nullsound: update initialization to allow music during attract mode

By convention, the BIOS calls NMI with command 02 to play a jingle
music during while the system ROM runs its attract mode animation.

Add the ability to call a user function right after nullsound
initialization, so one can play a user-provided NSS stream during
the attract mode.

Author: dciabrin

nullsound/bios-commands.s

@@ -65,8 +65,5 @@ snd_command_01_prepare_for_rom_switch::
 ;;; Reset sound driver
 ;;; reset stack and start the sound driver
 snd_command_03_reset_driver::
-        di
-        ld      sp, #0xffff
-        ld      hl, #snd_start_driver
-        push    hl
-        retn
+        ld      bc, #0
+        jp      snd_init_driver_from_nmi

nullsound/entrypoint.s

@@ -24,7 +24,8 @@
         .include "ports.inc"
         .include "ym2610.inc"
 
-        .equ    MAX_PENDING_COMMANDS, 64
+        .equ     START_OF_STACK, 0xfffd
+        .equ     MAX_PENDING_COMMANDS, 64
 
 
 
@@ -121,6 +122,19 @@ init_z80_and_wait:
         ret
 
 
+;;; Intialize and start the sound driver
+;;; This function is called during a NMI, this is the last
+;;; function called from the NMI before starting the driver
+;;; ------
+;;; [ bc ]: optional hook, called after the driver is initialized
+snd_init_driver_from_nmi::
+        di
+        ld      (#POST_INIT_HOOK), bc
+        ld      sp, #START_OF_STACK
+        ld      hl, #snd_start_driver
+        push    hl
+        retn
+
 
 ;;; Sound driver
 ;;; ------------
@@ -150,6 +164,17 @@ snd_start_driver::
         ld      (command_fifo_current), a
         ld      (command_fifo_pending), a
 
+        ;; if there is a post-hook function configured, run it
+        ;; now before entering the main loop
+        ld      hl, (#POST_INIT_HOOK)
+        ld      a, h
+        or      l
+        jr      z, snd_mainloop
+        ld      bc, #snd_mainloop
+        push    bc
+        push    hl
+        ret
+
 snd_mainloop:
         ;; process pending commands if any
         call    snd_process_pending_commands

nullsound/helpers.inc

@@ -37,13 +37,19 @@ cmd_jmptable_padding:
         .endm
 
         ;; private helper to generate z80 opcode to stay idle in RAM
+        .lclequ RESET_WAIT_PC,0xfffe
+        .lclequ OPCODE_JR_TO_SELF,0xfe18
         .macro  prepare_wait_in_ram_opcodes
         ;; create a jmp-to-self instruction at the top of the RAM
         ;; to ensure the 68k can bank-switch z80 code without impact
-        ld      bc, #0xfe18     ; jr <self>
-        ld      (#0xfffe), bc
+        ld      bc, #OPCODE_JR_TO_SELF     ; jr <self>
+        ld      (#RESET_WAIT_PC), bc
         ;; ;; prepare the stack to pop into the jump-to-self in RAM
         ld      hl, #0xfffe
         ld      sp, hl
         push    hl
         .endm
+
+        ;; address of an optional hook function that gets called
+        ;; once the driver finished its initialization
+        .lclequ POST_INIT_HOOK, RESET_WAIT_PC