Well I know I said I wasn't going to do any more high score mods for a while. That was until I picked up a Vanguard machine a couple of weeks ago, and just last week got talking to someone else who'd also just got one and we were both having a moan about the lack of a high score save function.
Vanguard, for people who don't know the game, is probably best described as a bit similar to Scramble. Although it has horrible collision detection and firing, and plenty of bugs, I think it's awesome, love the game and have fond memories of playing it when I was a kid. But whilst it has a 7-position high score table, it doesn't save it on power-down, and lacks any kind of free play mode.
It also runs on 6502 hardware, and I've probably only programmed a couple of hundred bytes of 6502 code in my life. But I wanted to get the additional functions on my game, so armed with "Programming the 6502" book by Rodnay Zaks, and a couple of excellent online resources, I set out to learn some of the workings of the game to enable me to write the mods.
The high score save was actually easiest. The game uses a 1Kx8 4118 (socketed!) for programme memory which needs to be swapped out for a Dallas DS1220 module, then I just needed to bypass the code that clears the high score memory on startup. Add some code that does clear the high score memory when you hold down certain controls on power-up and that was it done.
The enhanced free play mod (maintains the attract demo whilst in free play mode) was more tricky. There is an unused DIP switch that I've used to set freeplay mode on and off. Vanguard relies on an NMI being generated when a coin is entered, so the programme doesn't have to keep checking for coins in software. As my code preloads the game with the right number of credits depending on P1 or P2 start buttons being pressed, I had to write a number of checks for the buttons being pressed during the demo mode. I then changed the code that writes the number of coins inserted onto the screen, and have it shown only in non-freeplay mode.
Finally when in freeplay mode, I didn't want coins being registered, so had to intercept the NMI routine to not accept coins unless in normal (paid) mode.
Happy to say I got all this working today, programmed and tested it in Mame, and extensively tested it on real hardware, I think all the bugs are out.
The code needs just 3 of the game ROMs changing, and a Dallas NVRAM module to save the high scores.
Obligatory pictures below, thanks as usual for everyone who appears in the high score table
Feature summary:-
High score table saved on power-down
Default high score table by holding down certain controls on power-up
Enhanced freeplay (maintains attract mode) selectable using unused DIP switch
Coins only accepted in paid mode
Coins only displayed on screen in paid mode
Correctly handles continue function in both paid and freeplay modes
Reduced the high score table scores from 10000 to 5000 to make it easier to get a high score!
Attract screen (freeplay mode, note no coin count displayed)
Attract screen (paid mode - coins displayed)
High score table (saved to NVRAM)
On real hardware
Modifications to the board
Vanguard, for people who don't know the game, is probably best described as a bit similar to Scramble. Although it has horrible collision detection and firing, and plenty of bugs, I think it's awesome, love the game and have fond memories of playing it when I was a kid. But whilst it has a 7-position high score table, it doesn't save it on power-down, and lacks any kind of free play mode.
It also runs on 6502 hardware, and I've probably only programmed a couple of hundred bytes of 6502 code in my life. But I wanted to get the additional functions on my game, so armed with "Programming the 6502" book by Rodnay Zaks, and a couple of excellent online resources, I set out to learn some of the workings of the game to enable me to write the mods.
The high score save was actually easiest. The game uses a 1Kx8 4118 (socketed!) for programme memory which needs to be swapped out for a Dallas DS1220 module, then I just needed to bypass the code that clears the high score memory on startup. Add some code that does clear the high score memory when you hold down certain controls on power-up and that was it done.
The enhanced free play mod (maintains the attract demo whilst in free play mode) was more tricky. There is an unused DIP switch that I've used to set freeplay mode on and off. Vanguard relies on an NMI being generated when a coin is entered, so the programme doesn't have to keep checking for coins in software. As my code preloads the game with the right number of credits depending on P1 or P2 start buttons being pressed, I had to write a number of checks for the buttons being pressed during the demo mode. I then changed the code that writes the number of coins inserted onto the screen, and have it shown only in non-freeplay mode.
Finally when in freeplay mode, I didn't want coins being registered, so had to intercept the NMI routine to not accept coins unless in normal (paid) mode.
Happy to say I got all this working today, programmed and tested it in Mame, and extensively tested it on real hardware, I think all the bugs are out.
The code needs just 3 of the game ROMs changing, and a Dallas NVRAM module to save the high scores.
Obligatory pictures below, thanks as usual for everyone who appears in the high score table
Feature summary:-
High score table saved on power-down
Default high score table by holding down certain controls on power-up
Enhanced freeplay (maintains attract mode) selectable using unused DIP switch
Coins only accepted in paid mode
Coins only displayed on screen in paid mode
Correctly handles continue function in both paid and freeplay modes
Reduced the high score table scores from 10000 to 5000 to make it easier to get a high score!
Attract screen (freeplay mode, note no coin count displayed)
Attract screen (paid mode - coins displayed)
High score table (saved to NVRAM)
On real hardware
Modifications to the board
