Adventures in NES Bank Switching: Episode 4 (Hokuto no Ken 3)

[BlackPaladin]

I may have hit a roadblock in my attempt at translating Hokuto no Ken 3 NES when it comes to implementing DTE.  I'll explain what I've done so far.

First off, using the debugger, I looked for the "sweet spot" when it comes to the game's text. ("B1 A2" in 0x3D8E7).  Where 0x3D8E5-0x3D8E8 is ("A0 00 B1 A2"), added "20 7B FF EA" (JSR $FF76 / NOP) and pasted "A0 00 B1 A2" onto a small area of unused nearby space (0x3FF86) with a "60" (RTS) right after.  I tested it out, and that seems to work fine currently. 

But here's the thing, this space doesn't have enough space for DTE code and/or accompanying DTE table.  However, there is unused space in $2FC5D-$2FFFF.  Because of this, I had to do bank switching.  After looking up some information at nesdev on Mapper 1 (the mapper this game uses) and help from a couple of people on Discord, the register for text may have been discovered.  Here's the code from the debugger...

Spoiler

  03FFDF: A2 0E     LDX #$0E
  03FFE1: A9 FF     LDA #$FF
  03FFE3: 8D 51 04  STA $0451 = #$00
  03FFE6: 8A        TXA
  03FFE7: 8D FF FF  STA $FFFF = #$C9
  03FFEA: 4A        LSR
  03FFEB: 8D FF FF  STA $FFFF = #$C9
  03FFEE: 4A        LSR
  03FFEF: 8D FF FF  STA $FFFF = #$C9
  03FFF2: 4A        LSR
  03FFF3: 8D FF FF  STA $FFFF = #$C9
  03FFF6: 4A        LSR
  03FFF7: 8D FF FF  STA $FFFF = #$C9
  03FFFA: A9 00     LDA #$00
  03FFFC: 8D 51 04  STA $0451 = #$00
  03FFFF: 60        RTS -----------------------------------------

[close]

Well, using this information, I also looked up what code to use when it comes to bank swapping.  Here's the code I came up with (with help from a couple of people on Discord). This was pasted over 0x3FF86. (Where "A0 00 B1 A2 60" is)  Here's how I interpreted how this code is supposed to work...

Code Select Expand

  03FF86: A2 0B     LDX #$0B   <- Loads Bank 0B
  03FF88: 8A        TXA        <- Moves X Register to A
  03FF89: 20 E3 FF  JSR $FFE3  <- Jumps to Bank Switching subroutine
  03FF8C: 4C 5F FC  JMP $FC5F  <- Jumps to $2FC5F in Bank 0B
  03FF8F: A2 0F     LDX #$0F   <- Loads Bank 0F,
  03FF91: 8A        TXA        <- Moves X Register to A
  03FF95: 60        RTS        <- Ends Routine
I also pasted "20 7B FF EA 60" (JSR $FF76 / NOP / RTS) onto 0x2FC5F.  I had figured if I had done this, I can use the free space in $2FC5D-$2FFFF so that I can repurpose the DTE code I had used in Last Armageddon into this game as well as add an accompanying DTE table.  Anyway, when I put this into practice, this is what happened...



Yeah, not what I had expected. 

I don't know what went wrong here.  I thought this worked.  Can someone help me with what's going wrong?  (Thanks to leina and Gyroballer for helping out and getting me to this point so far.)

[Cyneprepou4uk]

Code Select Expand

03FF8C: 4C 5F FC  JMP $FC5F  <- Jumps to $2FC5F in Bank 0B
Unless your mapper is in 32k PRG mode (which is unlikely), you're jumping to the last fixed bank, not to bank 0B.

In 16k PRG mode you can swap 8000-BFFF area only.

[BlackPaladin]

Code Select Expand

03FF8C: 4C 5F FC  JMP $FC5F  <- Jumps to $2FC5F in Bank 0B
Unless your mapper is in 32k PRG mode (which is unlikely), you're jumping to the last fixed bank, not to bank 0B.

In 16k PRG mode you can swap 8000-BFFF area only.

So this means that this game has the following information in the mapper...

