Hardware Reference
In-Depth Information
contents, although for the character representation we have three extra bytes. This
is convenient, because it is not clear from the start whether the data will be used as
signed or unsigned integers, or as a string.
The middle right window is used for input and output. The first line is for
error output of the tracer, the second line for input, and then there are some lines
left for output. Error output is preceded by the letter ''E'', input by an ''I'', and
standard output by a ''>''. In the input line there is an arrow ''->'' to indicate the
pointer which is to be read next. If the program calls
read
or
getchar
, the next input
in the tracer command line is going into the input field. Also, in this case, it is nec-
essary to close the input line with a return. The part of the line which has not yet
processed can be found after the ''->'' arrow.
Usually, the tracer reads both its commands and its input from standard input.
However, it is also possible to prepare a file of tracer commands and a file of input
lines to be read before the control is passed to the standard input. Tracer command
files have extensions
.t
and input files
.i
. In the assembly language, both uppercase
and lowercase characters can be used for keywords, system subroutines and pseu-
doinstructions. During the assembly process, a file with extension
.$
is made in
which those lowercase keywords are translated into uppercase and carriage return
characters are discarded. In this way, for each project, say,
pr
we can have up to
six different files:
1.
pr.s
for the assembly source code.
2.
pr.$
for the composite source file.
3.
pr.88
for the load file.
4.
pr.i
for preset standard input.
5.
pr.t
for preset tracer commands.
6.
pr.#
for linking the assembly code to the load file.
The last file is used by the tracer to fill the upper right window and the program
counter field in the display. Also, the tracer checks whether the load file has been
created after the last modification of the program source; if not it issues a warning.
Figure C-11 lists the tracer commands. The most important ones are the single
return command, which is at the first line of the table and which executes exactly
one processor instruction, and the quit command
q
, at the bottom line of the table.
If a number is given as a command, then that number of instructions is executed.
The number
k
is equivalent to typing a return
k
times. The same effect is achieved
if the number is followed by an exclamation mark,
!
,oran
X
.
