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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; ...


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 ...


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 ...


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

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

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

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 ...


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 ...


1

Some background: TVs of the area before video recorders, satellite receivers etc became common and worked best with a baseband video connection, did have little reason to offer such a connection. Neither would such a connection have been trivial to implement or retrofit. TVs of that era were very commonly of a so called live chassis design, which made it ...


1

The MCE2VGA, based on Luis Antoniosi’s design, can convert CGA, EGA, MDA and Hercules signals to VGA: connect a TTL output to it, and it will produce a VGA-compatible signal. It can even emulate composite CGA! The first link includes video reviews so you can see it in action.


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