Commodore Amiga A1000, Hardware Reference Manual

Page: 202 / 380

As mentioned above, the blitter normally has a higher priority than the processor for DMA
cycles. There are certain cases, however, when the blitter and the 68000 can share
memory cycles. If given the chance, the blitter would steal every available memory cycle.
Display, disk, and audio DMA take precedence over the blitter, so it cannot block them
from bus access. Depending on the setting of the blitter DMA mode bit, commonly
referred to as the "blitter-nasty" bit, the processor may be blocked from bus access. This
bit is called DMAF BLITHOG and is in register DMACON.
If DMAF_BLITHOG is a 1, the blitter will keep the bus for every available memory cycle.
This could potentially be every cycle.
If DMAF_BLITHOG is a 0, the DMA manager will monitor the 68000 cycle requests. If the
68000 is unsatisfied for three consecutive memory cycles, the blitter will release the bus
for one cycle.
BLITTER BLOCK DIAGRAM
Figure 6-13 shows the basic building blocks for a single bit of a 16-bit wide operation of
the blitter. It does not cover the line-drawing hardware.
o The upper left comer shows how the first - and last - word masks are applied to the
incoming A-source data. When the blit shrinks to one word wide, both masks are applied.
o The shifter (upper right and centre left) drawing illustrates how 16 bits of data is taken
from a specified position within a 32-bit register, based on the A shift or B shift values
shown in BLTCON0 and BLTCON1.
o The minterm generator (centre right) illustrates how the minterm select bits either allow
or inhibit the use of a specific minterm.
o The drawing shows how the fill operation works on the data generated by the minterm
combinations. Fill operations can be performed simultaneously with other complex logic
operations.
o At the bottom, the drawing shows that data generated for the destination can be
prevented from being written to a destination by using one of the blitter control bits.
o Not shown on this diagram is the logic for zero detection, which looks at every bit
generated for the destination. If there are any 1-bits generated, this logic indicates that
the area of the blit contained at least one 1-bit (zero detect is false.)
- Blitter Hardware 193 -