Can you spot the difference?
It was pretty easy to add the memory since we still have some internal ram available. Just added 8k ram to the entity when the vic20 is accessing blk1 or blk2.
Found a few minutes today to have a look at the cartridge system. As it was implemented the vic didn’t start the rom when doing a reset. As found in the documentation linked in the previous post the vic expects some magic bytes at a specific address to autostart a cartridge:
$A000 Low-byte of a 16-bit "hard start" address (to begin at)
$A001 High-byte of the 16-bit "hard start" address
$A002 Low-byte of 16-bit "warm start" address (restore key)
$A003 High-byte of 16-bit "warm start" address
Address______Hex value_____Decimal value____ASCII values
$A004 $41 65 Capital "A"
$A005 $30 48 Digit "zero"
$A006 $C3 195 Reverse "C" character
$A007 $C2 194 Reverse "B" character
$A008 $CD 205 Reverse "M" character
It looks like that those dumped roms (.prg) have two extra bytes in the beginning. Shifting the access to those roms two bytes up made it work:
As usual the source code can be found in my github repository.
Looks like that there’re different dumps available. Found a side with .a0 cartridges which doesn’t need the shift.
Recently I tried to implement the ram expansion for the Apple II+ but failed again. The test suite didn’t recognize the additional ram or the additional memory access interfered with the disc access which lays in the sdram as well.
A little disappointed I was in need of a simple new project for my little free time. The vic20 was the obvious choice as it has low demand on memory and should be easy to port. But I was wrong.
The first difficulty was to find the right sources. Unfortunately there were no sources on the fpgaarcade site where it derives from. An Internet search revealed several places with several different implementations of the same original sources.
I started with the Papilio port but didn’t succeed. I got a screen output but only with a lot of “@”s.
Even though the sources were almost identically I got more success with the de2 sources. I used BRAM instead of SRAM and the vic20 on the MiST was alive:
But there’s still a lot do to to make it usable. The refresh rate is a little odd and needs a little tuning.
An internal test cartridge and one uploaded via the MiST osd didn’t start.
Due to this the sound output isn’t tested. The vic20 is only unexpanded and needs some memory expansions.
And finally the boot time is rather long and I dunno what’s doing there (testing ram?).
Fortunately I found some resources on the net which might help me to got those topics done:
The sources can be found here: