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Krill
Registered: Apr 2002
Posts: 2839 |
Alt-history no-cost design changes with great value
Which things in the C-64 could have been implemented or connected differently without conceivable extra cost, for coding advantages?
Thinking of things like shuffling the chip register bits like VIC's $d011 and $d016 differently (such that some effects can be achieved with fewer register writes or less twiddling).
Or putting some IO register to $01 (and move the memory configuration somewhere else, somehow).
Maybe also having different PLA memory configurations (not necessarily more).
Or connecting external signals to the CIA port pins in a different order.
Discuss! =) |
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ws
Registered: Apr 2012
Posts: 228 |
If, let's say, the screen ram adress would be set, and always the color ram adress would accordingly be set to a mirrored/inverted high adress of that value, like for example:
set screen ram to $0400, color ram would be at $8400,
or if you set screen ram to $8400, color ram would be at $0400 (explanation to make the "mirror" point clear here), thus being able to leave the color ram chips away, wouldn't that actually have reduced cost?
sacrificing just 1000 bytes - while actually, with screen color being read from normal ram, both nybbles could have been used? And even if you didn't change the bus so all 8 bits would be read (i understand that would be necessary?) - why did they add actual ram for the colors in the first place? Just to add 1000 bytes? |
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tlr
Registered: Sep 2003
Posts: 1714 |
Quote: If, let's say, the screen ram adress would be set, and always the color ram adress would accordingly be set to a mirrored/inverted high adress of that value, like for example:
set screen ram to $0400, color ram would be at $8400,
or if you set screen ram to $8400, color ram would be at $0400 (explanation to make the "mirror" point clear here), thus being able to leave the color ram chips away, wouldn't that actually have reduced cost?
sacrificing just 1000 bytes - while actually, with screen color being read from normal ram, both nybbles could have been used? And even if you didn't change the bus so all 8 bits would be read (i understand that would be necessary?) - why did they add actual ram for the colors in the first place? Just to add 1000 bytes?
The reason is memory bandwidth. The color ram has a separate 4-bit data bus. If you were to read the color data from the main ram instead, then you'd need to find 40 more cycles in every bad line. |
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ws
Registered: Apr 2012
Posts: 228 |
@tlr: ah, i understand. thank you for explaining! |
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Oswald
Registered: Apr 2002
Posts: 5017 |
one more pla and one more color ram chip could solve the problem, anyway I dont think we would get much boost from double bufferable color ram. the best scrolling games which milk every performance there is are good enough. |
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tlr
Registered: Sep 2003
Posts: 1714 |
For the record: the plus4 has color data fetched from the main ram. This is solved by having two bad lines each char row. I guess you wouldn't want that for the c64? :) |
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cadaver
Registered: Feb 2002
Posts: 1153 |
Oswald: Right, double buffer still means you lose the CPU cycles, and in most cases you can race the raster or split the update in halves. Selective non-update FTW :) |
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Jammer
Registered: Nov 2002
Posts: 1289 |
Quoting cadaverOswald: Right, double buffer still means you lose the CPU cycles, and in most cases you can race the raster or split the update in halves. Selective non-update FTW :)
Yeah. Too bad we can't do anything like this with dedicated sample channel which would handle playback timing internally ;) |
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ChristopherJam
Registered: Aug 2004
Posts: 1377 |
VSP fix... and a bit that lets you put the sprite pointers at (eg) end of bank regardless of where the screen is, so that AGSP routines don't scroll the sprite pointers onto the visible area. |
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chatGPZ
Registered: Dec 2001
Posts: 11107 |
Regarding sprite pointers i always wondered how they ended up in regular RAM - you'd think those are registers. |
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tlr
Registered: Sep 2003
Posts: 1714 |
Quote: Regarding sprite pointers i always wondered how they ended up in regular RAM - you'd think those are registers.
I guess that would have taken up more die space. They are only needed when the sprite data is fetched, and there are 4 read slots for each sprite anyway. Packing it to 3 slots/sprite would require more logic too.
It's a good thing the pointers can be switched with single writes, we should all be happy! |
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