Minus Zero Degrees

Yeah, and it cost $10,000, right? Just like every other PC and workstation AT&T ever sold. If you want to compare apples to apples, look at the Tecra 500CS from 1996 that was my first laptop in '97 - it cost me $700 practically new. (Then as now, ex-fleet machines are a great way to go, as long as you pick ones that were issued to people who hate computers.) For what this was doing half a decade later, it doesn't make the 1991 model look too shabby, although I concede nothing in those days had as much as one one-millionth of the price/performance of almost anything you can pick off a shelf today. https://www.minuszerodegrees.net/manuals/Toshiba/Tecra/Toshiba%20Tecra…

That's some oddly specific trivia, but the index page for that site looks like a great resource of technical information on the original PC line of computers. http://minuszerodegrees.net/index.htm

Valid arguments, and after buying the first model I didn't follow the Amiga story after it was clear it was going to fail. Doom requiring GPUs would seem to be adding to the argument that for a very long time period as these things go, their specialization paid off given the volume in which they sell. If/when GPUs get almost entirely subsumed into CPUs (I don't have any 3D requirements, and I'm typing this on my first, 1 year old computer with integrated into the CPU graphics), I don't think it'll invalidate their "success by specialization". However, I think I'll still go with my thesis on the Amiga: the failure to gain sufficient market share and therefore volume, plus poor to bad management, doomed it just like the Lisp Machines. And perhaps the NRE costs on the chipset, perhaps the experience of doing it and facing having to do it again, deterred the managers from going for 3D. As for modern PCs ... I'm not sure I really buy it. Look at all the discrete TTL in the original PC motherboard: http://mit-a.com/disp/mb1.shtml Look at all the TTL in standard cards for it: http://www.minuszerodegrees.net/5150_5160/cards/5150_5160_cards.htm And a 2nd generation 3com Ethernet card: http://sishardware.com/unt/20622-3com_etherlink_ii_tp_6330___01_10mbps… All computers, especially all designed for volume production, looked like that, and were filled with cards like that. I submit that the steady accumulation of all that logic…

Technically in MCGA mode VGA doesnt spread individual pixel bits among banks, pixels are kept "whole" as bytes in banks/planes. 13h is the only mode that doesnt benefit from all of that complicated EGA inherited machinery, yet it still became the most successful and practically synonymous with VGA :) Btw OG 1987 $595 IBM PERSONAL SYSTEM/2 DISPLAY ADAPTER https://www.minuszerodegrees.net/video/IBM%20Announcement%20Letter%201… VGA card used eight 64Kx4 DRAMs https://upload.wikimedia.org/wikipedia/commons/3/3f/IBM_VGA_graphics_c… to build its 4 banks. Afaik this ram is wired in parallel, at least the address bus part.

There were a bunch of predefined modes in the video BIOS, and with a little bit of assembler you'd issue an interrupt (a system call really) which would change the video mode. Then as the parent comment said, you could write to video memory directly and your writes would either be interpreted as ASCII character/attribute pairs in a text mode, or colour palette indices in a graphical mode. Most games at that time used mode 13h which was 320x200 with 8-bits per pixel which therefore indexed into a 256-colour palette (which could itself be redefined on the fly via reading from and writing to a couple of special registers - allowing for easy colour-cycling effects that were popular at that time). Here's a list of the modes: https://www.minuszerodegrees.net/video/bios_video_modes.htm

It's just an ugly ass syscall extension mechanism (so it has no direct equivalent in Linux lets say), it definitely looks bizarre in modern times. Int 2F is initially handled by DOS as a stub, but additional programs (like drivers and TSRs) can override INT 2F, put their bucket of functionality and then fallback to whatever the previous installed handler was (called chaining) for whatever they don't handle. This gives a glimpse into how much various crap could end up installed as an Int 2F handler: https://www.minuszerodegrees.net/websitecopies/Linux.old/docs/interrup… It was often used for feature/presence checks and usually nothing time critical as that chaining setup was most definitely not timing friendly.

5170 (IBM AT) was a 286 design. The 5150 (IBM PC) and 5160 (IBM PC XT) were the previous 8088-based systems. Sounds like it's described as "based on" because it's heavily reverse engineered, modified to use more readily available components where possible, and then improved to 20MHz capability over the original 5170's 6/8MHz. Lots of info on the 51xx series of machines is here: https://www.minuszerodegrees.net/index.htm

I remember setting those 7-segment LEDs up on my first PC build. On the motherboard there was a jumper which took the input from the turbo switch. To provide feedback to the user, some cases ran a wire from the turbo switch to a bi-color LED on the front of the case as well as to the motherboard to control the turbo state. On fancier cases they had multiple seven-segment LEDs which had a multi-state jumper for each segment, which decided to light that segment depending if the switch was on or off (example: http://www.minuszerodegrees.net/led_speed_display/LED%20display%20-%20… so you could display either the two different clock frequencies, or a "HI"/"LO" message, or anything you wanted. And connecting that switch input to the motherboard wasn't necessary for the LED to change, so I guess that is how your father's was setup :)

