The vga(4) manual page takes no position on these details. ropers <[email protected]> wrote:
> Privyet, > > On man 4 vga, it says: > > "This is the standard VGA text mode with 80 columns and 25 rows. (...) > Characters are 8 x 16 pixels large (...)." > > To my understanding, that is borderline technically correct, but misleading: > > A naïve calculation of the screen's pixel resolution from these data > would yield 640x400px. However, AFAIK, the VGA adapter still treats > text mode like the MDA adapter did[0], which means that though each > character's matrix is an eight pixel-wide bitmap (sometimes only seven > pixels are actually used), the adapter itself always renders a ninth > pixel for each text mode character (and scanline). Usually that pixel > is blank, but for characters C0h–DFh, it is a copy of what was in the > eighth pixel. This special-casing of characters C0h through DFh > (where horizontal box-drawing characters and the like are usually > located) can be turned off in VGA adapters[1], but is hardwired into > MDA cards. The long and short of it is that with a total of nine > pixel-columns being rendered for each character-column, the > pixel-resolution of the standard VGA text mode is 720x400, not > 640x400, as some have naïvely calculated. > > Why am I making noise about this, when the description on vga(4) is > TECHNICALLY correct? > Because I have noticed that ever since this edit[2], OpenBSD has been > cited at Wikipedia as evidence for the incorrect assertion that the > default VGA text mode was 640x400px. Evidently the editor in question > has made exactly the naïve (mis)calculation noted above. (By this > edit[3], the error had been propagated to the article's main text > modes table, leaving it further entrenched.) > > I don't intend to get involved in correcting this at Wikipedia (which > would probably lead to a life force-sucking edit war), but it bothers > me that OpenBSD's man page appears to be misleading on this point. > Though the page actually cited is from OpenBSD 3.9, the same wording > is still present in 6.9 and -current, with minor differences in > formatting. > > > ======================================================================== > Would people here be able to confirm that I'm not myself mistaken in > what I'm saying? And could anyone suggest how to word an edit so as > to concisely clarify the point and make the vga(4) man page less > misleading? I'm not always good at being concise myself. > ======================================================================== > > > * I think the 80x25 & 80x25bf, as well as the 80x50 & 80x50bf modes > all are 720x400px. If the 80x25 mode in particular is not 720x400px, > then it is not the standard VGA text mode (as is claimed). > > * The 80x40 & 80x40bf modes confuse me. It's unclear to me if they > render a ninth pixel-column and thus whether their screen width is 640 > or 720. Arithmetically, 8x10px characters with an 80x40 text mode > ought to yield 640x400px, but the only information I could find is > here[4], and it suggests a display resolution of 640x480, which is > neither here nor there. It would make sense with two extra blank > scanlines per line of text, but then again, 480 instead of 400 could > be a typo. I'm also confused by these modes because I'm much more > familiar with 80x43 text modes and I'm low-key wondering whether 40 > instead of 43 is an error. This page[5] makes no mention of an 80x40 > text mode. I realise that is an altogether different project, but > still. > Second thought, I did find this[6], which suggests an 8x12px font for > an 80x40 text mode, but then, this is again an altogether different > project and may be nothing to do with OpenBSD's 80x40 text modes. I'm > just wondering how VGA-typical are those? > > * If the only difference of the 80x24 & 80x24bf modes from their > 25-line counterparts is the omission of the last row of text, then > maybe these are 720x384px? I really don't know though. > > * I think the "bf" in the *bf modes is for "big font" or "bigger > font", but I could not find confirmation of that. Also, if I'm > reading this right, then it says under BUGS that these modes have not > been tested and won't work with a monochrome monitor. (It's a little > unclear to me how that's meant.) > > > I have tried looking at source code, but I don't really know where to > look, and at least in /src/sys/dev/ic/vga.c, I've not been able to > identify anything that specifies the correct size of the actual > default 720x400 VGA text mode, or that would account for the 9th > pixel-column in each character. I've not been able to use the source > to answer my own questions. > > Any help or clarifications would be great appreciated. > > Thanks and regards, > Ian > > (Ian Ropers) > > > PS: Btw., can anyone elucidate the etymology of the "ic" in > /src/sys/dev/ic/? Is it because these devices all have integrated > circuits, or does the "ic" stand for something else? > > > [0] http://www.seasip.info/VintagePC/mda.html#memmap > > [1] "LGA - Line Graphics Enable > This field is used in 9 bit wide character modes to provide continuity > for the horizontal line characters in the range C0h-DFh. If this field > is set to 0, then the 9th column of these characters is replicated from > the 8th column of the character. Otherwise, if it is set to 1 then the > 9th column is set to the background like the rest of the characters." > --http://www.osdever.net/FreeVGA/vga/attrreg.htm#10 > > [2] > https://en.wikipedia.org/w/index.php?title=VGA_text_mode&diff=950771231&oldid=950759222 > > [3] > https://en.wikipedia.org/w/index.php?title=VGA_text_mode&diff=1022622974&oldid=1021428913#PC_common_text_modes > > [4] https://opencores.org/projects/interface_vga80x40 > > [5] http://www.columbia.edu/~em36/wpdos/videomodes.txt > > [6] http://javiervalcarce.eu/html/vhdl-vga80x40-en.html >
