Page 340 - Programming Microcontrollers in C

Table Look-Up 325

instruction. For example, an integer multiply instruction might require
five clock cycles. In that case, an instruction pipe five instructions long
would be implemented within the integer ALU. Therefore, a multiply
instruction would be launched into the instruction pipe for execution.
In the meantime, other instructions could execute while the multiply is
progressing through the pipe.
Compiler optimization for these machines must take into account
all of the pipes, the separate ALUs, and other unique operations when
creating the required assembly code. Clearly, a straightforward cre­
ation of code in the order that might seem to be natural to a
programmer will not necessarily create the quickest code for a RISC
machine. Here, speed optimization consists of arranging the code so
that, as nearly as possible, the maximum number of instructions are
launched each clock cycle. This rearrangement of your code will
make it extremely difficult to examine the assembly code version of
the program and even make sense of it. So long as the results are not
altered, the optimizer for a RISC machine will move instructions
around in the code stream to accomplish this end. Therefore, from a
practical sense, any debugging on a RISC machine will probably be
done with a source level debugger, and not an assembly language
version of the program.
The chip used in Chapter 8 is of the MCORE family of RISC
microcontrollers. We will see more of the above comments in that
chapter.

EXERCISE
1. Sometimes two input values are needed to specify a parameter.
For example, if you recall from Chapter 5, the change in pulse on
time to properly control the motor speed was given by

∆p
∆p= 3809500
–
c 2
p
create a sparse two-dimensional look-up table whose inputs are p and
∆p and which provides ∆p as the result of three interpolations. Is the
c
look-up table better than the calculation in any way—less code, quicker,
etc.? Why would you use a look- up table in a problem like this one?