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TWW
Registered: Jul 2009
Posts: 545 |
Cartridge - C64 Game System, System 3 Memory Management
Does these cartridges allow memory management in terms of normal ROM/RAM/IO? I.E. Can I use the RAM under $a000-$bfff, $d000-$dfff and $e000-$efff?
Reason I ask i coz I made a frame for this cartridge in CRT format and it loads and executes fine. However I am only able to access ROM and IO.
http://vice-emu.sourceforge.net/vice_15.html#SEC319 |
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TWW
Registered: Jul 2009
Posts: 545 |
Quote: Quoting TWWYes, strangely enough I initially did set the DDR for the MPU. However when I did, the cartridge ROM would not appear at all.
Maybe you just set the DDR before $01? The correct sequence is:LDA #$37
STA $01
LDA #$2F
STA $00
When doing cartridge setup you really should have thoroughly examined the kernal startup code first so you know what's originally there.
Well to be fair it writes #$e7 ;-)
Not sure why it would write to the port prior to setting the DDR but I guess it's like aligning the valves before opening the manifoil (metafor intended).
And yes I see no that I was too caught up in the GAME/EXROM lines to actually see that there is no memory configuration allowing what I wanted (All RAM + CART @ $8000 + IO).
Nevertheless this only means (practically) that one can not access the RAM under LO/HIROM or IO from CARTROM.
Thanks for the guidance to all + other input. |
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tlr
Registered: Sep 2003
Posts: 1790 |
Quoting TWWWell to be fair it writes #$e7 ;-)
True, although the effect is the same.
Quoting TWWNot sure why it would write to the port prior to setting the DDR but I guess it's like aligning the valves before opening the manifoil (metafor intended).
Yes, something like that.
The way it works is that upon reset all bits in $00 are set to inputs. The pullups inside the 6502 will then keep the three lower bits effectively high (=$x7). When you set the correct DDR value the contents of $01 will start to affect those bits. If they haven't been set to the desired configuration, boom! |
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