ram-usage.h

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/* Copyright (c) 2025  Georg-Johann Lay
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#ifndef _UTIL_RAM_USAGE_H_
#define _UTIL_RAM_USAGE_H_
 
#include <stdint.h>
#include <bits/attribs.h>
 
#ifdef __cplusplus
extern "C" {
#endif
 
/** \file */
/** \defgroup util_ram_usage <util/ram-usage.h>: Determine dynamic RAM usage
    \code
    #include <util/ram-usage.h>
    \endcode
 
    This header provides a single function, _get_ram_unused(), that can
    be used during development to get a rough estimate of how much RAM might
    be used by an application.  This works as follows:
 
    -# The startup code paints the RAM location with a specific value.  This
    happens in \ref sec_dot_init ".init3", and only when _get_ram_unused()
    is actually used.  The coloring extends from #__heap_start (which is
    the RAM location right after static storage) up to and
    including \c RAMEND as defined in <avr/io.h>.
 
    -# The application calls _get_ram_unused() and determines how much
    of the colored bytes are still intact.  This can be used to calculate
    a <b>lower bound</b> of how much RAM is used by the application.
 
    \since AVR-LibC v2.3
*/
 
/** \ingroup util_ram_usage
 
    Determines how much of the initial RAM coloring is still intact.
    It can be used to get a <b>lower bound</b> of how much RAM might be
    used by an application.
 
    The function is written in such a way that it has a small register foot
    print, so that it is not a big issue to call it in an #ISR.
    Though that's not required for the intended purpose:
    _get_ram_unused() can simply be called after some time has elapsed
    and enough ISRs and other functions have been invoked.
 
    \return The value returned by _get_ram_unused() is an <em>upper bound</em>
    for how much bytes of RAM are still
    <em>unused at the time of invocation</em>.<br>
    The return value will never increase with time (except for very rare
    occasions where #__ram_color_value is written to the top of the stack).
 
    \par Limitations
    - The start of the coloring is hard coded as #__heap_start, which
    is the beginning of the RAM area after static storage.
    - The algorithm assumes that the stack is located <em>after
    static storage</em> and grows downwards towards #__heap_start.
    - The current implementation is <b>not compatible with malloc</b> et al.
    (alloca() is no problem though, since it allocates on the stack.)
*/
#ifdef __DOXYGEN__
extern inline uint16_t _get_ram_unused (void);
#else
extern __ATTR_ALWAYS_INLINE__ __ATTR_GNU_INLINE__
uint16_t _get_ram_unused (void)
{
  register uint16_t __r24 __asm("r24");
  __asm ("%~call _get_ram_unused"
         : "=r" (__r24) :: "r30", "r31");
  return __r24;
}
#endif /* !DOXYGEN */
 
#ifdef __DOXYGEN__
 
/** \name Symbols */
 
/** \ingroup util_ram_usage
    A symbol defined in the default linker script.
    It points one byte past static storage (\ref sec_dot_data ".data",
    \ref sec_dot_bss ".bss", \ref sec_dot_noinit ".noinit"). */
extern __heap_start;
 
/** \ingroup util_ram_usage
    A weak symbol that defaults to  <tt>RAMEND + 1</tt>.
    It points one byte past the last location that is colored by
    the startup code.
    It can be adjusted by say<br>
\code
    avr-gcc ... -Wl,-defsym,__ram_color_end=<value>
\endcode
    in the link command, or by means of a global inline assembly statement
    like:
\code
    __asm (".global __ram_color_end\n"
           "__ram_color_end = <value>");
\endcode */
__ram_color_end;
 
/** \ingroup util_ram_usage
    A weak symbol that defaults to 0xaa.
    It represents the "color" that's being used by the startup code to
    paint the RAM, and the value that _get_ram_unused() will check against.
    It can be adjusted by say
\code
    avr-gcc ... -Wl,-defsym,__ram_color_value=<value>
\endcode
    in the link command. */
__ram_color_value;
 
#endif /* DOXYGEN */
 
#ifdef __cplusplus
} // extern "C"
#endif
 
#endif /* _UTIL_RAM_USAGE_H_ */