Fixed PSG drum phasing issue + moved SCC player state to 0xee00

The PSG kick/snare drum transient could sometime occur during the 'off'' phase of the oscillator, which would cause the transient to not be there, or to lose its bite. We fix this by changing the frequency of those drums to '0' so that the oscillator is permanently in the 'on 'state. (note: for the snares this only applies to the initial 'click' part of the sound, not the noise part)
I managed to find some memory for the SCC player's state in 0xee00-0xf100. This is where Vampire Killer normally stores the stack, so I moved the stack to 0xfaf0 which is an unused area of memory (normally used by MSX BASIC). This way we no longer need to change page 0 to RAM, which allows us to remove a lot of complicated slot manipulation code and potential compatibility issues.

notes.txt

@@ -1,11 +1,11 @@
 TODO:
-- Switch to another ROM that has 'stage select'?
-  - Reverse engineer and reapply turbofix, sprite flickering fix, jump button fix, R800 enable
 - Re-work all music with proper speakers
-- drums sound distorted in R800 mode. a lot better in Z80 although not perfect
-  maybe the player is too fast somehow and it misses a transient?
-- SCC doesn't sound loud enough and/or SFX sound too loud (on my TurboR)
-- game over music should have sharper instrument
+    - heart of fire - lead instrument should be louder
+    - stage clear theme - not bright enough
+      needs to be higher octave
+    - out of time need brighter lead
+      (one at the start is ok)
+    - ending music needs brighter instruments
 
 
 turbofix patch (19 dec 2017)
@@ -19,6 +19,46 @@ sidenote: the turbofix patch causes graphics corruption on the ending
 titles (when you complete the game)
 
 
+kick drum fix notes
+-------------------
+The Konami PSG kick and snare rely on a click noise generated by a PSG envelope.
+The snare consists of a click, followed by noise. The bass drum is just a click
+(a slightly longer one). The problem is that if the oscillator is in the 'off'
+state at the start of the envelope, you don't hear the initial 'click'.
+This means that the sound of the kick/snare is not consistent.
+The fix is to set the oscillator frequency to '0', which makes it output
+a solid '1'. This way the sound of the click is consistent.
+
+SCC player reverse engineering notes:
+
+229c <- contains bit of PSG register that need to be set
+229d/e/f <- contains the values of PSG registers 11, 12, 13
+
+
+6b51 <- where the jump table is used
+6b5d <- jump table of addresses for drum set routines???
+6bed <- kick drum routine
+
+
+PSG frequency eg. register 0, 1 values are stored in IX+a/b, e.g. 200A, 200B
+
+
+66EE <- frequency is set here
+if I NOP this routine out and force the frequency to 0000 then the kick/snare are consistent
+but we lose the tonal part of some of the drums
+
+the frequency that 66EE sets comes from  IX+10/11
+(these are basically copied from 10/11 -> a/b)
+
+6509 <- 10/11 frequency is set here
+  for the kick drum, HL = 7485?
+  kick drum settings are at 7484:
+  7484 = 0A envelope frequency
+  7485 = 94 flags + frequency (high bits)
+  7486 = 50 frequency low bits
+
+
+
 music corruption when changing songs
 ------------------------------------
 when changing songs, the music must first be stopped using command 00

patch.py

@@ -31,7 +31,17 @@ def patch_mapper(rom):
             rom[offset + 2] += 0x10
 
 PATCH_IGNORE_LIST = [0x20daa, 0x23340, 0x20beb]
-CHANNEL_OFFSET = -0xc0  # 0e00h -> 02000h
+#CHANNEL_OFFSET = -0xc000  # 0e00h -> 02000h
+CHANNEL_OFFSET = +0x0e00  # 0e00h -> 0ee00h
+
+
+def offset_address(rom, index, offset):
+    """Modify an address in the ROM by a certain amount"""
+    addr = rom[index] + rom[index + 1] * 256
+    addr += offset
+    rom[index] = addr & 255
+    rom[index + 1] = addr >> 8
+
 
 def patch_music_channel_locations(rom):
     """Patch scc player to move channel data locations"""
@@ -41,10 +51,10 @@ def patch_music_channel_locations(rom):
         if (rom[offset] == 0xdd and
             rom[offset + 1] == 0x21 and
             (rom[offset + 3] & 0xfc) == 0xe0):
-            rom[offset + 3] += CHANNEL_OFFSET
+            offset_address(rom, offset + 2, CHANNEL_OFFSET)
         if (rom[offset] in [0x01, 0x11, 0x21, 0x32, 0x3a, 0x22, 0x2a] and
             (rom[offset + 2] & 0xfc) == 0xe0):
-            rom[offset + 2] += CHANNEL_OFFSET
+            offset_address(rom, offset + 1, CHANNEL_OFFSET)
 
