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#### 7.7.9 DWW high resolution graphics

The DWW, or Double-W 1, board is a high resolution graphics board for PET models 30xx. It attaches to the internal expansion connector. It would not physically fit in 20xx, 40xx or 80xx models because their connectors are physically and logically different. Apart from this, it requires address space at $EC00-$EFFF, which makes it unfit to work in 40xx and 80xx models, which have only 256 bytes of I/O space at $E800.

The DWW board consists of 8 KiB of RAM and a PIA to control various options. The RAM can be mapped either linearly at $9000-$AFFF, or bank-switched in 8 banks of 1 KiB each at $EC00-$EFFF.

It seems that in the demo programs, the BASIC versions like to POKE in 60200, where the machine language programs use $EB00.

    60200 Port A or DDR A           $EB28
    $EBx0 1  0 \
          2  1 - RAM block at $EC00 (0-7)
          4  2 /
          8  3 charrom         0 = off 1 = on
          16 4 hires           0 = on  1 = off
          32 5 extra charrom   0 = on  1 = off

    60201 Control Register A: bit #3 (worth 4) controls if 60200
    $EBx1 accesses the Data Direction Register A (0) or Port A (1).

    60202 Port B or DDR B
    $EBx2 0 = RAM is visible from $9000 - $AFFF
          1 = RAM is bank-switched in blocks of 1 K in $EC00 - $EFFF

          [Control Register B is never mentioned, so putting 1 in this
           address would access the DDR, creating an output line, which
           after RESET is default 0...]

Typical initialisation sequence:

        poke 60201,0        poke 60200,255          (all outputs)
        poke 60201,4        poke 60200,24 or 25 (16 + 8 + 1)

Demo programs on disk PBE-110A, 110B, 111A, and 111B. (PBE = PET Benelux Exchange, the Dutch PET user group)

The memory mapping is a bit strange. It seems each 1 K block contains the pixeldata for 1 bit-line of each text line. This is probably so that the addressing of the RAM can borrow part of the addressing logic/signals of the text screen. (The screen addressing cycles through 0-39, then increases the line (= byte offset) which is fetched from the character ROM; for the graphics, the screen position selects the byte in a KiB and the char ROM offset selects which KiB of graphics RAM).

My notes say: to set a pixel:

    RE = INT(Y/8): LY = Y - 8*RE    (or Y AND 7)
    BY = INT(X/8): BI = X - 8*BY    (or X AND 7)

when memory mapped to $9000:

        L = 36864 + 1024 * LY + 40 * RE + BY
        POKE L, PEEK(L) OR 2^BI

when memory mapped to $EC00:

        POKE 60200,LY + 40 (or 8?)
        L = 60416 + RE * 40 + BY
        POKE L, PEEK(L) OR 2^BI

Unfortunately there is no logical means of expanding the memory to 16 K, so even in an 80 columns PET the resolution will be the same.

#### Footnotes

##### (1)

Dubbel-W bord, designed by Ben de Winter and Pieter Wolvekamp

Next: HRE high resolution graphics, Previous: Changing screen colors, Up: PET-specific commands and settings [Contents][Index]