PRG ROM: 16 x 16KiB = 256 KiB
 CHR ROM: 0 x  8KiB = 0 KiB
 ROM CRC32: 0x4e7cad28
 ROM MD5:  0x39b549ef5d37dd661c0aeddca2b84c9f
 Mapper #: 1
 Mapper name: MMC1
 Mirroring: Horizontal
 Battery-backed: Yes
 Trained: No
 NES2.0 Extensions
 Sub Mapper #: 0
 Total WRAM size: 8 KiB
 Total VRAM size: 8 KiB
 WRAM backed by battery: 8 KiB
 VRAM backed by battery: 0 KiB

There's no way I can bank swap to $2FC5F?  Does it mean if I wanted to bank swap, it has to be in $8000-$BFFF, $18000-1BFFF, $28000-$2BFFF, $38000-3BFFF, etc.?

[Cyneprepou4uk]

Seems that you still don't understand what PRG bankswitching is all about.

Basically, CPU can access 0000-FFFF range via memory bus, and not more than that. 8000-FFFF is used for PRG, which is 32k, and CPU can access PRG within that range only (some mappers allow 6000-7FFF as well). However, games can have larger PRG than 32k, and by default CPU cannot access 0x2FC5F or whatever. So by using mappers you can connect different PRG banks to memory bus via their registers.

For example, your game allows you to connect 16k banks to 8000-BFFF range. If you connect bank 00 and execute JMP $8000, you will be jumping to 0x00010. If you connect bank 01 and execute the same JMP $8000, this time you will be jumping to 0x04010.

Here you can see how after the last write to FFFF register, memory at 8000-BFFF is changing to a different one in Hex Editor. I can "Go Here In ROM File" to see which PRG bank is connected at the moment. At the same time it's still 8000-BFFF for CPU, which means that your instructions must operate with 8000-BFFF range in order to do something with connected bank.

Also, your game's mapper configuration connects last 16k of PRG to C000-FFFF, and you can't swap it to another bank. It's called a fixed bank. So your JMP $FC5F will always jump to 0x3FC6F despite what is loaded into 8000-BFFF.

A fixed bank is used as a transitional place between bankswitching, because bad things will happen if you execute bankswitching code while being inside that bankswitching area (unless you know what you're doing). Also it contains common code for other banks in order for them to have a constant access to (math calculations for example). And of course it has interrupt vectors and their handlers. And sometimes DPCM music data.

[BlackPaladin]

Seems that you still don't understand what PRG bankswitching is all about.

Basically, CPU can access 0000-FFFF range via memory bus, and not more than that. 8000-FFFF is used for PRG, which is 32k, and CPU can access PRG within that range only (some mappers allow 6000-7FFF as well). However, games can have larger PRG than 32k, and by default CPU cannot access 0x2FC5F or whatever. So by using mappers you can connect different PRG banks to memory bus via their registers.

For example, your game allows you to connect 16k banks to 8000-BFFF range. If you connect bank 00 and execute JMP $8000, you will be jumping to 0x00010. If you connect bank 01 and execute the same JMP $8000, this time you will be jumping to 0x04010.

Here you can see how after the last write to FFFF register, memory at 8000-BFFF is changing to a different one in Hex Editor. I can "Go Here In ROM File" to see which PRG bank is connected at the moment. At the same time it's still 8000-BFFF for CPU, which means that your instructions must operate with 8000-BFFF range in order to do something with connected bank.

Also, your game's mapper configuration connects last 16k of PRG to C000-FFFF, and you can't swap it to another bank. It's called a fixed bank. So your JMP $FC5F will always jump to 0x3FC6C despite what is loaded into 8000-BFFF.

A fixed bank is used as a transitional place between bankswitching, because bad things will happen if you execute bank switching code while being in that bankswitching area (unless you know what you're doing). Also it contains common code for other banks in order for them to have a constant access to (math calculations for example). And of course it has interrupt vectors and their handlers. And sometimes DPCM music data.

I guess this means I cannot use the same DTE hack I used in Last Armageddon in this game as the sweet spot is in a fixed bank and cannot be moved and bank-switched out of it.

That's unfortunate.  I thought I could add DTE, but it looks like using KingMike's documentation on the subject (which I used for LA) cannot be used here.  *sigh*  Disappointed...

[Cyneprepou4uk]

Looks doable to me. I saw your screenshot in Discord



You read a byte and make a decision on whether DTE needs to be used. If yes, you swap bank and do your stuff, then swap back to read next byte. 900 free bytes in bank 0B should be enough.

Based on the screenshot, code which is reading text is located in a fixed bank, which means that you will have access to it from bank 0B if necessary. I don't know what are you complaining about.

[BlackPaladin]

Now, I'm confused.  Can you contact me on Discord?