Flickering
This release of PCjs implements FLICKERING.
Yes, the beloved flicker of the original IBM Color Graphics Adapter, or CGA. And I’m not talking about the barely perceptible flicker produced by the CGA monitor’s 60Hz refresh rate. That flicker was tolerable. In fact, many people who didn’t need either color or graphics would opt for IBM’s Monochrome Display Adapter, or MDA, because the green phosphor of IBM’s monochrome monitor had such high persistence that the 60Hz refresh rate was virtually imperceptible.
No, the capital-letter FLICKER that I’m talking about was a side-effect of what IBM’s ROM BIOS did every time the CGA’s screen had to be scrolled: completely turn off the video signal OFF, rearrange the contents of video memory, and then turn the video signal back ON. IBM adopted this approach because otherwise you would see an even worse effect: random video interference or “snow”.
This all came about because the CGA’s memory wasn’t “dual-ported”: if the CPU accessed the CGA’s memory at the same that the display hardware was accessing it (to refresh the screen 60 times per second), the CPU would win and the display hardware would lose, resulting in garbage appearing on the screen for a brief instant.
PC software could avoid that conflict by waiting until the CGA hardware was in a retrace period: either the brief horizontal retrace period that occurs as the CRT repositions the electron beam from the right side of the screen back to the left side, or the slightly longer vertical retrace period that involves moving the beam from the bottom to the top.
However, IBM decided that, since neither of those retrace periods were long enough to perform a full-screen scroll, the easiest solution was to simply turn the screen off for the duration. The result, which many of us suffered with for years, was FLICKER.
One of those interesting solutions was a program from 1986 called FlickerFree, the purpose and value of which would be almost completely lost on someone today if they had never seen or used an original IBM PC with a CGA.
In fact, FlickerFree proved to be a useful PCjs test case, because it relied on a quirk of CGA hardware that I hadn’t originally implemented: frame buffer wrap-around. FlickerFree resolved the flickering-scroll problem by treating the CGA’s 16Kb of video memory as a giant ring buffer: instead of moving the contents of video memory “upward”, like IBM’s ROM BIOS did, FlickerFree would simply shift the video memory start address “downward”. After all, one 25x80 screen of character data required only 4000 bytes, and the CGA had 16,384 bytes of memory.
At first glance, that approach would seem to merely delay the inevitable, because when the start address nears the end of the buffer, the entire buffer still has to be scrolled. But, as it turned out, the CGA’s CRT controller starts reading from the top of video memory after it reaches the bottom. So all software has to do is write new lines at the top of the video memory again. If IBM had thought of that trick 5 years earlier, back in 1981, think of all the years of shared flicker-grief that could have been avoided!
Today, in honor of all that flickering, I’m pleased to announce that PCx86 now offers FLICKERING on all IBM PC Color Graphics configurations. If you feel the need to override it, you can add flicker=0 to the URL of your favorite machine; e.g.:
In fact, you can set the flicker property to any value from 0 to 1; the default is 0.5. This value is applied to the opacity property of the PCx86 <canvas> element, allowing you to mimic whatever degree of “phosphorescent persistence” you prefer.
You can thank me later.