From: merlin@maths.tcd.ie (Merlin Hughes) Subject: Re: Manipulation of screenmodes Date: 31 Jul 92 01:34:04 GMT To program the VIDC, you need to be in supervisor mode, and you poke the new data into location $3400000. There are 4 words there mapped to the VIDC (I think), so you can STM up to 4 registers at a go. eg: SWI"OS_EnterOS" -- Supervisor mode MOV0,#&3400000 -- Address of VIDC MOV1,# -- Data to program VIDC with STMIA0,{1} -- Send it to the VIDC TEQP15,#0:DCD0 -- User mode The VIDC register you are programming is encoded into the top byte of the byte you store, the data is in the rest of the word. The register numbers are separated by 4, ie the first register is &00, the second &04, etc. ie: --Word-- High Byte Low Byte 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 x x x x x x 0 0 x x x x x x x x x x x x x x x x x x x x x x x x \_____________/ \_______________________________________________/ VIDC Address VIDC Data VIDC registers are: 00 \ .. > Video Palette Logical Colour 0 .. 15 3C / 40 Border Colour 44 \ .. > Cursor Palette Logical Colour 1 .. 3 4C / -- bit 12=Supremacy, 11..8=Blue, 7..4=Green, 3..0=Red -- (different slightly in 8BPP modes, but same region) 60 \ .. > Stereo Image Register 7 , 0 .. 6 7C / -- bit 2..0=Position 80 Horizontal Cycle Register 84 Horizontal Sync Width Register -- bit 23..14=Data 88 Horizontal Border Start Register 8C Horizontal Display Start Register 90 Horizontal Display End Register 94 Horizontal Border End Register -- bit 23..14=Data 98 Horizontal Cursor Start Register -- bit 23..13=Data, 12..11 must be 0 except in Hi-Res mode 9C Horizontal Interlace Register A0 Vertical Cycle Register A4 Vertical Sync Width Register -- bit 23..14=Data A8 Vertical Border Start Register AC Vertical Display Start Register B0 Vertical Display End Register B4 Vertical Border End Register -- bit 23..14=Data B8 Vertical Cursor Start Register BC Vertical Cursor End Register -- bit 23..14=Data C0 Sound Frequency Register -- bit 7..0=Data E0 Control Register -- bit 15..14= Test Mode; 00=>Normal Operation, 01..11=>Test Mode 0, 1, 2 -- bit 8=Test Mode; 0=>Normal Operation, 1=>Test Mode 3 -- bit 7=Composite Sync; 0=>Vertical, 1=>Composite -- bit 6=Interlace Sync; 0=>Off, 1=>On -- bit 5..4=DMA Request; 00..11=>End Of Word 0/4, 1/5, 2/6, 3/7 -- bit 3..2=Bits Per Pixel; 00..11=>BPP 1, 2, 4, 8 -- bit 1..0=Pixel Rate; 00..11=>MHz 8, 12, 16, 24 - Ok, those are the registers. If you are already in the type of mode you want to be in, and you just want to change the dimensions, you only need to reprogram the H&V Border/Display Start/End Registers. If not, you'll have to fiddle with the other relevant sounding ones. ( >From mode 13, programming the following registers with the following data values will get a 384*208 shaped mode.. H Disp Start = 40 H Disp End = 40 + (384/2) V Disp Start = 63 V Disp End = 63 + 208 You may have to push the borders out a bit. ) You'll probably get somewhere by experimentation... Something to note is that RISC OS reprograms stuff about the palette and the pointer quite regularly, so you'll have difficulty doing things with them. For full information, get the VLSI data book, or if you want me to, I'll type in the rest of what is said about the relevant registers. I'm not going to type it in now because I'm tired. I assume you'll assume you have enough screen memory.. People may object to your reprogramming VIDC for a few reasons.. Some people will say it isn't future compatible. I'll believe that one when I see a new video chip in the Arc - I think you can afford to forget about that. Different types of monitors however, are a valid reason for not doing the reprogramming yourself. People with multisyncs and VGAs and so on, sometimes have their own base modes defined. That way, different shaped modes will still be of the preferred users style (New modes are defined with a standard base mode, and then a VIDC list with the list of registers that need to be changed for the new mode. Normally these will only include new values for the H&V Border/Display Start/End registers.) People like this will hate your for changing mode yourself. Anyway, enjoy. --SICK--Merlin Hughes-- /| This is only the beginning. merlin@maths.tcd.ie //| We will regroup. We will prepare. //_| When the time comes, we will emerge and take Acorn.. Deus et machina. / | our place as the supreme masters of the universe.