Radio Shack TRS-80 Model II Reference Manual page 256

Model II TRSDOS
The first byte in a block identifies the block type, as follows:
Contents of Byte
#
Hex
01
05
02
03
00, 04, 06 - FF
Details of Block Structure
Block Type
Program data
Comment block
Trailer block - jump after loading
Trailer block - return to caller after
loading
Reserved for system use-loader will
fail if one of these codes is used.
Program data blocks consist of the following:
Byte
#
1
2
3-4
5-end
Notes:
Contents
Block Identifier
(= 1)
Length - number of bytes of program data plus two
for load address
Load address - where the following program data
starts loading into
RAM, LSB-MSB
format.
Program data
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 address plus
256
bytes of data), but it must
be translated into the range
0-255.
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