Monitor Types
Acorn machines support a number of Monitor Types. The configured monitor type is used to determine the supported video modes on RISC OS 3.11 and earlier.
RISC OS 3.5 and later (used on RiscPC and later machines) instead define the supported modes via a Mode Definition File.
Overview
The configured monitor type is stored in the MonitorType CMOS variable. Nine types are defined.
Normal (type 0)
Normal monitors support TV-standard modes of 50 Hz vertical and 15 kHz horizontal scan rates. The highest supported resolution is 768x288 (MODEs 33-36), with the typical desktop mode being 640x256 (MODEs 12 and 15).
Type 0 monitors are sometimes referred to as "Standard" or "CGA".
Acorn-supplied Normal monitors:
Multiscan (type 1)
Multiscan monitors support modes with scan rates of 15 kHz and above. RISC OS does not define any refresh rate limits for multiscan monitors, so every mode except MODE 23 (hi-res mono only) is available.
When used with Arthur, a resolution of 640x512 at 50 Hz (MODEs 18-21) was typical. On RISC OS, resolutions of 640x480 at 60 Hz and 800x600 at 56 Hz are also commonly used.
The 36 MHz pixel clock required for 800x600 is not provided on machines prior to the Archimedes 540. This can be rectified with a VIDC enhancer.
Acorn-supplied Multiscan monitors:
Hi-res mono (type 2)
Hi-res mono monitors support a single resolution of 1152x896 at 64 Hz (MODE 23). This mode is provided by high speed shift registers external to VIDC on the Archimedes 440, Archimedes 400/1 series and Archimedes 540 machines, outputting to the BNC connectors provided on these computers. As a result hi-res mono support is limited to these particular machines.
The mouse pointer is not rendered particularly well in hi-res mono mode; the hardware cursor pixels are 4x as wide as the rest of the screen. This affects Arthur and RISC OS. X on RISC iX uses a software pointer and does not suffer from this issue.
Acorn did not supply any hi-res mono monitors.
VGA (type 3)
VGA monitors support modes with 31 kHz horizontal and either 60 or 70 Hz vertical scan rates. They are limited to physical resolutions of 350, 400 and 480 lines. The typical desktop resolution is 640x480 at 60 Hz (MODEs 25-28).
On RISC OS 3, low resolution modes (256 lines) are supported via letterboxing; VIDC is programmed for a 350 line mode with the display occupying the middle 256 lines. Such modes run at a 70 Hz refresh rate instead of the 50 Hz rate used on type 0 and 1 monitors. As a result, most games will run 40% faster than intended on a VGA monitor.
On RISC OS 2, low resolution modes are not available when a VGA monitor is in use.
On machines prior to the Archimedes 540, VGA modes are implemented with a clock that is 4.7% slower than the VGA standard (24 MHz vs 25.175 MHz). The resulting video output is not compatible with all VGA monitors. This can be rectified with a VIDC enhancer.
Acorn did not supply any VGA monitors.
SuperVGA (type 4)
SuperVGA monitors support modes of 31 kHz and above horizontal scan rates. In practice this adds the resolution of 800x600 at 56 Hz (MODEs 29-31) to those supported by VGA monitors.
SuperVGA monitors are not supported prior to RISC OS 3.
The 36 MHz pixel clock required for 800x600 is not provided on machines prior to the Archimedes 540. This can be rectified with a VIDC enhancer.
Acorn-supplied SVGA monitors:
LCD (type 5)
The LCD monitor type is intended for the internal display on the A4. This DSTN display physically supports a single resolution of 640x480 with 15 grey scales, with a refresh rate of 82 Hz; the display is scanned at 41 Hz by VIDC then buffered and scanned out to the dual LCD panels at double rate by the LC ASIC. LC will also scale 320-wide modes for this display.
Low resolution modes are handled similarly to on VGA displays; the video output is vertically centered on the screen.
LCD monitors are not supported prior to RISC OS 3.
Mode Definition File (type 7)
TODO
NTSC (type 8)
TODO
EDID (type 16)
TODO