The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

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| |
| TYPE DATA AREA |
| ---- ---------------------------------- |
| 01 Object code load block |
| 02 Transfer address |
| 04 End of partitioned data set member |
| 05 Load module header |
| 06 Partitioned data set header |
| 07 Patch name header |
| 08 ISAM directory entry |
| 0A End of ISAM directory |
| 0C PDS directory entry |
| 0E End of PDS directory |
| 10 Yanked load block |
| 1F Copyright block |
| |
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Figure 8-1: Load Module TYPE Codes
Any code above X'1F' is invalid as a record type. In addition, any code not listed in
figure 8-1 is reserved for future use.
Let's look at a sample file. Start by listing the first sector of the FLOPPY/DCT utility
via the command: LIST FLOPPY/DCT (H). Notice that it starts out with:
05 06 46 4C 4F 50 50 59 1F 2A 43 6F ...
. . F L O P P Y . . C o ...
stretched across the screen. What you have here is a load module header (TYPE=05). The
length byte (LENGTH=06) follows the TYPE code. The 6-byte DATA AREA field is the header
name. All records follow this "fielding" order. A record is organized with a TYPE,
LENGTH, DATA sequence. The X'1F' begins the second record. It happens to be a copyright
record with a LENGTH of X'2A' or 42 decimal bytes. Incidentally, the TYPE=1F record is
generated automatically by the "COM" pseudo-op in PRO-CREATE, the assembler used to
develop and maintain the operating system.
Note that each record begins with the TYPE code and the first byte following the end of a
record is always the TYPE code of the next record. The only exception is when a TYPE code
indicates the end of a file. If you look further in the record displayed at relative
position X'34', or if you count 42 bytes down from the "C" of "Copyright", you will see:
01 02 00 2C D5 ...
The record TYPE is a load block (TYPE=01), and the length of the data area is X'02', or
258 data bytes. Yes, we previously stated that the length ranged up to 256 and here we
have 258! This TYPE-01 record is a special case. The two-byte field following the LENGTH
is the starting load address for the rest of the field. Since the LENGTH value includes
the 2-byte load address, a length of X'03' would indicate only one load byte. A length of
X'04' would indicate two load bytes. A length of X'FF' would indicate 253 load bytes. A
length of X'00' would indicate 254 load bytes. To be able to have a data area with up to
256 bytes of loadable data, the LENGTH values of X'01' and X'02' are indicative of 255
and 256 load bytes respectfully. This is accomplished by having the system loader
decrement the length value by two when reading a load address. The resultant value
becomes the true length of the loadable data.
If you let the program listing proceed to the end of the file, the last four bytes should
appear as:
8-6