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  1. DESKTOP x1k1
  2.                                 ╔═════════════════╗
  3.                                 ║   DMA-FDC/HDC   ║
  4.                                 ║ bit assignments ║
  5.                                 ╚═════════════════╝
  6.                                  Atari Direct Memory Access for
  7.                                  Floppy Disk Controller and Hard Disk Controller
  8.               ┌─────────────────┐
  9.               │ Status Register │     b2=data requested
  10.               │                 │        0:no
  11.               │                 │        1:yes
  12.               │                 │     b1=sector counter equals zero
  13.               │                 │        0:yes
  14.               │                 │        1:no
  15.               │                 │     b0=error
  16.               │                 │        0:yes
  17.               │                 │        1:no
  18.               └─────────────────┘
  19.              ┌──────────────────┐
  20.              │ Control Register │     b8=Read/Write
  21.              │                  │        0:read
  22.              │                  │        1:write
  23.              │                  │     b4=FDC-HDC/SC
  24.              │                  │        0:FDC-HDC
  25.              │                  │        1:sector counter
  26.              │                  │     b3=FDC/HDC
  27.              │                  │        0:FDC
  28.              │                  │        1:HDC
  29.              │                  │  b2-b1=[b1-b0]=FDC/HDC register
  30.              │                  │                00:FDC-SR/CR
  31.              │                  │                01:FDC-TKR
  32.              │                  │                10:FDC-SCR
  33.              │                  │                11:FDC-DTR
  34.              └──────────────────┘
  35.  ┌──────────────────────────────┐
  36.  │ Direct Memory Access Counter │[b23-b0]=direct memory access counter
  37.  │                              │         (low-middle-high for writing)
  38.  └──────────────────────────────┘
  39.                ┌────────────────┐
  40.                │ Sector Counter │  b7-b0=sector counter
  41.                │                │        (1 sector=512 bytes)
  42.                └────────────────┘
  43.  
  44.  
  45.  
  46.  
  47.  
  48.  
  49.                            ┌─────────────────────────┐
  50.                            │           DMA           │
  51.                            │   ┌─────────────────┐   │
  52.                 ┌────────┐ │   │ 16 bytes buffer │   │  ┌─────────┐ ┌──────┐
  53.                 │ Memory ├─────┼─────────────────┼──────┤ FDC/HDC ├─┤ Disk │
  54.                 └────────┘ │   │ 16 bytes buffer │   │  └─────────┘ └──────┘
  55.                            │   └─────────────────┘   │
  56.                            └─────────────────────────┘
  57.