[Coco] Video band width and accessing video memory
zambotti at iinet.net.au
Tue Jun 4 00:08:21 EDT 2019
>But it *is* feasible to do a 160x200 mode in 256 colors.
Yes but Mr X of the Hidden 256 Color Mode myth stated:
It was 320 x res by 200 y res by 256 cols.
Which means 320bytes_per_line * 200lines = 64000 bytes.
Halve that if video can access 2 bytes in one cycle equals 32000.
Divide that figure into 1788000 and you get 55.875.
So that’s 55 frames per second max you should expect, which is not 60.
Even if Mr. X was mistaken regarding Y being 200 when recalculated at 192 Y the max frames per second is 58. And again that is not 60.
So unless the system can tolerate a vsync of 0.0172ms instead of the expected 0.0167ms you are not going to get a lot of video output.
To get 60 fps with a 320pixel wide 256 col mode the Y must be restricted to 186 lines and the CoCo doesn't have such a mode.
So with the memory bandwidth limitations I doubt this hidden mode exists as the designers would have known these timing limitations before they started.
From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of Dave Philipsen
Sent: Monday, 3 June 2019 10:36 PM
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Subject: Re: [Coco] Video band width and accessing video memory
But it *is* feasible to do a 160x200 mode in 256 colors.
> On Jun 2, 2019, at 11:18 PM, Walter Zambotti <zambotti at iinet.net.au> wrote:
> Very interesting!
> That’s what I expected.
> The video hardware must access 2 bytes per cycle (or on one edge of the cycle).
> And the video frequency must always be 1788000 regardless of CPU speed.
> And if that is the case then the maximum accessible bytes would be double 29800 or 59600.
> Now if you divide that figure by:
> 320 horizontal pixels of 4 bits (160 bytes) then the max theoretical vertical resolution is 372.
> 320 horizontal pixels of 8 bits (320 bytes) then the max theoretical vertical resolution is 186.
> And 186 does not match any known CoCo vertical resolution.
> So a hidden CoCo 3 graphics modes that allows 256 colors must simply not exist as there is insufficient bandwidth to display 320x200x8bit. As I calculated the max height could only be 186(.25).
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