HDMA

References

1. ^ abThe SNES has 8 DMA channels, each of which can run as HDMA, writing values from a table to a PPU register at set scanlines. Each table entry starts with a byte that either writes once and waits that many lines ($01-$80) or writes a new value on each of the next $n-128$ lines ($81-$FF); $00 ends the table. An indirect table holds pointers to the data instead of the data.WikiSNESdev Wiki, “DMA registers”
   How we know

   The SNES also features HDMA (Horizontal-blank DMA) which runs in the background and can be set up to automatically write values to hardware registers at specific scanlines, allowing for effects.

   The SNES contains 8 separate DMA "channels"

   $00: Stop processing HDMA on that channel for the rest of the frame. $01-$80: Write once, then wait for X scanlines $81-$FF: Write every scanline for X-$80 scanlines, repeat mode

   The tables can either directly contain the values (Direct mode) or specify 16-bit pointers that are then used to get the values (Indirect mode).

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger

2. ^ abThe Ceres haze is an indirect HDMA table on the sub-screen backdrop color, used for color math. Its first entry covers 64 lines and 15 more cover 8 lines each; while the room fades in, the routine raises the bands' intensities over 16 frames, ending with 0 at the top and 1 to 15 in the bands below, a gradient down the screen.CodeInsaneFirebat/sm_disassembly
   How we know

   10016    JSL.L Spawn_HDMAObject                                               ;88DDD0;
   10017    db $40,$32                                                           ;88DDD4;
   10018    dw InstList_CeresHaze_ColorMathSubScnBackColor_RidleyAlive_0         ;88DDD6;
   

   990;;         $32: Colour math subscreen backdrop colour (8-bit, sometimes used with write-twice)
   

   10098PreInstruction_CeresHaze_ColorMathSubScnBackColor_FadingIn:
   ⋮
   10109    LDA.W HDMAObject_Var0,X                                              ;88DE3F;
   10110    CMP.W #$0010                                                         ;88DE42;
   10111    BEQ .done                                                            ;88DE45;
   ⋮
   10115    LDA.W HDMAObject_Var1,X                                              ;88DE4B;
   10116    STA.B DP_Temp14                                                      ;88DE4E;
   10117    LDA.W HDMAObject_Var0,X                                              ;88DE50;
   10118    LDX.B #$0F                                                           ;88DE53;
   ⋮
   10120  .loop:
   10121    STA.B DP_Temp12                                                      ;88DE55;
   10122    ORA.B DP_Temp14                                                      ;88DE57;
   10123    STA.L HazeColorMathSubscreenBackdropColorHDMADataTable,X             ;88DE59;
   ⋮
   

   10247IndirectHDMATable_CeresHaze_ColorMathSubScnBackdropColor:
   10248    db $40 : dw HazeColorMathSubscreenBackdropColorHDMADataTable         ;88DF03;
   10249    db $08 : dw HazeColorMathSubscreenBackdropColorHDMADataTable+1
   ⋮
   10263    db $08 : dw HazeColorMathSubscreenBackdropColorHDMADataTable+$F
   10264    db $00
   

   Checked against the evidence by its author, an AI model, 2026-10-25.

   Checked by a second AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-25.

   In the claim ledger

3. ^ abcIn rooms whose water is set to wave, an indirect HDMA channel writes BG2's X scroll on every line. Above the water each line points at the plain scroll; below it the lines walk through a ring of 16 values, the scroll plus a displacement of 0, 1, 1, 0, 0, −1, −1, 0 repeating, so the background ripples a pixel either way in an 8-line pattern that shifts one line every 6 frames.CodeInsaneFirebat/sm_disassembly
   How we know

   LDA.W FX_LiquidOptions … AND.W #$0002 … BEQ .notWavy … JSL.L Spawn_HDMAObject … db $42,$0F … dw InstList_Water_BG2_Xscroll

   982;;         $0F: BG2 X scroll (16-bit, write-twice)
   

   7303;;; $C46E: Wave displacement table - water ;;;
   7304WaveDisplacementTable_Water:
   

   7524Setup_Water_BG2_Xscroll_DataTable_Wavy:
   ⋮
   7554  .loop:
   

   7468; the calculation 1FFh - [A] at $C5CE is equivalent to FFh + (Y position of screen on FX)
   

   8365;;; $CF46: Indirect HDMA table - water BG2 X scroll ;;;
   8366IndirectHDMATable_WaterBG2XScroll:
   ⋮
   8368    db $81 : dw WaterBG2XScrollHDMADataTable-4
   

   8623    db $81 : dw WaterBG2XScrollHDMADataTable
   ⋮
   8638    db $81 : dw WaterBG2XScrollHDMADataTable+$1E
   

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger