Details Of Block Structure - Radio Shack TRS-80 Operation Manual

2.
Comment
blocks.
These
contain
documentation
for the
programmer.
Comment
blocks
are
not loaded or
examined by
the loader.
Comment
blocks are
prefixed
by two
bytes
of header information.
3.
Trailer blocks.
Each program
file
ends with a
trailer
block.
It
marks
the
end of
file
and
tells
the Printer Controller
to transfer
control
to
a specified address. Trailer
blocks
are
always four bytes long.
The
length of the blocks
will
vary,
depending on
the
type of block and
the
amount
of
information
in
that block.
The
blocks must be contiguous: there
can be no unused
bytes
after the
end
of
one block and
the
beginning of the next.
The
first
byte in a
block
identifies the
block type
as
follows:
Contents of Byte
#
Block Type
Hex
01
Program
data
05
Comment
block
02
Trailer
block
—
jump
after
loadin
all
others
ignored
Details of
Block
Structure
Program data blocks consist of
the following:
Byte
#
Contents
1
Block
Identifier
(=1)
2 Length
—
number
of bytes of
program
data plus
two
for
load address
3-4 Load
address
—
where
the
following
program
data
starts
loading into
RAM, LSB-MSB
format.
5-end
Program
data
Notes:
1.
The block
identifier
always equals binary
"1"
for
a
program
data block.
2.
The
length byte gives the
number
of bytes
in the rest
of the block
—
following
the
two-byte
load address. This
sum may
range from three (two-byte addresses plus
one
byte of data)
to
258 (two-byte addresses
plus
256
bytes of
data),
but
it
must be
translated into the
range 0-255.
To
do
this,
take the
number
of
program
bytes
and increment
by
two.
Note
that
values
greater
than
255
"wrap
around"
to 0,
1,
and
2.
Here
is
a table:
Number
of
bytes
after
the address
Use
this
value
for
(program
data only)
the length byte
itself
1
3
2
4
3 5
253
255
254
255
1
256
2
22