Commodore Amiga A1000, Hardware Reference Manual

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that is given in words. The height is stored in the upper ten bits of the BLTSIZE register,
with zero representing a height of 1024 rows. Thus, the largest blit possible with the
current Amiga blitter is 1024 by 1024 pixels. However, shifting and masking operations
may require an extra word be fetched for each raster scan line, making the maximum
practical horizontal width 1008 pixels.
NOTE
To emphasize the above paragraph: Blit width is in words with a zero representing 64
words. Blit height is in lines with a zero representing 1024 lines.
The blitter also has facilities, called modules, for accessing images smaller than the entire
bitplane. Each of the four DMA channels has 16 bit modulo register called BLTxMOD. As
each word is fetched (or written) for an enabled channel, the address pointer register is
incremented by two (bytes, or one word.) After each row of the blit is completed, the
signed 16-bit modulo value for that DMA channel is added to the address pointer. (A row
is defined by the width stored in BLTSIZE.)
NOTE
The modulo values are in bytes, not words. Since the blitter can only operate on words,
the least significant bit is ignored. The value is sign-extended to the full width of the
address pointer registers. Negative modules can be useful in a variety of ways, such as
repeating a row by setting the modulo to the negative of the width of the bitplane.
As an example, suppose we want to operate on a section of a full 320 by 200 pixel bitmap
that started at row 13, byte 12 (where both are numbered from zero) and the section is
10 bytes wide. We would initialize the pointer register to the address of the bitplane plus
40 bytes per row times 13 rows, plus 12 bytes to get to the correct horizontal position.
We would set the width to 5 words (10 bytes). At the end of each row, we would want to
skip over 30 bytes to get to the beginning of the next row, so we would use a modulo
value of 30. In general, the width (in words) times two plus the modulo value (in bytes)
should equal the full width, in bytes, of the bitplane containing the image.
- 166 Blitter Hardware -