33
votes
How did the IBM 5153 color display detect and modify the signal to make low-intensity yellow into "brown"?
Dr. H. Holden describes the general workings of the IBM 5153 colour conversion circuit (which converts the digital RGBI signals to analogue RGB levels) on pages 21-24 of his document "Fitting an ...
- 23k
22
votes
Accepted
How do CGA emulators for Hercules graphics work?
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 ...
- 195k
21
votes
What video connections were common in Europe?
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 ...
- 1,832
20
votes
Accepted
Do all VGA cards implicitly support CGA and EGA?
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 ...
- 110k
20
votes
What video connections were common in Europe?
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 ...
- 195k
19
votes
Accepted
Could CGA synchronize with vertical blank?
It is possible to synchronise with vertical blank, but it involves polling (as does avoiding snow on CGA). Two bits are important in the status register, read from 0x03DA:
bit 0 is 1 when the CPU can ...
- 110k
17
votes
How do CGA emulators for Hercules graphics work?
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 ...
- 110k
16
votes
Accepted
What video connections were common in Europe?
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 ...
- 2,098
16
votes
Was there ever a functional EGA emulator for the Hercules graphics card?
There can not be an EGA emulator for many reasons.
Emulating CGA works, because in graphics mode, Hercules has a 64k frame buffer at address B0000h - BFFFFh, and for a single page, only 32k from ...
- 24.2k
14
votes
How to use the "darker" CGA palette using x86 Assembly?
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 (...
- 110k
14
votes
Accepted
What really prevented PC games from using hardware scrolling on CGA
According to https://en.wikipedia.org/wiki/Adaptive_tile_refresh:
CGA (the previous generation of PC graphics hardware) lacks features for scrolling in hardware
so PC games started featuring ...
- 195k
14
votes
What really prevented PC games from using hardware scrolling on CGA
I did it for a game, but it was a mess.
It was a port of a 2-way fine scroller... by making it a 1-way scroller, we trashed the game pretty badly... it was definitely not faithful to the original. ...
12
votes
What is an XRGB video signal? (NOT the modern XRGB mini-DIN.)
The Chroma Pro was a series of screens introduced after 1985. I remember using a CM 2000 in the late 80s.
RGBI is essentially IBM CGA.
RGB is Analogue RGB.
I used both modes with IBM and an Atari ST.
...
- 195k
11
votes
Could CGA synchronize with vertical blank?
The synchronization is possible via polling, but it could also be done by interrupt even when no hardware interrupts were provided. It worked on any card that had a "real-time" status ...
11
votes
How did the IBM 5153 color display detect and modify the signal to make low-intensity yellow into "brown"?
The circuitry does exactly what you described, it detects the bit pattern for that specific color that would be "dark yellow", and when the bit pattern matches, an extra transistor is turned ...
- 24.2k
10
votes
Why did CGA RGBI output leave DAC to the monitor?
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,...
- 195k
9
votes
Accepted
IBM 5153 monitor vertical resolution
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 ...
- 195k
9
votes
How can I adapt a digital CGA/C128 signal (RGBI DE-9) to a modern monitor?
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)...
- 17.2k
9
votes
Do all VGA cards implicitly support CGA and EGA?
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!", ...
- 1,026
8
votes
Accepted
Motorola 6845 and bitwise graphics
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 ...
- 24k
8
votes
Is the IBM 5153 color display compatible with the Tandy 1000 16 color modes?
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 ...
- 32.3k
7
votes
How to use the "darker" CGA palette using x86 Assembly?
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 ...
- 6,460
7
votes
Accepted
How do I configure a V6366 graphics controller for 640x400 B/W?
You can find a description of a lot of the V6366 features that actually get used in typical laptops in the Zenith TurbosPort technical reference manual.
That manual clearly names the register bits ...
- 6,743
7
votes
What is an XRGB video signal? (NOT the modern XRGB mini-DIN.)
https://groups.google.com/g/comp.sys.apple2/c/1In2WJZ9U6M?pli=1
From my reading of that post linked above, XRGB is a variation of RGB that maps to NTSC artifact colors, rather than the usual 16 seen ...
- 764
6
votes
How can I adapt a digital CGA/C128 signal (RGBI DE-9) to a modern monitor?
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 ...
- 32.3k
6
votes
Motorola 6845 and bitwise graphics
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 ...
- 110k
6
votes
VGA to CGA (RGBI) signal
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 ...
- 1,248
6
votes
How do I configure a V6366 graphics controller for 640x400 B/W?
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
votes
Accepted
Why did CGA RGBI output leave DAC to the monitor?
Conjecture: Cable material that could cleanly transfer three analog video signals, plus sync pulses and identification pins, over several feet was probably not a cheap, mass produced item at the time ...
- 5,530
Only top scored, non community-wiki answers of a minimum length are eligible