Thread View: rec.games.programmer
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Started by jskubick
Thu, 16 Apr 1992 02:38
Linear VGA RAM possible?
Author: jskubick
Date: Thu, 16 Apr 1992 02:38
Date: Thu, 16 Apr 1992 02:38
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Is it possible to forget segmentation altogether and treat RAM on the VGA card as 1 contiguous chunk of memory on a 486 (assuming that I'm planning to write a program using 486 assembly language in protected mode with linear ram addressing)? Am I wrong in assuming that a 486 can map the RAM on a VGA card (well, an S3 card to be exact) to a 1 meg contiguous area addressable by the 486 as normal RAM? This is not merely an academic question. I just bought a 486 PC 2 months ago, but I used to be a fairly adept Amiga ML programmer (well, I still am, though I'm a PC owner now...) I'm planning to write an R-Type - type game this summer that will put a 33mhz 486 and S3 card to good use (I'm not planning to sell the game commercially, so I don't have to care whether it runs on anything less). All of the books I've read on VGA programming mention accessing VGA ram in 64k segments, though they all seem to be at least 2-3 years old, and I'm hoping that they simply don't mention linear VGA ram access because 3 years ago the 486 hadn't yet come into general use. Thanks for your help!
Re: Linear VGA RAM possible?
Author: Christopher Hecker
Date: Thu, 16 Apr 1992 05:21
Date: Thu, 16 Apr 1992 05:21
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No, you can't remap the memory so that it looks linear. There was some talk on BIX a while back about using the paging registers in concert with the mmu on the 386 to do this, but the speed hit wouldn't be worth the programming simplicity you would gain. Chris
Re: Linear VGA RAM possible?
Author: Jason Lowe
Date: Thu, 16 Apr 1992 23:28
Date: Thu, 16 Apr 1992 23:28
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I believe this may actually be possible. I was programming SVGA assembly code a while back, and I do remember reading something about the Tseng Labs ET4000 chip set (SpeedStar I believe uses these) can map its address space into higher memory. I am pretty sure the ET4000 is the only Tseng chip that claims to do this. Ahh, here I found the text I read this from: Excerpt from _Programmer's Guide to the EGA and VGA Cards, Second Edition_ by Richard F. Ferraro: (page 932) The [Tseng Lab ET4000] can be configured to access memory in a linear or in a segmented fashion through the SLS field. This linear addressing mode forces all display memory to be continuous. .... If the SLS Field is 1, this linear mapping scheme is enabled. If 0, the standard VGA segmented memory mapping scheme is enabled. ....it is possible to place the VGA into high memory where there is room to place one Megabyte of display memory into a contiguous address space. This allows the ET4000 to ignore the bank selection registers. [text goes into more detail..] If you have an ET4000, this seems possible according to Ferraro. I could not find, however, any mention of exactly what address this linear space shows up at, or even how to specify it yourself. I was interested in using this linear mode, so I called Speedstar. The person I talked to seemed to not know the chipset could even do this. He suggested calling Tseng Labs direct. If anyone finds out how to do this, or if it is truely possible, please let me know. -- Jason Lowe | // | Take the Oath. Watch Ren and Stimpy! stimpy@uiuc.edu | \X/ Amiga! | "And look, he's beating up on our cheese!"
Re: Linear VGA RAM possible?
Author: David M. Erazmus
Date: Fri, 17 Apr 1992 02:26
Date: Fri, 17 Apr 1992 02:26
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jdl47513@uxa.cso.uiuc.edu (Jason Lowe) writes: >I believe this may actually be possible. I was programming SVGA assembly >code a while back, and I do remember reading something about the Tseng Labs >ET4000 chip set (SpeedStar I believe uses these) can map its address space >into higher memory. I am pretty sure the ET4000 is the only Tseng chip that >claims to do this. Ahh, here I found the text I read this from: >Excerpt from _Programmer's Guide to the EGA and VGA Cards, Second Edition_ >by Richard F. Ferraro: >(page 932) >The [Tseng Lab ET4000] can be configured to access memory in a linear or in a >segmented fashion through the SLS field. This linear addressing mode forces >all display memory to be continuous. .... If the SLS Field is 1, this linear >mapping scheme is enabled. If 0, the standard VGA segmented memory mapping >scheme is enabled. ....it is possible to place the VGA into high memory where >there is room to place one Megabyte of display memory into a contiguous >address space. This allows the ET4000 to ignore the bank selection registers. >[text goes into more detail..] The chip may be capable of linear addressing, but it's still up to the card manufacturer to support this feature. With an ISA card, I don't think it's possible due to the fact that the ISA bus does not support 32-bit addressing. If there is a video system that does this, chances are it is either for the MCA, EISA, or is a motherboard implementation. I believe Ferraro mentions the advantage of using an ET4000 on the motherboard. It would simply use the motherboard's 32-bit memory bus and I suspect it would really cook! "More than 64K! Who could possibly want more than 64K!?!" - an IBM engineer. Dave Erazmus Champaign, IL
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