System information

The second form of the T command is similar to the first, except that execution is traced for n
steps (n is a hexadecimal value) before a program breakpoint occurs. A breakpoint can be forced
in the trace mode by typing a rubout character. The CPU state is displayed before each program
step is taken in trace mode. The format of the display is the same as described in the X command.
You should note that program tracing is discontinued at the CP/M interface and resumes after
return from CP/M to the program under test. Thus, CP/M functions that access I/O devices, such
as the disk drive, run in real-time, avoiding I/O timing problems. Programs running in trace mode
execute approximately 500 times slower than real-time because DDT gets control after each user
instruction is executed. Interrupt processing routines can be traced, but commands that use the
breakpoint facility (G, T, and U) accomplish the break using an RST 7 instruction, which means
that the tested program cannot use this interrupt location. Further, the trace mode always runs the
tested program with interrupts enabled, which may cause problems if asynchronous interrupts are
received during tracing.
To get control back to DDT during trace, press RETURN rather than executing an RST 7. This
ensures that the trace for current instruction is completed before interruption.
4.2.11 The U (Untrace) Command
The U command is identical to the T command, except that intermediate program steps are not
displayed. The untrace mode allows from 1 to 65535 (0FFFFH) steps to be executed in
monitored mode and is used principally to retain control of an executing program while it reaches
steady state conditions. All conditions of the T command apply to the U command.
4.2.12 The X (Examine) Command
The X command allows selective display and alteration of the current CPU state for the program
under test. The X command takes the forms:
X
Xr
where r is one of the 8080 CPU registers listed in the following table.
4.2 DDT Commands CP/M Operating System Manual
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