Notes for inverted mode

[KevinCathcart]

This "issue" exits to provide Karkat with notes about spawning in a single entrance cave (specifally c-shaped house) for inverted mode. Please read this carefully.

Doing this in the approach we discussed consists of the randomizer front end writing out one row to the starting point table, (to specify the new starting point), and one row to the exit table (to update the Houlihan exit). Due to the column-major table design writing out one row to each of those tables consist of a bunch of really small writes, but it is still conceptually one record.

While the front end would be responsible for filling values into the two table rows mentioned, you will obviously want to have a local copy of the values to be set for testing purposes. You will want to stick the below in a file like sandbox.asm. Fully commented version is available

;----------------------------------
; Start at C-Shaped house
;----------------------------------
org $02D8D2 : dw $11c
org $02D8E0 : db $23, $22, $23, $23, $19, $19, $19, $19
org $02D918 : dw $1900
org $02D926 : dw $2300
org $02D934 : dw $233e
org $02D942 : dw $1940
org $02D950 : dw $0177
org $02D95E : dw $017F
org $02D96C : db $03
org $02D973 : db $00
org $02D97A : db $FF
org $02D981 : db $00
org $02D988 : db $00
org $02D98F : db $02
org $02D996 : dw $0DE8
org $02D9A4 : dw $0054
org $02D9B2 : db $07

;--------------------------------------------------------------
; Make Houlihan exit at C-Shaped House
;--------------------------------------------------------------
org $02DB68 : dw $0003
org $02DBC9 : db $58
org $02DC55 : dw $09d8
org $02DCF3 : dw $0744
org $02DD91 : dw $02ce
org $02DE2F : dw $0797
org $02DECD : dw $0348
org $02DF6B : dw $07b3
org $02E009 : dw $0353
org $02E06A : db $0a
org $02E0B9 : db $f6
org $02E145 : dw $0DE8
org $02E1E3 : dw $0000

In addition to that, it is neccesary to add a hook that triggers when loading the link's house staring point, that will load up the values needed to allow leaving the starting area without crashing. In order to make it generic I've designed the hook to simply load values for the houlihan exit.

org $02D9B9 ; <- 159B9 - Bank02.asm:11089  (LDA $7EF3C8)
JSL AllowStartFromSingleEntranceCave

AllowStartFromSingleEntranceCave:
; 16 Bit A, 16 bit XY
; do not need to preserve A or X or Y
    LDA $7EF3C8 : AND.w #$00FF ; What we wrote over
    BEQ +
      BRL .done
    +
        PHA
            LDA #$0016 : STA $7EC142 ; Cache the main screen designation
            LDA $02DCF3 : STA $7EC144 ; Cache BG1 V scroll
            LDA $02DD91 : STA $7EC146 ; Cache BG1 H scroll
            LDA $02DE2F : !ADD.w #$0010 : STA $7EC148 ; Cache Link's Y coordinate
            LDA $02DECD : STA $7EC14A ; Cache Link's X coordinate
            LDA $02DF6B : STA $7EC150 ; Cache Camera Y coord lower bound.
            LDA $02E009 : STA $7EC152 ; Cache Camera X coord lower bound.
            LDA $02DC55 : STA $7EC14E ; Cache Link VRAM Location

            ; Handle the 2 "unknown" bytes, which control what area of the backgound
            ; relative to the camera gets loaded with new tile data as the player moves around
            ; (because some overworld areas like Kak are too big for a single VRAM tilemap)
            
            LDA.l $02E06A : AND.w #$00FF
            BIT.w #$0080 : BEQ + : ORA #$FF00 : + ; Sign extend
            STA.l $7EC16A

            LDA.l $02E0B9 : AND.w #$00FF
            BIT.w #$0080 : BEQ + : ORA #$FF00 : + ; Sign extend
            STA.l $7EC16E

            LDA.w #$0000 : !SUB.l $7EC16A : STA $7EC16C
            LDA.w #$0000 : !SUB.l $7EC16E : STA $7EC170

            LDA $02DBC9 : AND.w #$00FF
            STA $7EC14C ; Cache the overworld area number
            STA $7EC140 ; Cache the aux overworld area number
        PLA
    .done
RTL

That hook is safe to run unconditionally, because multi-entrance caves ignore all those values, and they will all be overwritten the next time the player enters any overworld->underworld door.