Texas Instruments 990 Operation Manual page 145

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Tag characters 3 and 4 are used for external references. Tag character 3 is used when the last
appearance of the symbol in the second field is in relocatable code. Tag character 4 is used when
the last appearance of the symbol is absolute code. The hexadecimal field contains the location
of the last appearance. The symbol in the second field is the external reference. Both fields are
used by the linking loader to provide the desired linking to the external reference.
For each external reference in a program, there is a tag character in the object code, with a
location, or an absolute zero, and the symbol that is referenced. When the object code field
contains absolute zero, no location in the program requires the address that corresponds to the
reference (an IDT character string, for example). Otherwise, the address corresponding to the
reference will be placed in the location specified in the object code by the linking loader. The
location specified in the object code similarly contains absolute zero or another location. When it
contains absolute zero, no further linking is required. When it contains a location, the address
corresponding to the reference will be placed in that address by the linking loader. The location
of each appearance of a reference in a program contains either an absolute zero or another
location into which the linking loader will place the referenced address.
Figure 6-2 illustrates the chain of the external reference EXTR. The object code contains the
following tag and fields:
4COOEEXTR
At location COOE, the address COO A points to the preceding appearance of the reference. The
chain includes both absolute and relocatable addresses and consists of absolute address COOE,
COOA, C006, and C002, relocatable addresses 029E, 029A, and 0298, absolute addresses BOOE,
BOOA, B006, and B002, and relocatable addresses 0290 and 028E. Each location points to the
preceding appearance, except for location 028E, which contains zero. The zero identifies location
028E as the first appearance of EXTR, the end of the chain.
Tag characters 5 and 6 are used for external definitions. Tag character 5 is used when the
location is relocatable. Tag character 6 is used when the location is absolute. Botfi fields are used
by the linking loader to provide the desired linking to the external definition. The second field
contains the symbol of the external definition.
Tag character 7 precedes the checksum, which is an error detection word. The checksum is
formed as the record is being written. It is the 2's complement of the sum of the 8-bit ASCII
values of the characters of the record from the first tag of the record through the checksum tag,
7.
Tag characters 9 and A are used with load addresses for data that follows. Tag character 9 is
used when the load address is absolute. Tag character A is used when the load address is
relocatable. The hexadecimal field contains the address at which the following data word is to be
loaded. A load address is required for a data word that is to be placed in memory at some
address other than the next address. The load address is used by the loader.
Tag characters Band C are used with data words. Tag character B is used when the data is
absolute; an instruction word or a word that contains text characters or absolute constants, for
example. Tag character C is used for a word that contains a relocatable address. The hexadecimal
field contains the data word. The loader places the data word in the memory location specified
in the preceding load address field, or in the memory location that follows the preceding data
word.
Tag character F indicates the end of record. It may be followed by blanks.
6-3
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