Risc PC Monitor Definition Files Explained

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.

You are not advised to edit monitor definition files directly.

The 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

is the number of rasters displayed vertically (pixels)

is the width of the horizontal sync pulse

is the width of the horizontal back porch

is the width of the left border

is the number of pixels displayed horizontally (which is normally the same as the x-resolution)

is the width of the right border

is the width of the horizontal front porch

is the width of the vertical sync pulse

is the width of the vertical back porch

is the width of the top border

is the number of rasters displayed vertically (pixels)

is the width of the bottom border

is the width of the vertical front porch

is the pixel rate in kHz

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

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.

Glossary of terms

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              32,000
                 32 bpp          16 Million

Limiting Factors

The following describes these interrelationships and offers guidance on the limits for the screen mode definition. As values are changed, the MakeModes application automatically checks that the mode will be displayable, if it is not, MakeModes will inform you. However, much time will be saved when attempting to create new modes if you understand and work within the limits imposed rather than using a 'trial and error' method.

Listed below are the three main limiting factors which are discussed in this document.

Monitor Limitations

The Pixel rate is one of the principal variables that you will use in the MakeModes application. It can be used to increase or decrease a modes frame rate. Because a modes Line rate is directly affected by its Frame rate, altering the Pixel rate will change this value as well.

The Pixel rate is calculated from:

Memory / Bandwidth Limitations

With no VRAM fitted the Risc PC will be forced to use DRAM which has a slower access time than dedicated Video RAM. This greatly increases the amount of time spent by the processor updating the screen and therefore limits the amount of memory that can be used as screen RAM. This limitation in turn limits the maximum resolution and pixel depth that can be used. On a machine with 1MB VRAM, the maximum amount of screen memory that the system can process is 80MB/sec, with 2MB of VRAM, this figure doubles giving a maximum video bandwidth of 160MB/sec. Without VRAM the maximum video bandwidth is 40MB/sec. In order to provide a safety margin, the bandwidth limit file stored in the !Boot directory on the Risc PC hard disc sets the maximum bandwidths to 38MB/sec, 76MB/sec and 152MB/sec respectively.

Is is important to be aware of both the memory and bandwidth calculations as it is possible to define a mode that fits within the available video RAM but exceeds the machines maximum video bandwidth. The MakeModes application will warn you if a proposed screen mode will exceed either of these limitations.

The following table shows an example of the screen modes and colour combinations (pixel depths) that can be used with the three different VRAM options.

Calculating required bandwidth for a screen mode

	Bits per pixel	Calculation used

	1   bpp		Video Bandwidth = (pixel_rate / 1000000) / 8
	2   bpp		Video Bandwidth = (pixel_rate / 1000000) / 4
	4   bpp		Video Bandwidth = (pixel_rate / 1000000) / 2
	8   bpp		Video Bandwidth = (pixel_rate / 1000000)
	16  bpp		Video Bandwidth = (pixel_rate / 1000000) * 2
	32  bpp		Video Bandwidth = (pixel_rate / 1000000) * 4

These calculations will give a result in MB/sec.

This total is then used in conjunction with the number of bits per pixel (bpp) to calculate the amount of memory required.

		Bits per pixel	Calculation used

		1   bpp		VRAM = pixel_total / 8
		2   bpp		VRAM = pixel_total / 4
		4   bpp		VRAM = pixel_total / 2
		8   bpp		VRAM = pixel_total
		16  bpp		VRAM = pixel_total * 2
		32  bpp		VRAM = pixel_total * 4

The horizontal back porch (hbpch) and and horizontal front porch (hfpch) values define the time, in units of pixels, from the end of the hsync pulse to the start of the horizontal left border (hlbdr) and the end of the horizontal right border and to the end of the horizontal scan line respectively. Amongst other things the hsync+hbpch time period is used by the VIDC20 to switch between cursor and video data. If the combined time period is not long enough, the screen display will be corrupted as the VIDC20 will be unable to fetch video data before the screen display starts.

The horizontal left border and horizontal right border values define the time period, in units of pixels, for the left and right screen borders. The horizontal left border and horizontal back porch values can be altered to position the picture correctly.

The horizontal display (hdisp) value is the horizontal screen resolution i.e the number of pixels displayed across the screen.

The vertical sync (vsync) value is the width, in units of raster, of the sync width pulse. If this pulse is not wide enough, the monitor will be unable to sync to the signal.

The vertical porches (vbpch, vfpch), vertical borders (vtbdr, vbbdr) are the vertical equivalents of the horizontal values but are all measured in units of raster.