chuck@umbc5.umbc.edu (Chuck Rickard) (01/12/91)
Just thinking about the memory involved in displaying a 1024x768 256 color picture... 1024 pixels x 256 colors = 1 K bytes per row 1 K bytes x 768 rows = 768 K bytes So, the most memory required to display such a picture is 768K, right? What is done with the other 232K left over in the 1 meg of video memory? Chuck Rickard (chuck@umbc5.umbc.edu) Chuck Rickard (chuck@umbc5.umbc.edu)
raster@itsgw.rpi.edu (Jerry D Bain) (01/12/91)
chuck@umbc5.umbc.edu (Chuck Rickard) writes: >Just thinking about the memory involved in displaying a 1024x768 256 color >picture... >1024 pixels x 256 colors = 1 K bytes per row >1 K bytes x 768 rows = 768 K bytes >So, the most memory required to display such a picture is 768K, right? >What is done with the other 232K left over in the 1 meg of video memory? Yes, all video cards that require 1meg of memory for 1024x768x256 are wasting memory. Usually the memory gets fragmented due to the strange paging techniques the various chip sets use to get access to the meg of memory through the 64k VGA "window"... I once asked an engineer at an *unnamed* video board manufacturer what the deal was... He responded that the inefficient use of memory was a result of an attempt at getting the base cost down on the control circuitry. With the paging that is used (on most boards), the memory gets fragmented as a result of the attempt to simplify the calculation of addresses that are on power of 2 boundries.
phil@brahms.amd.com (Phil Ngai) (01/12/91)
In article <4839@umbc3.UMBC.EDU> chuck@umbc5.umbc.edu writes: |Just thinking about the memory involved in displaying a 1024x768 256 color |picture... |... |So, the most memory required to display such a picture is 768K, right? |What is done with the other 232K left over in the 1 meg of video memory? Nothing. Do you want your money back? -- militia: 1.a. A citizen army, as distinct from a body of professional soldiers. b. The armed citzenry, as distinct from the regular army.
silver@xrtll.uucp (Hi Ho Silver) (01/13/91)
In comp.sys.ibm.pc.hardware, chuck@umbc5.umbc.edu typed: $Just thinking about the memory involved in displaying a 1024x768 256 color $picture... $1024 pixels x 256 colors = 1 K bytes per row $1 K bytes x 768 rows = 768 K bytes Yeah, that's the correct answer, though the method isn't 100% on. The usual method of achieving more than one bit depth is to use bitplanes; in this case, there would be a series of eight 1024x768x1 arrays. Some boards no doubt do it otherwise, though. $What is done with the other 232K left over in the 1 meg of video memory? Which other 232K? There's another 256K there. Oh well, in any case, nothing. It's exactly the same as what happens with most other modes on lesser boards (e.g. nearly half of 512K in 640x480x256 mode). It's hardly something to get worried about; the cost difference, if any, would be quite small, and could even work to the advantage of a 1M board. $Chuck Rickard $(chuck@umbc5.umbc.edu) $ $Chuck Rickard $(chuck@umbc5.umbc.edu) BTW, your .signature is appearing twice. -- __ __ _ | ...!nexus.yorku.edu!xrtll!silver | always (__ | | | | |_ |_) >----------------------------------< searching __) | |_ \/ |__ | \ | if you don't like my posts, type | for _____________________/ find / -print|xargs cat|compress | SNTF
phil@brahms.amd.com (Phil Ngai) (01/16/91)
In article <4842@umbc3.UMBC.EDU> chuck@umbc5.umbc.edu.UUCP (Chuck Rickard) writes: |Second, I had heard that some programs such as QEMM and DR-DOS were going to |have drivers which would claim unused memory in the video card and map that |into usable space (on a 386, of course). Have you heard anything like this? Yes and no. The amount of video card memory exposed to DOS is limited. The 1 meg on the card is accessed by bank switching. QEMM can give you access to the memory that is directly exposed but doesn't do the bank switching, nor would there be any reason to except as a weird type of EMS. Such memory would be extremely slow in any case. QEMM could do this but doesn't since it's not worth the bother. (Real, fast) memory is cheap and this kind of stuff is a waste of time. -- militia: 1.a. A citizen army, as distinct from a body of professional soldiers. b. The armed citzenry, as distinct from the regular army.