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21

How exactly did those programs work? They were based on the ability of the Hercules card use the CGA's memory space. The original MDPA (*1) had (only) 4 KiB of memory mapped at B0000h, while the CGA had 16 KiB of Memory at B8000h. The Hercules card in contrast had 64 KiB of memory (*2), starting at B0000h. To avoid conflicts with a CGA in the same system ...


21

In the UK in the 1980s and the early 1990s, no video input at all was common. Early video recorders and set top boxes (for the new digital channels) would use RF out. You would disconnect the aerial from the TV, connect it to the VCR or STB, and connect the VCR or STB to the TV with another aerial cable. The TV would see it as an extra channel. You might ...


20

All VGAs support the “official” CGA and EGA modes, so most CGA and EGA games work fine. However, compatibility can only be relied upon at the BIOS level; a VGA can be implemented without strict hardware compatibility with the older standards. This affects some CGA games in particular. There is a CGA test tool available. It was written following this ...


19

Up until ~1980 the only connectors TV sets had were Belling-Lee type antenna in. So most VCR and next to all home computers did use RF modulated output toward the TV set (*1). This included even cable networks. While Type F connectors where used for more sophisticated equipment, households were still fitted with Belling-Lee. Professional equipment of the ...


16

Before SCART, European TVs often had not video input at all or S-Video on 6-pin or 8-pin DIN sockets (not Mini-DIN). Cinch composite was uncommon until VCRs came out. After that the old S-Video connectors died out and it was most common for cheap TV sets to have only an antenna and one composite in, better TV sets had a SCART input very early. Component ...


14

The basic idea is to exploit the fact that both CGA and Hercules use the 6845. I have the source code of an emulator at hand, so I can say what it does: it handles the mode-setting function of interrupt 0x10; for all non-Hercules modes, it includes tables of register settings to use to program the Hercules for the appropriate resolution and framebuffer ...


10

Your code is correct; a yellow prompt means that you’re using the red/green/brown palette. However, to get the low intensity variant, you also need to call interrupt 10h service 0Bh with BX set to 0 (black background, low intensity; strictly speaking, you can have any background — the bottom four bits, 3–0 — and the fifth bit, bit 4, controls the intensity; ...


9

TLDR: It's a soft spot for optimization around the ability to display 25 lines of text. (And why this is important has been discussed some time ago in an answer to your question about why 80x25 became standard) Preface: As usual with such decisions there are many factors involved - and most of them are not hard but variable within a certain frame and in ...


9

The other answers are good, but one specific area where VGA cards are generally incompatible with CGA is with respect to CGA's Composite mode. In the video "CGA Graphics - Not as bad as you thought!", The 8-Bit Guy demonstrates the CGA Composite mode and explains that, after about 1987 or so, new software written specifically for CGA Composite Mode became ...


8

Basically the 6845 was a bunch of counter registers, which you could program by defining end values (and sometimes start values), and use to generate RAM addresses and CRT sync signals. There was also a separate row address, so the original intention clearly was for character based displays: With a bit of additional components, you'd read some RAM content ...


8

One strategy is to convert the digital to analog, then the analog to VGA. CGA consists of digital red, green, blue, and intensity (RGBI) 5V signals, plus horizontal (15.75 kHz) and vertical (59.92 Hz) sync. VGA consists of analog red, green and blue signals (0.7V peak to peak), plus horizontal (31.46875 kHz) and vertical (59.94 Hz) sync (RGBHV). The first ...


6

Yes, most EGA and CGA games work just fine with a VGA card. VGA isn't 100% backwards compatible with EGA and CGA but in practice I don't think many games tried to get too fancy with these older cards. Gaming on IBM PC compatibles didn't really become a thing until VGA became standard on PCs, before that most IBM PC games were ports from other platforms. A ...


6

The Tandy 1000 CGA output was essentially like the IBM CGA electrically, and graphics modes were compatible. There was, however, an important difference in text mode. In text mode, the CGA used the middle 200 scan lines of a 262-line frame to display 25 rows of 8 scan lines each, with generous borders on the top and bottom. Using 8 lines per character ...


6

CGA and Hercules Graphics use the 6845. In graphics mode, each pixel is controlled individually, with no involvement from the character generator. Graphics mode is controlled by bit 1 of the 03D8h control register; when it’s set, the framebuffer is treated as containing bitmap information, and when it’s cleared, the framebuffer is processed by the character ...


6

I assume you want to connect machine emulating CGA image through VGA to the native CGA monitor. It is of course possible, but mind cost and quality. To formally downgrade VGA to CGA you need scan frequency go down from 31 kHz to 15 kHz, and thus dot clock will half too, but you will need to read two rows and perform intelligent generation colors of the ...


