Commodore Amiga A500 Technical Reference Manual page 206

This pair of registers contains the 1 8 bit starting address (location)
of Audio channel
X (X =
0,1,2.3)
DMA
data. This is not a pointer reg-
ister and therefore only needs to be reloaded if a different memory
location is to be outputted.
BLT
X
PTH
Blitter pointer to
X
(high
3
bits)
BLT
X
PTL Blitter pointer to
X
(low
1 5
bits)
This pair of registers contains the 1 8 bit address of Blitter source
(X
=
A,B,C) or dest.
(X
=
D) DMA
data. This pointer must be preload-
ed with the starting address of the data to be processed by the blit-
ter. After the Blitter is finished it will contain the last data address
(plus increment and modulo).
LlNE DRAW: BLTAPTL is used as an accumulator register and must
be preloaded with the starting value of (2Y-X) where
YIX is the line slope. BLTCPT and BLTDPT (both H and
L)
must be preloaded with the starting address of the
line.
BLT
X
MOD Blitter Modulox
This register contains the Modulo for Blitter source
(X =
A,B,C) or
Dest
(X
=
D).
A
Modulo is a number that is automatically added to the
address then points to the start of the next line. Each source or desti-
nation has its own Modulo, allowing each to be a different size, while
an identical area of each is used in the Blitter operation.
LlNE DRAW: BLTAMOD and BLTBMOD are used as slope storage
registers and must be preloaded with the values
(4Y-
4X) and (4Y) respectively. Y/X
=
line slope BLTCMOD
and BLTDMOD must both be preloaded with the width
(in bytes) of the image into which the line is being
drawn (normally 2 times the screen width in words).
BLTA' F WM Blitter first word mask for Source
A
BLTALWM
Blitter last word mask for Source
A
The patterns in these two registers are "anded" with the first and
last words of each line of data from Source
A
into the Blitter. A zero
in any bit overrides data from Source A. These registers should be
set to all "ones" for fill mode or for line drawing mode.
BLT
X
DAT Blitter source
X
d a b
register
This register holds Source
X
(X =
A,B,C) data for use by the Blitter. It
is normally loaded by the Blitter
DMA
channel, however, it may also
be preloaded by the microprocessor.