Compressed Absolute Format Object Code - Texas Instruments 990 Operation Manual

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Tag character D is followed by a load bias (offset) value. The loader uses this value instead of
the load bias computed by the loader itself. The loader adds the load bias to all relocatable entry
addresses, external references, external definitions, load addresses, and data. The effect of the D
tag character is to specify the area of memory into which the loader loads the program. The tag
character D and the associated field must be placed ahead of the object code generated by the
assembler.
Correction of object code may require only changing a character or a word in an object code
record. The user may duplicate the record up to the character or word in error, replace the
incorrect data with the correct data, and duplicate the remainder of the record up to the 7 tag
character. Because the changes the user has made will cause a checksum error when the
checksum is verified as the record is loaded, the user must change the 7 tag character to F.
When more extensive changes are required, the user may write an additional object code record
or records. Begin each record with a tag character 9 or A followed by an absolute load address
or a relocatable load address, respectively. This may be an address into which an existing object
code record places a different value. The new value on the new record will override the other
value when the new record follows the other record in the loading sequence. Follow the load
address with a tag character B or C and an absolute data word or a relocatable data word,
respectively. Additional oata words preceded by appropriate tag characters may follow. When
additional data is to be placed at a nonsequential address, write another load address tag
character followed by the load address and data words preceded by tag characters. When the
record is full, or all changes have been written, write tag character F to end the record.
When additional memory locations are loaded as a result of changes, the user must change the
hexadecimal field following the tag character 0 that contains the number of bytes of relocatable
code. For example, when the object file written by the assembler contained 1000
16
bytes of
relocatable code, and the user has added 8 bytes in a new object record, additional memory
locations will be loaded. The user must find the
a
tag character in the object code file and
change the value following the tag character from 1000 to 1008; he must also change the 7 tag
character to F in that record.
When added record~ place corrected data in locations previously loaded, the added records must
follow the incorrect records. The loader processes the records as they are read from the object
medium, and the last record that affects a given memory location determines the contents of
that location at execution time.
The object code records that contain the external definition fields, the external reference fields,
the entry adtlress field, and the final program start field must follow all other object records. An
additional field or record may be added to include reference to a program identifier. The tag
character is 4, and the hexadecimal field contains zeros. The second field contains the first six
characters of the IDT character string. External definitions may be added using tag character 5
or 6 followed by the relocatable or absolute address, respectively. The second field contains the
defined symbol, filled to the right with blanks when the symbol contains less than six characters.
6.4 COMPRESSED ABSOLUTE FORMAT OBJECT CODE
Absolute format object code provides the user with a compact object code which can be loaded
more rapidly than standard 990 code.
6-5
Digital Systems Division .