5

But the other CGA output option was NTSC, and that involved doing DAC on the card after all. I think here the basic logic error of your question is hidden. Colour in NTSC is neither an analogue level, nor tied to intensity. Intensity the base b&w part of a colour TV signal and formed independent as level (I wouldn't dare call it a DAC, it just emits two ...


5

TVs in Czechoslovakia (mainly TESLA brand) Till approximately 1985 TVs had only the antenna input. See1, 2 later almost all the colour TVs had composite input small devices had DIN-6 connectors with composite input (used also by Grundig and Sony) large devices had SCART connectors with composite input probably none of the TVs had RGB pins connected it ...


5

The easiest approach is probably to convert to a component video signal. If you pass the R, G, B, and I signals through a couple inverting buffers (so you have buffered versions of complemented and non-complemented signals) and formulated a composite sync signal, you could then use resistor dividers to generate Y, U, and V signals which could be fed into a ...


4

For the home market, TVs were often used because of their easy availability: most households already had one, and the image was often good enough for the low resolution of early machines. Professional users would often buy a purpose-made computer monitor, which would become essential as display resolutions increased and computers entered the GUI era. I'll ...


4

Another partial answer: Contrary to my first impression (by analyzing just the INT 10 handler it provides), the BIOS does in fact contain code that interfaces with the extended registers of the V6366 video controller. But first let's take a look at the schematics at page 22/23: Obviously, the only video RAM chip is the 32Kx8 SRAM U6, so the V6366 is ...


4

On the original IBM PC BIOS and true compatibles, the foreground intensity can be set using INT 10h with AH = 0Bh and BH = 00h. The low four bits of BL give the background colour, and bit 4 gives the foreground intensity. This is documented in (for example) the PC XT technical reference, page A-53. So after selecting your palette, this should switch to low ...


3

CGA 40-column text mode is 320x200 16-colors and 80-column text mode is 640x200 16 colors. Those are a completely standard feature of CGA and therefore a completely normal thing for a CGA monitor to display. But because they're text mode, you can't set arbitrary pixels to arbitrary colors, you have to use the character cells. Tandy and PCjr advanced ...


3

Basically yes. TGA (1000EX or similar) signal output (colour/intensity and sync signals) and connector pinout is upward compatible to CGA, thus compatible with CGA monitors. Both produce the very same RGBI signal using the same timing and encoding. TGA just employs more memory, thus being able to supply more colours at higher resolutions. After all, TGA is ...


3

One thing I have seen is that with the reprogrammability of the 6845 CRTC on the CGA - and moreso its workalike on the EGA - you can indeed put more than 200 lines on the 5153 if you really want to. To make it work on CGA you either have to reduce the horizontal resolution, or use 40-col text mode, because of memory concerns, but it's doable. However it's ...


3

If you have a genuine CGA card, it should have a composite video output. I would recommend simply using that. For one, lots of devices still have a composite input, and the cables are very common. One thing that's often forgotten is that, for graphics, CGA composite video looks much better than CGA's notoriously horrible digital graphics. Games written ...


3

With great help by user Michael Karcher we found the following details poking through the V6366 controller's innards: # ports 102 (outside of V6366, the decoded I/O select line is called VDCS) Bit 5: 00: Plasma panel active 20: External monitor active 3d4 CRTC index port 3d5 CRTC data port (6845 compatible, except for "interlace & skew" ...


2

In case you aim for a single box to convert digital RGB video to HDMI, there's boxes around like this that converts from SCART (which is RGB) to HDMI. I use one somilar to the one in the link to connect most of my old comuters (Sinclair Spectrum and QL) to a modern HDMI monitor. If your monitor has USB connectors, you can also conveniently power the ...


2

Partial answer: I was unable to find [the low level programming] documentation. [...] Perhaps they can give a clue as to what ports to look for. The datasheet you linked says the V6366 is compatible to the MC6845 except interlacing and skew, and also compatible to CGA, MDA and Hercules. That already gives a good idea what the registers look like (...


1

Just as a correction to a bit of the above: Hercules cards use regular MDA monitors. Their 350-line, nominal 50Hz mode (actually a bit below) isn't anything to do with regular broadcast monitors. They scan at 18.4kHz, which is a good way above that of typical TVs and low-rez monitors at 15.5~15.9kHz (and has the nice side effect of being effectively ...


1

I still have a small CRT TV in my house (belongs to my landlord) which has a composite video input on the front (which can still be used with a Raspberry Pi), and a SCART socket on the back. The latter is used with a digital TV receiver, as the analogue TV signal has long since been switched off. For context, this is in Finland. In the UK, I remember the ...


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