64k ought to be enough - a demo for the PCjr

[riq]

Hi,

We (PVM) have just released a demo for the PCjr (works even on the 64K-RAM only version[*]):

video: https://www.youtube.com/watch?v=uakDpJns9LA
(thanks monzamess for the capture).

Binary download:
http://pungas.space/pvm-64ko.zip (only run on real hardware, no emulators)

source code here:
https://gitlab.com/ricardoquesada/pcjr-flashparty-2018

and here is high-level documentation that explains how the different tricks were done:
https://retro.moe/2018/09/26/64k-ram-ou ... -the-pcjr/

I hope you like it.

[*]: there is bug displaying 32k RAM video modes when the PCjr has only 64k RAM.

[Trixter]

This is a wonderful demo and an even more wonderful technical write-up. I'm very pleased! You even managed to squeeze in 640x200x4.

320x200x16 doesn't work in a 64K PCjr because there isn't enough memory bandwidth. I'm guessing you found this out the hard way :) so while the demo technically works on a 64K PCjr, it only works correctly on a 128KB PCjr. But that is the most common stock config so that's cool.

I'll try to do a decent capture of it this weekend.

[riq]

This is a wonderful demo and an even more wonderful technical write-up. I'm very pleased! You even managed to squeeze in 640x200x4.

Thanks.

320x200x16 doesn't work in a 64K PCjr because there isn't enough memory bandwidth.

Do you know why is this happening? Since the demo, even with 128KRAM, never uses the upper 64K RAM. Why is there a bandwidth issue? Does the 64k RAM extension offer something else besides RAM?

I'll try to do a decent capture of it this weekend.

Much appreciated.

[Trixter]

Do you know why is this happening? Since the demo, even with 128KRAM, never uses the upper 64K RAM. Why is there a bandwidth issue? Does the 64k RAM extension offer something else besides RAM?

I'm paraphrasing from the tech ref, but the VGA chip accesses both 64k banks in parallel to enable the high bandwidth needed for 320x200x16.

I'm assuming you got doubled pixels when you tried to do 320x200x16 with only 64K? Or was it corrupt in some other way?

[riq]

I'm paraphrasing from the tech ref, but the VGA chip accesses both 64k banks in parallel to enable the high bandwidth needed for 320x200x16.

Ahhhhhh...
From Tech ref:

The system board 64K RAM is mapped at the bottom
of the 1Mb address space. The system board 64K
RAM is mapped to the next 64K bytes of address space
if the 64kb Memory and Display Expansion option is
not installed. If read or written to, this higher block of
address space will look just like the low-order 64k-byte
block. This means the bottom 128K bytes of address
space is always reserved for RAM.

...

When inserted, the memory expansion option uses the
ODD memory space, while the system memory is
decoded as the EVEN memory. Thus, when used as
video memory, the memory expansion option has the
video attributes while the on-board system memory has
the video characters. This arrangement provides a
higher bandwidth of video characters.

That explains a lot of things.

[Trixter]

I am working on trying to capture this tonight via RGBI.

[Trixter]

The RGBI -> analog RGB portion of my capture chain broke. Exploring replacements. Sorry, this might be another week or two.

[riq]

The RGBI -> analog RGB portion of my capture chain broke. Exploring replacements. Sorry, this might be another week or two.

Thanks!

Would you mind sharing what devices are you using for the capture? I want to have my own chain as well... Thanks.

[Trixter]

CGA -> splitter cable (so I can hook up a monitor) -> gglabs CGA2RGB -> VGA gender changer -> VGA cable -> VGA to DVI adapter -> Epiphan DVI2PCIe with CGA video defined as a custom VGA mode.

I then run it through an avisynth software filter to snap noise to colors, and perform resizing and saving into something I can bring into Premiere.

[monzamess]

CGA -> splitter cable (so I can hook up a monitor) -> gglabs CGA2RGB -> VGA gender changer -> VGA cable -> VGA to DVI adapter -> Epiphan DVI2PCIe with CGA video defined as a custom VGA mode.

Which piece converts analog CGA to VGA? Or do you even need that in this chain?