ajenner

As trollbridge said, there isn't enough VRAM in the CGA to have 200 rows of 80 columns (2 bytes per cell - one for character and one for attribute). But it can be done in 40-column mode. As the CRTC can only do 128 rows unattended, it requires the CPU to be involved (reprogramming the CRTC several times per frame on particular scanlines) but 8088 MPH and Area 5150 did such things.

I haven't published it yet as there are still some rough edges to clear up, but if you email me ([email protected]) I'll send you the current work-in-progress (the same one that nand2mario is working from).

It's not - the "DA" means that the write happens to the ES segment, while "DS" means that the read happens from the DS segment. The use of different segments for the source and destination gives these instructions extra flexibility.

Yes, it pops the stack value into the CS register. But it doesn't update the PC as well - it'll continue to point to the offset (in the old CS) of the instruction after the "POP CS". So whatever code is in the new CS, it has to be "compatible" with the code in the old CS, in terms of the instructions starting in the right place. However, it's even more complicated than that because the prefetch queue is not flushed, so the exact address where it switches over will be unpredictable. For certain very specific scenarios it could potentially be useful to speed up conditional execution by eliminating those prefetch queue flushes, though.

There's a POP instruction in the loop that pops to a memory location addressed by a register. When that register contains the address of the final JMP instruction, the latter gets overwritten by a forward JMP.

I used the DOSBox debugger to debug several of the effects during the making of Area 5150, but other than that I used real hardware. 86Box is probably the most accurate one at the moment but still isn't accurate enough to run the entire demo correctly.

Most of the tricks in the demo should work unmodified (or could be made to work) on most CGA implementations of the era. Some clones had slightly different font ROMs so it would look a little bit wrong on those. The final lake effect (and the ripply picture shortly before it) use very tight cycle counting so probably won't work on anything except a genuine IBM PC/XT and CGA. Some effects (like the radial fire effect and the voxel landscape) should work on just about anything. The Amstrad PC1512 will have trouble with any effect that modifies the CRTC timing registers as it doesn't have a fully programmable CRTC and always generates a 15.7kHz/59.92Hz 640x200 image. I don't have personal experience with the Tandy 1000 and don't know how compatible it is off the top of my head.

Yes, I'm reenigne from the demo. Feel free to contact me at [email protected].

I'm not sure what you're doing with the colour mixing on that page but I'm wondering if you're just applying a gamma curve to the mixed result. This is what I meant:

sRGB interpolation:

  r_final = r_1*x + r_2*(1-x)
  g_final = g_1*x + g_2*(1-x)
  b_final = b_1*x + b_2*(1-x)

linear RGB interpolation:

  r_final = 255*((((r_1/255)^2.2)*x + ((r_2/255)^2.2)*(1-x))^(1/2.2))
  g_final = 255*((((g_1/255)^2.2)*x + ((g_2/255)^2.2)*(1-x))^(1/2.2))
  b_final = 255*((((b_1/255)^2.2)*x + ((b_2/255)^2.2)*(1-x))^(1/2.2))

The real gamma correction formula is actually slightly more complicated than that because it's linear up until the sRGB value is about 10 then then follows a ^2.4 curve but the difference is too small to notice.

As I remember it, the term "XT class" was used back in the day to distinguish between "AT class" PCs and those prior - i.e. to mean an IBM PC, XT or comparable machine. So (weirdly enough) the 5150 was considered XT class despite predating the XT (they're almost identical as far as software is concerned anyway). The term "PC class" wasn't a thing because PC came to be shorthand for any IBM PC/XT/AT or later x86 machine. Some more powerful machines like the Amstrad PC1512 (with an 8MHz 8086) were also considered XT class - these machines could run games like Bruce Lee and Digger which were designed for the PC/XT (and which were too fast on AT and later machines), though the gameplay was quicker than they were designed for so were extra-challenging.

The C000 segment was used for the EGA/VGA extension ROM. I'm guessing that using D000-EFFF would be unnecessary (because of the planar addressing squeezing 256kB of video memory into a 64kB address space), inconvenient (because the addresses wouldn't be contiguous - EGA and VGA were designed to coexist with either CGA or monochrome adapters in B000-BFFF) and (for VGA) insufficient - you'd still not have enough to map the entire 256kB of VRAM linearly. I also expect that IBM's engineers didn't want to take up all the extension ROM space because then it wouldn't be possible to add EMS cards, network cards, and whatever else ended up being mapped there. Though 192kB of write-only video memory in that space would be an interesting design!

The programmatic transitions between still images use linear RGB space, which is the correct way to interpolate between two colours. The maths behind it is pretty simple - essentially just reversing the gamma correction from normal (0-255) sRGB space before the interpolation and redoing it for the final colour (no need to get into the hairy areas of LAB or perceptual colour spaces for this). Once we know the colour we want, we choose the character (from a list of 6) and attribute which most closely matches that colour. Of course, it's all done using lots of lookup tables so that we can process several hundred character cells per frame.

Thanks! The 3D hills are no less 3D than any other 3D scene displayed on a 2D monitor. It's a heightmap (of a real place) rendered with correct perspective.

Getting the samples to output at the correct time during the end titles required a painstaking amount of cycle-counting, measuring, and iteration.