Friday, July 14, 2017

Where Esh's Aurunmilla stores its 'current stage'

There's been some discussion about an Esh's Aurunmilla bug that shows up on stage 7.

Out of curiosity, I dug into the ROM program a little bit to see how hard it would be to modify the program to start on stage 7 to make testing easier.  After all, no one wants to play all the way to stage 7 just to test a fix!

I found that the stage value is stored at 0xE424 + whatever value is at 0xE3C1.  Here is code at 0x3d06 that prints out the current stage (before each stage):

seg000:3D06 print_current_stage:                    ; CODE XREF: seg000:3B4C p
seg000:3D06                 push    af
seg000:3D07                 push    bc
seg000:3D08                 push    de
seg000:3D09                 push    hl
seg000:3D0A                 call    sub_0_BC7
seg000:3D0D                 ld      a, 1
seg000:3D0F                 call    sub_0_3AAC
seg000:3D12                 ld      b, 1
seg000:3D14                 ld      hl, stage_number_txt
seg000:3D17                 call    pre_print_text
seg000:3D1A                 ld      hl, 0E424h      ; prepare to load current stage data
seg000:3D1D                 ld      a, (0E3C1h)     ; load optional offset
seg000:3D20                 ld      c, a
seg000:3D21                 ld      b, 0
seg000:3D23                 add     hl, bc          ; HL now contains address that contains stage value
seg000:3D24                 ld      l, (hl)         ; load stage value into L
seg000:3D25                 ld      h, 0
seg000:3D27                 ld      b, 14h
seg000:3D29                 ld      c, 0Eh
seg000:3D2B                 ld      e, 3            ; 3 digits for the stage number
seg000:3D2D while_stage_not_printed:                ; CODE XREF: print_current_stage+3D j
seg000:3D2D                 push    bc
seg000:3D2E                 ld      b, 10           ; base 10 conversion
seg000:3D30                 call    hex2dec         ; Hex conversion
seg000:3D30                                         ; Src number is in HL.
seg000:3D30                                         ; Base will in B (ie 10 for base 10 conversion).
seg000:3D30                                         ; Returns the lowermost digit in A and HL will be have been divided by the base (ie if HL originally contained 3387 decimal, it will return as 338 decimal with 7 in A).
seg000:3D30                                         ;
seg000:3D33                 pop     bc
seg000:3D34                 push    de
seg000:3D35                 add     a, 30h ; '0'    ; convert to ASCII?
seg000:3D37                 ld      d, a
seg000:3D38                 ld      e, 9
seg000:3D3A                 ld      a, 1
seg000:3D3C                 call    print_letter    ; prints one letter to the screen
seg000:3D3F                 pop     de
seg000:3D40                 dec     b
seg000:3D41                 dec     b
seg000:3D42                 dec     e               ; decrement digits left count
seg000:3D43                 jr      nz, while_stage_not_printed
seg000:3D45                 pop     hl
seg000:3D46                 pop     de
seg000:3D47                 pop     bc
seg000:3D48                 pop     af
seg000:3D49                 ret

The stage value is incremented around 0xD7D:

seg000:0D7D loc_0_D7D:                              ; CODE XREF: seg000:0D6A j
seg000:0D7D                                         ; seg000:0D79 j
seg000:0D7D                 ld      hl, 0E424h      ; base address where current stage lives
seg000:0D80                 ld      a, (0E3C1h)
seg000:0D83                 and     a
seg000:0D84                 jr      z, loc_0_D87    ; this can increment the current stage
seg000:0D86                 inc     hl
seg000:0D87 loc_0_D87:                              ; CODE XREF: seg000:0D84 j
seg000:0D87                 inc     (hl)            ; this can increment the current stage

The stage value is initially set at:

seg000:0E69                 ld      hl, 0
seg000:0E6C                 ld      (0E424h), hl    ; set current stage to be 0

So to force the game to start on stage 7 instead of 0, one must do this:

Change the "ld hl,0" at 0xE69 to "ld hl,7".  Or change 0x21 0x00 0x00 to 0x21 0x07 0x00.

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