author Sam Lantinga <>
Sat, 12 Feb 2005 18:01:31 +0000
changeset 1035 974ba6ae0fa3
parent 955 d74fbf56f2f6
child 1332 de3d552b59b2
permissions -rw-r--r--
Date: Wed, 26 Jan 2005 13:37:09 GMT From: Peter Naulls Subject: RISC OS SDL Patches Sam, I've attached a diff of the latest changes to libSDL for RISC OS support. These changes are by Alan Buckley and myself. The most significant of these are: Optimised assembler blit rountines - I've attached the file src/video/riscos/SDL_riscosASM.s which is needed for this. Move to using /dev/dsp instead of its own audio implementation. This means that src/audio/riscos/SDL_drenderer.c should be removed Typo fixes. Mainly correct spelling of "RISC OS", but some from elsewhere too.

Readme for RISC OS port of SDL

This document last updated on 26th January 2005

This is a RISC OS port of the Simple Direct Media Layer (SDL) by Alan Buckley with contributions from Peter Naulls.

Details of the SDL can be found at

The source code including the RISC OS version can be obtained from

RISC OS makefiles, pre built libraries and many games and applications compiled for RISC OS using this library can be downloaded from The Unix Porting Project at

This is released under the LGPL see the file COPYING for details.

RISC OS port of SDL runtime information

Runtime requirements

This library currently needs a minimum of RISC OS 3.6. The source code for the library (and a lot of the programs built with it) also need long file names.

To use the audio you also need 16 bit sound and to have installed the DigitalRender module by Andreas Dehmel version 0.51 available from his
web site:
This is loaded when needed by UnixLib.

Note: As most programs ported from other OSes use high resolution graphics and a memory back buffer a machine with a StrongARM processor and 1 or 2MB of VRAM (or a better machine) is recomended.

RISC OS runtime parameters

Several environmental variables have been defined to make porting programs easier (i.e. By setting these variable you do not need to have source code differences between OSes).

They are all defined on an application basis.

The <appname> used below is found as follows:
1. Use the name of the program unless it is !RunImage
2. Check the folder specification for the folder !RunImage is run from. If it is a folder name use that name, otherwise if it is an environmental variable of the form <XXX$Dir> use the value of XXX.

The variables are:

SDL$<appname>$TaskName - The name of the task for RISC OS. If omitted then <appname> is used for the task name,

SDL$<appname>$BackBuffer - set to 1 to use a system memory back buffer for the screen in full screen mode. Some programs on other systems assume their is always a back buffer even though the SDL specification specifies this is not the case. The current RISC OS implementation uses direct writes to the screen if a hardware fullscreen is requested.

SDL$<appname>$CloseAction - set the action for the close icon. Again as programs don't match the specification you can set this to 0 to remove the close icon from the main window for applications where this does not affect the program.

SDL$<appname>$AudioBuffer - set the minimum size of the audio buffer in samples in steps of 512. If this is less than the minimum calculated from the parameters requested in the program by SDL_OpenAudio these will be used instead. If this parameter is not used or set to 0 the buffer size will be calculated to buffer 10 centisecond of sound. The time buffered can be calculated as specified size/frequency in seconds. The larger the buffer the more audio data that will be buffered, but this can lead to a lag between a sound being requested and it being heard, so the size should be kept to a minimum. You should only set this parameter if the sound is being missed out. 

RISC OS SDL port API notes

Current level of implementation

The following list is an overview of how much of the SDL is implemented. The areas match the main areas of the SDL.

video - Mostly done. Doesn't cover gamma, YUV-overlay or OpenGL.
Window Manager - Mostly done. SetIcon/IconifyWindow not implemented.
Events - Mostly done. Resize and some joystick events missing.
Joystick - Currently assumes a single joystick with 4 buttons.
Audio - Done
CDROM - Not implemented.
Threads - Done
Timers - Done

Thread support can be removed by defining DISABLE_THREADS and recompiling the library.

SDL API notes

This section contains additional notes on some specific commands.

  On RISC OS a fullscreen mode directly accesses the screen. This can be modified by the environmental variable (SDL$<appname>$BackBuffer) or by using the SDL_SWSURFACE flag to write to an offscreen buffer that is updated using SDL_UpdateRects.
  Open GL is not supported so SDL_OPENGL and SDL_OPENGLBLIT flags fail.
  SDL_RESIZEABLE and SDL_NOFRAME flags are not supported.

  In a wimp mode the screen colours are not changed for a hardware palette instead the RISC OS sprite colour mapping is used to get the best matching colours.

   Inverted colour is not supported.

   Currently this won't work if the application starts up in Fullscreen mode.
   Toggling to fullscreen will only work if the monitor is set up to support the exact screen size requested.

   Unicode translation used here is only really accurate for 7 bit characters.

SDL_NumJoysticks/JoystickName etc.
   Hardcoded to expect only 1 joystick with 4 buttons if the Joystick module is loaded.

   Timer used has only a centisecond accuracy. This applies to other time related functions.
   Modified to poll keyboard/mouse during the delay on the event thread.

Notes on current implementation

Keyboard and mouse are polled so if too long a time is spent between a call to SDL_PumpEvents, functions that use it, or SDL_Delay events can be missed.