine the polarity of the sync lines. VIDC uses its clock together with a set of internal dividers to provide a range of pixel rates.
The format of the "VIDC list" returned from Service_ModeExtension (&50) has been extended to allow the pixel rate and sync polarities to be specified.
On original Archimedes machines, the VIDC clock is fixed at 24MHz, and the pixel rate is only determined by VIDC's internal dividers, as specified in bits 0 and 1 of the Control Register (VIDC address &E0). This would be stored in the VIDC list as a word of the form &E00000xx.
RISC OS now supports two different format VIDC lists.
The original (type 0) VIDC list format is as follows:-
Offset Value
0 0
4 VIDC base mode
8 VIDC parameter
12 VIDC parameter
.. ..
n -1
The new (type 1) VIDC list format is as follows:-
Offset Value
0 1
4 VIDC base mode
8 VIDC parameter
12 VIDC parameter
.. ..
n -1
n+4 Extended parameter
n+8 Extended parameter
.. ..
m -1
where extended parameters are of the form
(0 << 24) + (pixel rate in kHz)
or
(1 << 24) + (sync polarity)
The sync polarity is defined as follows:-
bit 0 = 0 => HSync +ve (as on a standard Archimedes)
= 1 => HSync -ve
bit 1 = 0 => VSync +ve (as on a standard Archimedes)
= 1 => Vsync -ve
bits 2..23 must be zero
A pixel rate specifier in a type 1 VIDC list will override the settings of bits 0 and 1 of a Control Register specifier in the main body of the list. If no pixel rate is specified, then the VIDC clock is set to 24MHz, and the settings of the divider in the Control Register are used as normal.
The A540 hardware provides the following pixel rates:-
24000 kHz, 25175 kHz, 36000 kHz with a multiplier of 2/2
16000 kHz, 16783 kHz, 24000 kHz with a multiplier of 2/3
12000 kHz, 12587 kHz, 18000 kHz with a multiplier of 1/2
8000 kHz, 8392 kHz, 12000 kHz with a multiplier of 1/3
If the pixel rate specified is not achievable with the hardware on the
machine, the nearest available pixel rate is used.
Note: when specifying a pixel rate for a hi-res-mono display, the pixel rate
specified should be the actual pixel rate divided by 4, ie 24000 not 96000.
If no sync polarity is specified, a default of 0 is used (ie the same as a normal Archimedes).