Texas Instruments 990 Operation Manual page 70

~.o
_ _ _ _ _ _ _
~
945255-9701
executed, is depleted, the monitor returns control to the command processor. Otherwise the
monitor continues execution of the user program.
Error message:
MX04 Attempt to execute in trace mode when the instruction trace
overlay is not loaded.
Application notes: Be sure that the processor registers, are properly set before beginning
execution. The contents of the registers may be inspected with the Inspect Registers OR)
command and modified as needed with the Modify Registers (MR) command. The initial PC is
set by the loader when a user program that specifies an entry point is loaded. The starting
memory location of a program is specified in the END statement of the program; a label that
appears within the program for this purpose is referenced in the END statement. If two or more
user programs are competing for specification of the starting location, the last one loaded takes
precedence.
The user may regain control of the program which is executing under SIE or instruction trace by
pressing the escape (ESC) key on the terminal keyboard. If the user's program is using monitor
I/O support,pressing the' ESC key may cause an escape character to be returned to the program
rather than to the monitor; the user should be aware that the escape character may be handled
in these two different ways since the results of program operation maybe affected.
Interrupts are processed as they occur by the user program using the SIE mode of execution.
When running under the trace mode, interrupts and extended operations (XOPs) are executed.
directly.
When running under SIE, an IDLE assembly language machine instruction is handled like an NOP
instruction. The SIE level 0 interrupt causes the computer to continue execution.
The user must be aware of how the 733 ASR operates when he decides to enable interrupts
since interrupts can occur when character keys are pressed. PX9MTP is not interrupt driven;
therefore, significant problems may result. It is recommended that the interrupt mask be set if
possible so that the 733 ASR cannot interrupt.
The overhead when executing under SIE is approximately 100 instructions for each user
instruction. Using trace, the overhead is approximately 170 instructions for each user instruction.
It is often convenient to use the trace mode of execution when no information is being printed
(by setting a null trace type). This is similar to executing using the SIE processor except that
interrupts run at full processing speed. A variable trace can also be used to detect modification
of particular memory locations. (Variable trace is explained in paragraph 3.4.26.) .
Examples:
.RU
.RU 5
In the first example, the maximum number of instructions to be executed before returning to
command mode is the value used at the last entry into command mode, or is 0 initially or if the
previous RU command has exhausted the instruction count. The second example specifies an
instruction count of 5., .
3-32
Digital Systems Division