SNES: Optimizing a data bank swap during interrupt

[Sarah Shinespark]

I'm back at it with Arcana again! The game got a FastROM patch for loading from faster banks, but every interrupt it reverts to SlowROM again until RTI. I wanna fix that since it's running 243 SlowROM commands every frame.

Code Select Expand

$00/8335 C2 30       REP #$30                A:0820 X:0A04 Y:003D D:1E00 DB:7E S:1FEA P:envmxdIzc HC:0110 VC:225 FC:57 I:00
$00/8337 48          PHA                     A:0820 X:0A04 Y:003D D:1E00 DB:7E S:1FEA P:envmxdIzc HC:0140 VC:225 FC:57 I:00
$00/8338 DA          PHX                     A:0820 X:0A04 Y:003D D:1E00 DB:7E S:1FE8 P:envmxdIzc HC:0178 VC:225 FC:57 I:00
$00/8339 5A          PHY                     A:0820 X:0A04 Y:003D D:1E00 DB:7E S:1FE6 P:envmxdIzc HC:0216 VC:225 FC:57 I:00
$00/833A 0B          PHD                     A:0820 X:0A04 Y:003D D:1E00 DB:7E S:1FE4 P:envmxdIzc HC:0254 VC:225 FC:57 I:00
$00/833B F4 00 00    PEA $0000               A:0820 X:0A04 Y:003D D:1E00 DB:7E S:1FE2 P:envmxdIzc HC:0292 VC:225 FC:57 I:00
$00/833E 2B          PLD                     A:0820 X:0A04 Y:003D D:1E00 DB:7E S:1FE0 P:envmxdIzc HC:0340 VC:225 FC:57 I:00
$00/833F 8B          PHB                     A:0820 X:0A04 Y:003D D:0000 DB:7E S:1FE2 P:envmxdIZc HC:0384 VC:225 FC:57 I:00

$00/8340 4B          PHK                     A:0820 X:0A04 Y:003D D:0000 DB:7E S:1FE1 P:envmxdIZc HC:0414 VC:225 FC:57 I:00
$00/8341 AB          PLB                     A:0820 X:0A04 Y:003D D:0000 DB:7E S:1FE0 P:envmxdIZc HC:0444 VC:225 FC:57 I:00

$00/8342 E2 20       SEP #$20                A:0820 X:0A04 Y:003D D:0000 DB:00 S:1FE1 P:envmxdIZc HC:0480 VC:225 FC:57 I:00
$00/8344 AD 10 42    LDA $4210  [$00:4210]   A:0820 X:0A04 Y:003D D:0000 DB:00 S:1FE1 P:envMxdIZc HC:0510 VC:225 FC:57 I:00
$00/8347 E6 20       INC $20    [$00:0020]   A:08C2 X:0A04 Y:003D D:0000 DB:00 S:1FE1 P:eNvMxdIzc HC:0588 VC:225 FC:57 I:00
$00/8349 C2 20       REP #$20                A:08C2 X:0A04 Y:003D D:0000 DB:00 S:1FE1 P:envMxdIzc HC:0634 VC:225 FC:57 I:00
$00/834B AD 55 10    LDA $1055  [$00:1055]   A:08C2 X:0A04 Y:003D D:0000 DB:00 S:1FE1 P:envmxdIzc HC:0664 VC:225 FC:57 I:00
$00/834E D0 03       BNE $03    [$8353]      A:0000 X:0A04 Y:003D D:0000 DB:00 S:1FE1 P:envmxdIZc HC:0712 VC:225 FC:57 I:00
$00/8350 20 79 83    JSR $8379  [$00:8379]   A:0000 X:0A04 Y:003D D:0000 DB:00 S:1FE1 P:envmxdIZc HC:0736 VC:225 FC:57 I:00
......
$00/8373 AB          PLB                     A:0000 X:FFFE Y:000F D:0000 DB:00 S:1FE1 P:envmxdIZC HC:0634 VC:235 FC:57 I:00
$00/8374 2B          PLD                     A:0000 X:FFFE Y:000F D:0000 DB:7E S:1FE2 P:envmxdIzC HC:0670 VC:235 FC:57 I:00
$00/8375 7A          PLY                     A:0000 X:FFFE Y:000F D:1E00 DB:7E S:1FE4 P:envmxdIzC HC:0714 VC:235 FC:57 I:00
$00/8376 FA          PLX                     A:0000 X:FFFE Y:003D D:1E00 DB:7E S:1FE6 P:envmxdIzC HC:0758 VC:235 FC:57 I:00
$00/8377 68          PLA                     A:0000 X:0A04 Y:003D D:1E00 DB:7E S:1FE8 P:envmxdIzC HC:0802 VC:235 FC:57 I:00
$00/8378 40          RTI                     A:0820 X:0A04 Y:003D D:1E00 DB:7E S:1FEA P:envmxdIzC HC:0846 VC:235 FC:57 I:00


It saves A, X, Y, D, B to the stack, loads $0000 into D (sets the direct page to 0000), and loads K the bank byte into B the bank register. Problem is, despite the FastROM patch setting the ROM type to FastROM, the interrupt jumps to bank 00 instead of bank 80. So I want to set the bank to $80, figure I can put something at 8340 where I spaced it out. But, there's only 2 bytes there, so I need to jump somewhere else and take some of the following code with me.

My first thought, I might be over-complicating things. Do naturally fastrom games stay in FastROM during interrupts? Is there somewhere that handles the interrupt vector and sets the bank?

Second thought, from reading 65816 Basics I can only change the bank (K) by jumping long. So I jump to $80/Fxxx, PHK, PLB, start appending the overwritten code... and then what? If I return to run the rest, I'll be back to SlowROM. I could copy the entire subroutine to the end of the bank and jump there, but that seems silly. Any brighter ideas or something I missed? Trying to reduce the commands needed since this code's really prevalent.

[slidelljohn]

At 0xffea it has 35 83, change that to 80 ff, at 0xff80 put 5c 35 83 80 and you should be good.

[Sarah Shinespark]

I knew it, I knew I was overthinking things! You're the best 

[KingMike]

Yes, the interrupt routines are always bank 0.