Alto .Hardware Manual
Section 3: Emulator
23
Word Name
Remarks
O·
FUNCTION
Operation,
SourceType, Bank, etc
1
unused
2
DBCA
Destination
BCA
3
DBMR*
Destination
BMR
4
DLX*
Destination
LX
5
DTY*
Destination
TY
6
DW*
Destination W
7
DH*
Destination H
8
SBCA
Source
BCA
9
SBMR
Source
BMR
10
SLX*
Source
LX
'
..
11
STY*
Source
TY
12
GrayO
'
Four
words to
specify gray
block
...
13
Grayl
14
Gray2
15
Gray3
*These should all be positive values, although
if
DH<O
or
DW<O
then
BITBLT
is a
NOP.
Trick: since
BITBLT
uses all
of
the accumulators,
BCPL
programmers must save AC2, the stack
pointer, somewhere.
Put
it
in
word 1
of
the
BBTable,
since AC2 still points
at
the table after the
instruction finishes, making it easy to recover.
'The instruction
is
interruptable
as
it begins consideration
of
each scan line.
If
an
interrupt
happens, the state
of
its progress
is
saved in
Acl
and the
PC
is backed up so
that
on return from
the interrupt,
BITBLT
will finish its job. This
isthe
reason
w4y
Aclmust
be
zero when starting
the instruction.
DEFINITIONS
A bit map is a region
of
memory defined by
BCA
and
BMR,
where
BCA
is
the
base core address
(starting location) and
BMR
is the bit map raster
width
in words; the
number
of
scan lines is
irrelevant for
our
purposes.
(If
both
BMR
and
BCA
are even,
then
the
bit
map
may be displayed
on the screen using standard Alto facilities.)
A block is a rectangle within a bit map.
It
has four corners which
need
not fall on word
boundaries. A block is described by 6 numbers:
BCA
Bit map's base core address
BMR
Bit map's width
in
words
LX
Block's left X ("x offset" from first bit
of
scan-line)
TY
Block's top Y ("y offset" from first scan-line)
W Block's width in bits
H Block's height
in
scan-lines
Example: A block is used to designate a sequence
of
bits in memory, such as a 16 wide
14
high
region containing the bit pattern
of
a font character.
In
this . case,
BCA
points to the font
character,
BMR
is
1,
LX
and
TY
are
0,
W is 16, and H
is
14.
If
source
and
destination blocks
overlap, they
had
better have the same
BCA.
.
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