PROGMEM_*****宏定义如何理解？

cos12a

一堆宏定义无法理解，还请赐教：如下：

#if defined(__GNUC__) || defined(__DOXYGEN__)
# include <avr/pgmspace.h>
# define PROGMEM_LOCATION(type, name, loc) \
                type name __attribute__((section (#loc)))
# define PROGMEM_DECLARE(type, name) const type name __attribute__((__progmem__))
# define PROGMEM_STRING(x) PSTR(x)
# define PROGMEM_STRING_T  PGM_P
# define PROGMEM_T const
# define PROGMEM_PTR_T const *
# define PROGMEM_BYTE_ARRAY_T uint8_t*
# define PROGMEM_WORD_ARRAY_T uint16_t*
# define PROGMEM_READ_BYTE(x) pgm_read_byte(x)
# define PROGMEM_READ_WORD(x) pgm_read_word(x)

#elif defined(__ICCAVR__)
# include <pgmspace.h>
# ifndef __HAS_ELPM__
#  define _MEMATTR_ASF  __flash
# else /* __HAS_ELPM__ */
#  define _MEMATTR_ASF  __hugeflash
# endif /* __HAS_ELPM__ */
# define PROGMEM_LOCATION(type, name, loc) const _MEMATTR_ASF type name @ loc
# define PROGMEM_DECLARE(type, name) _MEMATTR_ASF type name
# define PROGMEM_STRING(x) ((_MEMATTR_ASF const char *)(x))
# define PROGMEM_STRING_T  char const _MEMATTR_ASF *
# define PROGMEM_T const _MEMATTR_ASF
# define PROGMEM_PTR_T const _MEMATTR_ASF *
# define PROGMEM_BYTE_ARRAY_T uint8_t const _MEMATTR_ASF *
# define PROGMEM_WORD_ARRAY_T uint16_t const _MEMATTR_ASF *
# define PROGMEM_READ_BYTE(x) *(x)
# define PROGMEM_READ_WORD(x) *(x)
#endif

在C内直接就这样定认数据：
PROGMEM_DECLARE(char const, header[]) = "Temperature";
PROGMEM_DECLARE(char const, cdc_help[]) =
        "\r\n"
        "Welcome to the XMEGA-A3BU Xplained Demo CDC interface!\r\n"
        "\r\n"
        "Key bindings for LCD menu control:\r\n"
        "  Enter      : Enter\r\n"
        "  Backspace  : Back\r\n"
        "  Arrow Up   : Up\r\n"
        "  Arrow Down : Down\r\n";
PROGMEM_DECLARE(char const, tz_00[]) =  "UTC: UK (GMT)";
PROGMEM_DECLARE(char const, tz_01[]) =  " +1: Oslo, Berlin";
PROGMEM_DECLARE(char const, tz_02[]) =  " +2: Istanbul";
PROGMEM_DECLARE(char const, tz_03[]) =  " +3: Nairobi";
PROGMEM_DECLARE(char const, tz_04[]) =  " +4: Moscow";
PROGMEM_DECLARE(char const, tz_05[]) =  " +5: Pakistan";
PROGMEM_DECLARE(char const, tz_06[]) =  " +6: Bangladesh";
PROGMEM_DECLARE(char const, tz_07[]) =  " +7: Thailand";
PROGMEM_DECLARE(char const, tz_08[]) =  " +8: Hong Kong";
PROGMEM_DECLARE(char const, tz_09[]) =  " +9: Tokyo";
PROGMEM_DECLARE(char const, tz_10[]) =  "+10: AEST";
PROGMEM_DECLARE(char const, tz_11[]) =  "+11: Solomon Island";
PROGMEM_DECLARE(char const, tz_12[]) =  "+12: Fiji";

就这样执行了：
        while ((val = PROGMEM_READ_BYTE(help))) {
                udi_cdc_putc(val);
                help++;
        }


这几句，真心不懂。
# define PROGMEM_LOCATION(type, name, loc) \
                type name __attribute__((section (#loc)))
# define PROGMEM_DECLARE(type, name) const type name __attribute__((__progmem__))

正点原子

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