Agnus/
                  Read/   Denise/
Register Address  Write   Paula         Function
-------- -------  -----   -------       --------
BLTCON0    040      W       A      Blitter control register 0
BLTCON1    042      W      A( E )  Blitter control register 1

                 These two control registers are used together to
                 control blitter operations.  There are two basic
                 modes, area and line, which are selected by bit
                 0 of BLTCON1, as shown below.

                    AREA MODE ("normal")
                 -------------------------
                 BIT# BLTCON0     BLTCON1
                 ---- -------     -------
                 15   ASH3        BSH3
                 14   ASH2        BSH2
                 13   ASH1        BSH1
                 12   ASA0        BSH0
                 11   USEA         X
                 10   USEB         X
                 09   USEC         X
                 08   USED         X
                 07   LF7          DOFF
                 06   LF6          X
                 05   LF5          X
                 04   LF4         EFE
                 03   LF3         IFE
                 02   LF2         FCI
                 01   LF1         DESC
                 00   LF0         LINE(=0)

                 ASH3-0  Shift value of A source
                 BSH3-0  Shift value of B source
                 USEA    Mode control bit to use source A
                 USEB    Mode control bit to use source B
                 USEC    Mode control bit to use source C
                 USED    Mode control bit to use destination D
                 LF7-0   Logic function minterm select lines
                 EFE     Exclusive fill enable
                 IFE     Inclusive fill enable
                 FCI     Fill carry input
                 DESC    Descending (decreasing address) control bit
                 LINE    Line mode control bit (set to 0)

                 LINE DRAW       LINE MODE (line draw)
                 LINE DRAW     ------------------------------
                 LINE DRAW     BIT# BLTCON0     BLTCON1
                 LINE DRAW     ---- -------     -------
                 LINE DRAW     15   START3      TEXTURE3
                 LINE DRAW     14   START2      TEXTURE2
                 LINE DRAW     13   START1      TEXTURE1
                 LINE DRAW     12   START0      TEXTURE0
                 LINE DRAW     11    1              0
                 LINE DRAW     10    0              0
                 LINE DRAW     09    1              0
                 LINE DRAW     08    1              0
                 LINE DRAW     07   LF7             0
                 LINE DRAW     06   LF6            SIGN
                 LINE DRAW     05   LF5             0 (Reserved)
                 LINE DRAW     04   LF4            SUD
                 LINE DRAW     03   LF3            SUL
                 LINE DRAW     02   LF2            AUL
                 LINE DRAW     01   LF1            SING
                 LINE DRAW     00   LF0            LINE(=1)
                 LINE DRAW
                 LINE DRAW     START3-0  Starting point of line
                 LINE DRAW               (0 thru 15 hex)

                 LINE DRAW     LF7-0     Logic function minterm
                 LINE DRAW     select lines should be preloaded
                 LINE DRAW     with 4A to select the equation
                 LINE DRAW     D=(AC+ABC).  Since A contains a
                 LINE DRAW     single bit true (8000), most bits
                 LINE DRAW     will pass the C field unchanged
                 LINE DRAW     (not A and C), but one bit will
                 LINE DRAW     invert the C field and combine it
                 LINE DRAW     with texture (A and B and not C).
                 LINE DRAW     The A bit is automatically moved
                 LINE DRAW     across the word by the hardware.
                 LINE DRAW
                 LINE DRAW     LINE   Line mode control bit (set to 1)
                 LINE DRAW     SIGN   Sign flag
                 LINE DRAW     0      Reserved for new mode
                 LINE DRAW     SING   Single bit per horizontal line for
                 LINE DRAW            use with subsequent area fill
                 LINE DRAW     SUD    Sometimes up or down (=AUD*)
                 LINE DRAW     SUL    Sometimes up or left
                 LINE DRAW     AUL    Always up or left

                 LINE DRAW     The 3 bits above select the octant
                 LINE DRAW     for line drawing:

                 LINE DRAW            OCT     SUD  SUL  AUL
                 LINE DRAW            ---     ---  ---  ---
                 LINE DRAW             0       1    1    0
                 LINE DRAW             1       0    0    1
                 LINE DRAW             2       0    1    1
                 LINE DRAW             3       1    1    1
                 LINE DRAW             4       1    0    1
                 LINE DRAW             5       0    1    0
                 LINE DRAW             6       0    0    0
                 LINE DRAW             7       1    0    0

                 LINE DRAW     The "B" source is used for
                 LINE DRAW     texturing the drawn lines.

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