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The C Users' Group Library 1994 August
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1985-08-21
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Mchip80 - Release 1.2, Feb 86
I now believe that some Mchip80 users will have difficulty in
certain situations that were not covered in a practical way in the
original documentation. Last year when I decided that Mchip80 and its
documentation was ready for release, I had personally reached a stage of
keen awareness of the C/80 compiler's style and habits in creating an
assembly (.MAC) file from the C source. It is now apparent that this
awareness is necessary for all Mchip80 users, and some may need these
helpful suggestions.
C/80 will initialize a float at every opportunity in a program,
right up to the last line. The original documentation was intended to
convey this meaning, and also to cover float constants, which are now
declared illegal for use with Mchip80. Here is an example of illegal
usage:
#define FCONSTA 123.4
float fvar1,fvar2,pow();
main() {
fvar1 = pow(FCONSTA,fvar2);
Here are two examples of pseudo float constants that are acceptable
with Mchip80. The first example is when the constant has a fraction part,
is in exponential notation, or for any other reason requires the function
atof(), Mchip80 version.
char fconsta[] = "123.4";
float fvar1,fvar2,fconv,atof(),pow()
main() {
fvar2 = pow((fconv = atof(fconsta)),fvar1);
The second example is when the constant value has no fraction, ie.,
can be expressed as an integer or long, and type conversion can do the
job.
int n;
float fconsta;
main() {
fconsta = n =123;
Both examples result in float variables that can be used as substite
constants, until they are written over with some other data. A great
awareness tool is the compiler '-t' switch, which can be useful on a
dummy "situation" program. The following program is 'unreal', having no
input or output, and is created solely for a '-t' switch exercise, and
resides on file 'specimen.c'.
float pow(); /* returns a float, says nothing about parameters */
float fvar1,fvar2;
main() {
fvar2 = pow(123.4,fvar1);
}
Use C/80 configured for floats and longs, with this command line:
c -t specimen=specimen
As promised in the Toolworks documentation, the compiler delivers a
.MAC file with all the source lines from 'specimen.c' at the appropriate
places, set up as comments. Print a hard copy of specimen.mac, put it
aside and read some more of this.
You can also use the '-t' switch exercise on one of your real
programs, and I suggest color coding the .MAC printout for visual clarity
and pattern detection. You will need four to six different coloured felt
tip highlighters. Your first impression right now might be here comes a
tedious and childish task, but give it a try if Mchip80 does strange
things to your program. First use blue on all the comment lines from the
source file, to push them into the background. Put red on all RET
instructions, and there won't be many of them. Select either yellow or
green for the CALL instructions. Then treat every PUSH and POP
instruction to the same color, perhaps orange. It is starting to get
interesting, because a pattern is showing. C/80 uses the stack before and
after calling a function. Read more about it in the Toolworks
documentation.
The most interesting lines are now in vivid black and white. Find
the segments where the program is handling floats or longs, it doesn't
matter which. Draw boxes around these segments in a color of your choice.
There should be CALLs to subroutines named 'slong' and 'llong', which are
in Mathpak, and which copy 32 bit variables between memory and registers
BC and DE. Mchip80 has no problem with 'slong' and 'llong', and is in
fact totally dependent on them.
Look again for another situation inside the float/long boxes, where
numbers are loaded "immediate" (instruction LXI) into registers BC and
DE, and where 'slong' and 'llong' are absent. Here the C/80 compiler has
created these numbers and initialized a float or a long. If a float
variable received this treatment, the original program has to be changed
before it will work with Mchip80. You can see this undesirable situation
on the hard copy of 'specimen.mac' printed earlier. In a second exercise,
change the file 'specimen.c', by replacing 123.4 with fvar2, and do the
'-t' switch exercise again. This version is correct for Mchip80.
The '-t' switch exercise is a great tool for exploring the interface
between C/80 and Mchip80, and after a little more use and experience, you
can almost phase it out.