Texas Instruments 990 Operation Manual page 53

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934255-9701
Description: PX9LAL loads program modules into the memory of the Model 990 Computer and
performs address modification for relocatable code. PX9LAL also performs the linking defined in
the program modules and prints a load map. After all modules have been loaded, control returns
to the monitor. Details of PX9LAL operation are contained in the next paragraph.
Example:
.LL
After the user has entered the command, a series of questions is printed. The answers provide
the information needed to complete the linking and loading. .
3.4.6.1 Description of PX9LAL Operation. PX9LAL loads the first object module supplied.
Subsequent object modules will be loaded only if the first six characters of the Program
Identifier (lDT) assembler directive character string of the module match an unsatisfied external
reference included in a previously loaded module. This makes it necessary for the user to
identify the modules desired since modules not referenced are not loaded. This also allows the
user to maintain a library of program modules on cassette. .
A program identifier (lDT) assembler directive allows a program name of up to eight characters,
but PX9LAL recognizes and prints only the first six. Therefore, the first six characters of all IDT
program names must be unique. .
End-ol-Module and End-ofFile Records. A module is terminated by an end-of-module separator
record, which is denoted by a colon as the firSt character of the record. The end-of-module
record is generated by the assembler when an "END" statement is encountered. The end-of-me
record is generated by the assembler when an end-of-me record is encountered by the assembler
on the source input. This enables the user to batch-assemble source and batch-load object
modules. The loader will, continue loading modules from one cassette until an end-of-file record
is encountered.
PX9LAL Symbol Table. The symbol table is built in the user area. The length of the symbol
table is determined by the number of symbols externally defined or referenced in the program
loaded by PX9LAL. Ten bytes of memory are required for 'a symbol table entry for each
symbol. PX9LAL builds the symbol table toward the low-order addresses in memory, beginning
at the top of user memory.
User Program Load Addresses. PX9LAL loads user programs into the user memory address space.
The user program may not be loaded. in a memory area with a higher address than the current
lowest address of the symbol table. If the user attempts this, an error message will be printed.
As described in Section VI, the object code may contain a load point preceded by a tag
character of D. The tag character and the associated load point must precede the other tag
characters and fields of the program module. PX9LAL loads the relocatable code of the module
beginning at the specified load point unless the load point is an odd addreSS (not on a word
boundary). In that case, PX9LAL loads the relocatable code beginning at the word\boundary
preceding the address. When no load point is specified for the first module loaded, PX9LAL
loads the relocatable code beginning at a default address (A0
16 ).
When no load point is specified
for a subsequent module, PX9LAL loads the relocatable code beginning at the first word
boundary following the last byte of the preceding module.
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Digital Systems Division .