Embedded Systems Design Europe - June/July 2008 - (Page 21)

memory Listing 7 Look up table (C, static allocation). int lut_static(int ind) { int N=1025; int n; static int lut[N]; // static allocation dynamic memory allocation without the associated problems: this solution sidesteps the problems with malloc and free but introduces extra complexity. As an example of variable-size arrays in MATLAB, consider the MATLAB ﬁnd function: given a vector x, the MATLAB statement “ind=ﬁnd(x)” returns a vector ind of indices corresponding to nonzero entries of x. The exact length of ind cannot be determined until run time. Find is a commonly used function in MATLAB, but the variable length of the output makes translation to C problematic, especially when dynamic memory allocation is prohibited. The example in Listing 8 initializes x with ten random numbers between 0 and 1.0, then returns the indices of elements in x that are greater than 0.5. The length of ind isn’t known until after the random numbers are generated. To avoid dynamic memory allocation, you can recognize that the length of ind will never exceed the length of x (=10). This gives you the maximum possible size for ind, which you allocate on the stack. You can also introduce a separate variable, ind_dim1, to track the actual size of ind. The resulting C translation of ﬁndrand is shown in Listing 9. The MATLAB language frees you from having to think about memory allocation and actively encourages the use of variable-size arrays. On the other hand, many algorithms developed in MATLAB are eventually ported to C on an embedded system, where memory, time, and hardware constraints limit how the C implementation allocates memory. In some cases, embedded projects prohibit the use of dynamic memory allocation entirely. It’s no wonder that translating from MATLAB to embedded C is a complicated task. Robert Yu (robert.yu@agilityds.com) is an applications engineer at Agility Design Solutions involved in the development and use of Agility MCS for automatic MATLAB-to-C translation tool in embedded applications. 21 Listing 8 Find and variable-size array. function ind=ﬁndrand() % % ﬁll x with random values % between 0 and 1.0 x=rand(10,1); % % length of ind unknown a priori ind=ﬁnd(x>0.5); Listing 9 C Translation of ﬁndrand with ﬁxed-size arrays. void _ﬁndrand(void) { double x[10]; int32 x_dim1; int32 ind[10]; int32 ind_dim1; // track actual size of ind int32 n; // ﬁll x with 10 random numbers my_rand(x, &x_dim1); // ind=ﬁnd(x>0.5) ind_dim1=0; for (n=0;n 0.5) { ind[ind_dim1]=n; // store 0-based index ind_dim1++; } } print_vector(“ind”, ind, NULL, 9, 2, 1, ind_dim1); return; } tion. If your MATLAB code uses variable-size arrays, how do you translate them to C? The natural solution is to allocate an equivalent array dynamically, but in an environment that prohibits dynamic memory allocation, you have several solutions for handling the variable-size array: • Rewrite the algorithm to eliminate all variable-size arrays: you can • emulate variable-size arrays by preallocating a maximum, ﬁxed-size array, then use extra “bookkeeping” variables and logic to track the actual array size during runtime. This method can be done in C or even MATLAB, although this goes against the grain of normal MATLAB coding style. Use a custom memory allocator in C, such as a pool manager, to allow www.embedded.com/europe | embedded systems design europe | JUNE – JULY 2008 http://www.embedded.com/europe

Table of Contents Feed for the Digital Edition of Embedded Systems Design Europe - June/July 2008

Embedded Systems Design Europe - June 2008 Contents Work in Progress to Define Compact PCI Plus Power.org Demonstrates New Tools Project Supports Multi-core System Programming Altium Links Electronic to Mechanical Design PLDs Look to Cut Power Budget and Costs Project to Provide Coverage Analysis Tool Microsoft Details Windows Embedded Update Cover Feature: Leveraging Virtual Hardware Platforms for Software Allocating Memory in MATLAB-to-C Code MDD & IDEs: Making the Twain Meet in Embedded System Designs Debugging Mixed Signal Designs for Infrequent & Random Events Why Open Source is the Natural Choice for High-security Systems Bringing the Benefits of Low Power CPUs to Modular Design New Products Advertising Contacts

Embedded Systems Design Europe - June/July 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.