Xerox Alto I Hardware Manual page 30

Alto Hardware Manual
Section 3: Emulator 24
BLOCK OPERATIONS
The basic block operations operate by storing some bits into a "destination block." The source
of these bits varies; often it is another block, the "source block." There are various functions
that BITBLT can perform.
The FUNCTION word of the BBTabie contains a number of fields:
FUNCTION[O-9]
FUNCTION[lO]
FUNCTION[ll]
FUNCTION[12-13]
FUNCTION[14-15]
Must be zero
Source block is in the alternate bank
Destination block is in the alternate bank
SourceType
Operation
The
operation
field specifies the operation to be performed on the source and destination blocks:
Operation Name Action
0
Replace
Destination Block +-
Source
1 Paint Destination Block +-
Source
OR
Destination
2 Invert
Destination Block +- Source
XOR
Destination
3 Erase
Destination Block +- (NOT Source)
AND
Destination
The
SourceType
specifies how the
Source
as used in the above 4 operations is to be computed.
The encodings are:
SourceType Meaning
o
1
2
3
Source
is a block of a bit map
Source
is the complement of a block of 'a bit map
Source
is the logical "and" of a source block and the "gray block" (see
below).
Source
is the "gray block.".
The "gray block" is conceptually a block of infinite extent in which a pattern of dots is repeated.
The pattern is specified by four words (GrayO through Gray3). These give the patterns to write
into the destination. block where called for, one gray word per scan line. The words will align
with destination block word boundaries, but BITBLT will use GrayO through Gray3 in the order in
which BITBLT processes scanlines (either top to bottom (DTY(STY) or bottom to top (DTY> STY».
The most common use of these gray values is to generate a uniform pattern. While the BITBLT
instruction takes care of going through these values appropriately, the table must be phased
properly to eliminate
seams.
Specifically, if ABC D are the desired 16-bit word-aligned values of
gray for scan-lines 0 1 2 3 (mod 4), then two adjustments must be made:
Let Q
=
DTY
+
l.
If DTY ( STY, then exchange Band D and let
Q
=
-(DTY
+
DR
+
2).
Rotate the pattern left (Le., A+- B, B +- C, etc) a total of (Q AND 3) times.
Set GrayO+-A, Grayl+-B, Gray2+-c, Gray3+-D
When the source is a block of bit map, the width and height parameters of the block are not
needed: the width and height. of the destination block are· also used as the width and height of
the source block. It is permissible for the source and destination blocks to overlap, such as when
sliding an existing block around within a bit map; BITBLT will move words in the order required
for the correct results. However, if the source and destination blocks do overlap, they must
belong to the same bit map (Le., DBCA=SBCA and DBMR=SBMR).