Page 221 - The Art of Designing Embedded Systems

208 THE ART OF DESIGNING EMBEDDED SYSTEMS

clude the chip select and wait state code in-line (not as a subroutine). Be
careful to initialize these selects at the very top of the module, to allow fu-
ture subroutine calls to operate, and since some debugging tools will not
operate reliably until these are set up.

Stack and Heap Issues

Always initialize the stack on an even address. Resist the temptation
to set it to a odd value like Oxffff, since on a word machine an odd stack
will cripple system performance.
Since few programmers have a reasonable way to determine maxi-
mum stack requirements, always assume your estimates will be incorrect.
For each stack in the system, make sure the initialization code fills the en-
tire amount of memory allocated to the stack with the value 0x55. Later,
when debugging, you can view the stack and detect stack overflows by
seeing no blocks of 0x55 in that region. Be sure, though, that the code that
fills the stack with 0x55 automatically detects the stack’s size, so a late
night stack size change will not destroy this useful tool.
Embedded systems are often intolerant of heap problems. Dynami-
cally allocating and freeing memory may, over time, fragment the heap to
the point that the program crashes due to an inability to allocate more
RAM. (Desktop programs are much less susceptible to this as they typi-
cally run for much shorter periods of time.)
So, be wary of the use of the malloc() function. When using a new
tool chain examine the malloc function, if possible, to see if it implements
garbage collection to release fragmented blocks (note that this may bring
in another problem, as during garbage collection the system may not be re-
sponsive to interrupts). Never blindly assume that allocating and freeing
memory is cost- or problem-free.
If you chose to use malloc(), always check the return value and safely
crash (with diagnostic information) if it fails.
When using C, if possible (depending on resource issues and pro-
cessor limitations), always include Walter Bright’s MEM package (www.
snippets.org/mem.txt) with the code, at least for the debugging.
MEM provides:

ISO/ANSI verification of allocationheallocation functions
Logging of all allocations and frees
Verifications of frees
Detection of pointer over- and under-runs.