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5th September 1994
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Support Group Application Note
Number: 254
Issue: 1.02
Author: DM
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Risc PC monitor definition files explained
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Notes:
This application note describes in detail Risc PC monitor definition files.
It should be read in conjunction with the !MakeModes documentation.
!MakeModes is an application which assists the creation and editing of Risc
PC monitor definition files.
The !MakeModes application and this application note assume a degree of
technical knowledge concerning video display generation and is not intended
for use by those unfamiliar with display technology.
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Applicable Hardware:
Risc PC range
Related Application Notes:
!MakeModes
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Copyright (C) 1994 Acorn Computers Limited
Every effort has been made to ensure that the information in this leaflet is
true and correct at the time of printing. However, the products described in
this leaflet are subject to continuous development and improvements and
Acorn Computers Limited reserves the right to change its specifications at
any time. Acorn Computers Limited cannot accept liability for any loss or
damage arising from the use of any information or particulars in this
leaflet. ACORN, ECONET and ARCHIMEDES are trademarks of Acorn Computers
Limited.
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Support Group
Acorn Computers Limited
Acorn House
Vision Park
Histon
Cambridge
CB4 4AE
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Introduction
------------
The Acorn Risc PC, unlike any previous Acorn machine, does not have
information on screen modes built into the operating system. Screen modes
are instead defined in a Monitor Definition File (MDF) which is loaded when
the machine boots up. This provides the user of a Risc PC with a much
greater degree of flexibility when it comes to choosing a monitor as screen
modes can now be individually tailored to suit the facilities offered by the
monitor.
Overview
--------
A Monitor Definition File which normally resides in the
!Boot.Resources.Configure Mon- itors directory is a plain text file which
defines all of the parameters necessary for the Risc PC to generate and
display RISC OS screen modes. An example segment of an MDF follows...
file_format:1
monitor_title:Acorn AKF60
DPMS_state:1
# 320 x 250 (70Hz)
startmode
mode_name:320 x 250
x_res:320
y_res:250
pixel_rate:12587
h_timings:36,14,12,320,12,6
v_timings:2,109,0,250,0,88
sync_pol:2
endmode
# 800 x 600 (75Hz)
startmode
mode_name:800 x 600
x_res:800
y_res:600
pixel_rate:49500
h_timings:80,46,42,800,42,46
v_timings:3,21,0,600,0,1
sync_pol:0
endmode
# 1024 x 768 (60Hz)
# High band
startmode
mode_name:1024 x 768
x_res:1024
y_res:768
pixel_rate:65000
h_timings:128,36,60,1024,60,36
# VESA:136,160,0,1024,0,24
v_timings:6,29,0,768,0,3
sync_pol:0
endmode
Each screen mode that you wish to use should be defined as above within the
monitor definition file. The application !MakeModes can be used to generate
monitor definition files from data supplied by the user. It is available
from your Acorn Dealer or local I.T. Centre. The application consists of a
disc containing !MakeModes, a ReadMe file and an accompanying manual.
You are not advised to edit monitor definition files directly.
parameters file_format, monitor_title and DPMS_state are set only once at
the beginning of an MDF. The command startmode tells the machine that a mode
definition follows. Below is a summary of all of the commands used in an
MDF.
file_format: format
monitor_title: title
DPMS_state: state
startmode
mode_name: mode_name
x_res: x-resolution
y_res: y-resolution
pixel_rate: pixel_rate
h_timings: hsync, hbpch, hlbdr,hdisp,hrbdr,hfpch
v_timings: vsync, vbpch, vtbdr, vdisp. vbbdr, vfpch
sync_pol: sync_polarities
endmode
where:
mode_name: is a textual name for the mode that will be used in the
display manager's mode menu. It is possible to prevent
defined screen modes from appearing in the modes menu by
simply leaving this field blank. Mode names are limited to
19 characters in length and may contain space characters.
x-resolution: is the number of pixels displayed across the screen
y-resolution: is the number of rasters displayed vertically (pixels)
hsync: is the width of the horizontal sync pulse
hbpch: is the width of the horizontal back porch
hlbdr: is the width of the left border
hdisp: is the number of pixels displayed horizontally (which is
normally the same as the x-resolution)
hrbdr: is the width of the right border
hfpch: is the width of the horizontal front porch
vsync: is the width of the vertical sync pulse
vbpch: is the width of the vertical back porch
vtbdr: is the width of the top border
vdisp: is the number of rasters displayed vertically (pixels)
vbbdr: is the width of the bottom border
vfpch: is the width of the vertical front porch
pixel_rate: is the pixel rate in kHz
sync_polarities:is a number indicating what kind of sync signals are
required. The kinds of sync signal are as follows:
0 hsync normal, vsync normal
1 hsync inverted, vsync normal
2 hsync normal, vsync inverted
3 hsync inverted, vsync inverted
All values on the h_timings line are in units of pixels, and all values on
the v_timings line are in units of raster lines.
Note: VIDC20 imposes restrictions on these parameters. In particular, all
the horizontal timing values must be in multiples of 2, and the horizontal
total (hsync + hbpch + hlbdr + hdisp + hrbdr + hfpch) must be a multiple of
4.
The diagram below gives a graphical representation of this information.
<Insert Drawfiles.Fig1>
Glossary of terms
-----------------
Before describing in detail how these parameters can be used to define a new
screen mode, it is necessary to understand some of the conventions that will
be used in this application note.
Video Bandwidth:
Video Bandwidth is the term used to describe the amount of video
information that a Risc PC can process over a specified time period.
Typically, video bandwidth will be specified as megabytes per second e.g. a
Risc PC fitted with 2MB VRAM has a maximum video bandwidth of 156MB/sec.
Dot Pitch:
The dot pitch is the physical spacing of the phosphor dots that a monitor
uses to generate a screen display. The smaller the dot pitch, the clearer
the picture. Some high resolution screen modes may appear blurred if the dot
pitch is not small enough to cope with the size of the pixels being
generated by the computer.
Line Rate (Horizontal frequency):
A monitor's line rate defines the speed at which a monitor can scan one
horizontal line. The line rate is normally specified as a range in kHz i.e.
the AKF60 has a line rate range of 30-50kHz.
Frame Rate (Vertical frequency):
The frame rate is the speed at which the monitor can refresh the screen. The
frame rate is normally specified as a range in Hz i.e. the AKF60 has a frame
rate range of 40-90Hz.
Pixel Depth:
Pixel depth dictates the number of colours that can be displayed on the
screen. Pixel depth is measured in bits per pixel (bpp). The following table
describes how bits per pixel relates to the number of colours that will be
displayed;
Bits per pixel Number of colours
1 bpp 2
2 bpp 4
4 bpp 16
8 bpp 256
16 bpp 32000
32 bpp 16 Million
Limiting Factors
------