 
 def patch_bios_psg_calls(rom):
@@ -78,7 +88,38 @@ def save_kss_file(filename, rom):
 # Nemesis 3 kick drum fix
 # See: https://www.msx.org/forum/msx-talk/general-discussion/nemesis-3-gofers-ambition-episode-ii-bass-drum-lost
 assert rom[0x21484] == 0x0a
-rom[0x21484] = 0xf0
+#rom[0x21484] = 0xf0  # make kick envelope longer
+# kick fix (maybe?)
+rom[0x21485] = 0x90
+rom[0x21486] = 0
+# snare fix (maybe?)
+rom[0x21487] = 0x90
+rom[0x21488] = 0
+
+# kick
+rom[0x213e0] = 0xc0
+rom[0x213e1] = 0
+
+# snare click
+rom[0x213ec] = 0xc0
+rom[0x213ed] = 0
+
+# snare2 click
+rom[0x21404] = 0xa0
+rom[0x21405] = 0
+
+# move the stack to 0xfaf0. this is a bit of memory
+# that is used by the MSX BASIC 'PAINT' command so it
+# is unused for our purposes. This then frees up the area
+# that Vampire Killer has reserved for the stack:
+# 0xee00 - 0xf100. We then use this area of memory
+# for the Nemesis 3 SCC player's state data
+rom[0x0078] = 0xfa
+rom[0x0077] = 0xf0
+
+# there are lots of these, and some of the drums
+# will iterate through a list of them over time
+
 
 # compile new music into ROM
 compile('mml/vkiller_scc.mml', rom, nemesis3, 0x1a000, 0x7510, 0x8000)

vkiller_scc.asm

@@ -23,10 +23,10 @@
 
 ; disable R800 mode
 ; R800 mode causes the PSG drums to sound inconsistent/weird
-        output vkiller_patch01db2.bin
-        nop
-        nop
-        nop
+;        output vkiller_patch01db2.bin
+;        nop
+;        nop
+;        nop
 
 
         output vkiller_patch01c8a.bin
@@ -124,7 +124,7 @@ write_psg:
 
         ; if the game is paused, don't write PSG ch7
         ; this is to prevent interference with the game paused jingle
-        ld      a, (02280h)
+        ld      a, (0f080h)
         and     2
         ret     nz  ; paused
 
@@ -152,69 +152,18 @@ not_register_seven:
         ret
 
 map_slots:
-        ; map RAM into bank 0000-3fff
-        ; returns original value of #ffff in a
-        ; returns original slot select in b/c
-        ld      c, 0a8h
-        in      b, (c)
-        push    bc
-        push    hl
-        ld      a, b
-        rlca    ; replicate slot in page 3 to page 0
-        rlca
-        and     03h
-        ld      l, a
-        ld      a, b
-        and     ~03h
-        or      l
-        out     (c), a
-
-        ; SOFAROM uses these addresses for SCC patching
-        ; to take into account that we map RAM into page 0, we need
-        ; to change these values. this is a SOFAROM specific hack
-        ld      (0f6ebh), a  ; primary slot select (game)
-        ld      a, (0f6eah)  ; primary slot select (SCC)
-        or      03h
-        ld      (0f6eah), a
-
-        ; get subslot from page 3 (c000-ffff) and apply to page 0 (0000-3fff)
-        ld      hl, 0ffffh
-        ld      a, (hl)
-        cpl
-        push    af
-        ld      b, a
-        and     011000000b
-        rlca    
-        rlca
-        ld      c, a
-        ld      a, b
-        and     011111100b
-        or      c
-        ld      (hl), a
-
         ; map music data using konami SCC mapper
         ld      a, 17
         ld      (09000h), a
         ld      a, 18
         ld      (0b000h), a
-
-        ld      a, 1
-        out     (0fch), a
-
-        pop     af
-        pop     hl
-        pop     bc
         ret
 
 music_start:
         di
 
         call    map_slots
 
-        push    bc
-        push    hl
-        push    af
-
         ; interpret the command
         ld      a, e
         cp      0xfa  ; 0xfa and higher = commands: fade out, pause, etc.
@@ -234,9 +183,9 @@ music_stop:
         ; clear music player state
         push    bc
         push    de
-        ld      hl, 02000h
-        ld      de, 02001h
-        ld      bc, 00300h
+        ld      hl, 0ee00h
+        ld      de, 0ee01h
+        ld      bc, 00280h
         ld      (hl), 0
         ldir
         pop     de
@@ -256,18 +205,10 @@ music_start_command:
 music_start_call:
         push    bc
         call    06003h  ; nemesis 3 song start function
+        call    06180h  ; initialize some state which fixes boss music cutting out
         pop     bc
 
 music_start_skip:
-
-        pop     af
-        ld      (0ffffh), a
-        pop     hl
-        pop     bc
-
-        ; restore BIOS ROM       
-        out     (c), b
-
         di
         ld      a, 14
         ld      (09000h), a
@@ -288,21 +229,12 @@ music_update:
 
         call    map_slots
 
-        push    bc
-        push    af
-
         call    06006h  ; nemesis 3 song update function
 
-        pop     af
-        ld      (0ffffh), a
-
         ; set a flag which vkiller uses to know whether music is playing
-        ld      a,(020c0h)
+        ld      a,(0eec0h)
         ld      (0c0a7h), a
 
-        ; restore ROM
-        pop     bc
-        out     (c), b
 ram_not_initialised:
         ld      a, 14
         ld      (09000h), a