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TSM
Registered: Jan 2007
Posts: 42 |
Hires bitmap RAM arrangement
I wanted to make a little program to display the "Doodle" picture file from this page:
http://www.editorix.org/congo/html/c64_image_formats__part_i.ht..
It loads at $1C00, so you have screen data at $1C00 and bitmap data at $2000. I set everything up very carefully (screen mode, base VIC address, screen RAM and bitmap RAM addresses) and the bitmap is correct but the colors are all messed up. If I move the screen data to $0C00 and change the address accordingly in $D018, then it works.
I wanted to display the picture without moving any data around; is this possible at all?
Strangely, if I save a snapshot with Vice at the messed-up screen and feed the file to "vicegrab", it will spit up a perfectly working .prg file (although with screen RAM @ $0C00)! |
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TSM
Registered: Jan 2007
Posts: 42 |
Thanks Graham, I did just that. Then, just the base address for the VIC had to be updated. |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Krill: But doesn't the VIC just see the RAM at $0000-$0fff when looking at $1000-$1fff in the Ultimax memory configuration, since there is only 4K of RAM available? Thanks for pointing out this very important and relevant fact, btw. :) |
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Oswald
Registered: Apr 2002
Posts: 5094 |
more cool facts: VICII will also display the contents of $0000 and $0001 instead of the CPU port. ;) there were also some wild speculations in a c= hacking (iirc) on how to write there. (have VICII read the data to be written while cpu sets bus to write) |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
nothing to speculate there... i have written a little test program recently that does just that, check this :) |
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Oswald
Registered: Apr 2002
Posts: 5094 |
its hard to see what it does (counts upwards, screen is at $0000?) so you have prooved this writing method works? and vice even emulates it? weird it does, the perequisites sound like they are only needed for this scenario. |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
vice emulates it for quite some time :) its not very hard to do either, the VIC fetches from $3fff in border area. so all you need to do is wait for border, write the value you want to go to $00 to $3fff, and write (whatever) value to $00. same for $01 respectively. check readme and source here |
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Krill
Registered: Apr 2002
Posts: 2980 |
Quote: Krill: But doesn't the VIC just see the RAM at $0000-$0fff when looking at $1000-$1fff in the Ultimax memory configuration, since there is only 4K of RAM available? Thanks for pointing out this very important and relevant fact, btw. :)
In Ultimax mode, the CPU can only access the lowmost 4 kB of internal RAM, but VIC sees internal RAM beyond that, including the $1000..$1fff and $9000..$9fff ranges.
(However, it reads external memory at $3000..$3fff of each 16 kB VIC bank, and can even be forced to only see external memory, but that is another story.)
You can confirm this by putting graphics in the usually non-displayable memory range as the OP did, and then freeze using a freezer cartridge. When freezing, it will briefly switch into Ultimax mode, revealing the graphics normally hidden underneath the character generator ROM.
Action Replay and similar cartridges allow you to programmatically enable the Ultimax mode. With some code below $1000, disable interrupts and write #$03 to $de00, and the graphics are revealed permanently.
Sources:
http://skoe.de/docs/c64-dissected/pla/c64_pla_dissected_a4ss.pdf
http://codebase64.org/doku.php?id=base:memory_management
http://ar.c64.org/wiki/Hiding_kilobytes_C%3DHacking_Issue_7.txt
http://www.floodgap.com/retrobits/ckb/secret/ultimax.html
http://ar.c64.org/wiki/Action_Replay |
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Oswald
Registered: Apr 2002
Posts: 5094 |
Quote: vice emulates it for quite some time :) its not very hard to do either, the VIC fetches from $3fff in border area. so all you need to do is wait for border, write the value you want to go to $00 to $3fff, and write (whatever) value to $00. same for $01 respectively. check readme and source here
So VICII will leave the data on the bus it has read, and the CPU will drive the R/W line to write, but not change bus contents since it pushes the data internally, correct ?
what other stuff may have required VICE to emulate what happens to the memory on such a low level ? |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: So VICII will leave the data on the bus it has read, and the CPU will drive the R/W line to write, but not change bus contents since it pushes the data internally, correct ?
what other stuff may have required VICE to emulate what happens to the memory on such a low level ?
A similar "feature" is the last OP-code read will become d8xx-color in the FLI-bug area etc. You simply need to emulate the memory bus behavior correctly and not just set a byte in some array. |
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Oswald
Registered: Apr 2002
Posts: 5094 |
thanks, that explains. |
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