Enlightenment may come from looking at an IBM-PC MDA (Monochrome Display Adapter) board: http://www.minuszerodegrees.net/5150_5160/cards/ibm_mda.jpg ...or a VT100 main-board: http://i.imgur.com/Xae4be6.png 80% of these boards are "the graphics part." Dozens of chips! This is what the sibling commenters mean when they're saying that the video signal is "discrete logic" (as opposed to integrated logic.) It'd be a bit of an ask to have a hobbyist build one of those themselves (adding an 800% complexity increase to what is otherwise a very approachable hobby project.)

Compare the IBM PC/AT "Guide to Operations" of the same time: http://www.minuszerodegrees.net/manuals/IBM_5170_Guide_to_Operations_1… No colour, and of course a CLI doesn't need a lesson in how the UI works, but plenty of technical information.

http://www.minuszerodegrees.net/manuals/IBM_5150_Technical_Reference_6… This is the technical manual for it

Do you remember seeing jumpers on the motherboard for that? The only ones I recall seeing were on the display in the case itself, like this this: http://www.minuszerodegrees.net/led_speed_display/LED%20display%20-%20…

Nitpicking: The CGA 8x8 font had a number of "foreign" (non-ascii) characters that could be displayed in 640x200 or 320x200 text modes. All (?) European though. https://raw.githubusercontent.com/viler-int10h/CGA-font-compare/images… From IBM's technical manual: * 16 special characters for game support * 15 characters for word-processing editing support * 96 characters for the standard ASCII graphics set * 48 characters for foreign-language support * 48 characters for business block-graphics support (for the drawing of charts, boxes, and tables using single and double lines) * 16 selected Greek characters * 15 selected scientific-notation characters (https://minuszerodegrees.net/oa/OA%20-%20IBM%20Color%20Graphics%20Moni… ...although https://en.wikipedia.org/wiki/Code_page_437#Internationalization reaches the conclusion that it only e.g. English, Germany and Swedish are fully supported due the lack of upper-case accented latin characters. Huh, TIL.

> You're right, sort of the oddest thing about IBM's basic bidirectional mode (which is pretty much equivalent to byte mode in 1284) is that support for it went away only briefly. I sort of wonder about the internal events at IBM here, one would think they were probably starting to develop applications for bidirectional mode at the same time they stopped including it, since they brought it back pretty quickly. You got me curious about this and so I looked into it a bit further. I managed to find IBM's technical documentation[0] for the original printer original printer port card and it looks like it only sort of supports bidirectional use of the data pins. It supports reading the state of the data pins, but it seems this was intended to be used for diagnostic purposes. There's no way to disable the output drivers for these pins like there are on later bidirectional ports. The manual does mention that if something happens to drive the pins the result of reading the port will be that value ORed with whatever was written previously (though I've seen claims elsewhere that it's actually more of an AND), but it also cautions that driving these pins externallly is "in violation of usage ground rules". Apparently some folks did use it this way though and perhaps that's what inspired making it properly supported in the PS/2. It's unclear where this idea that the original PC had this feature, but it went away came from. I looked at the manual for the AT's serial/parallel combo…

That mode is also non-square, with an aspect of 0.6:1. The IBM PC and its various video cards in the early days used a number of tricks to keep the cost down, one of those tricks is to keep the horizontal and vertical sync signals the same for different modes as much as possible. (Recently I’ve started to appreciate just how many tricks they used to keep the cost down, without sacrificing hardware reliability in the process!) For both 320x200 and 640x200, HSYNC ran at 15.75 kHz, VSYNC ran at 60 Hz. When you go from 320x200 to 640x200, all that happens is the pixel clock (the rate at which you read out from RAM) is doubled, so you get exactly two pixels horizontally packed in where there used to be one pixel. The older hardware, like EGA video cards, can only generate one other HSYNC speed: 21.8 kHz, for special 350-line modes. When VGA came out, it doubled the HSYNC frequency and, for these modes, would just read each row out twice. http://www.minuszerodegrees.net/video/bios_video_modes.htm So SCREEN 1 should have 1.2:1 ratio, and SCREEN 2 should have a 0.6:1 ratio. One of the better ways to verify this kind of thing is to compare box art from DOS video games to screenshots taken from DOSBox or sprites extracted from the data files. You can see that the screenshots from DOSBox match the sprite files, if you leave aspect unadjusted, but if you look at the box art from the retail packaging, you will (often) see artwork and screenshots with the correct aspect ratio, just…