为什么v8引擎 reserved_semispace_space 64位 是 16M 呢

看v8 gc机制， 有个疑问，请教大家

//semispace_size_ should be a power of 2 and old_generation_size_ should be a multiple of Page::kPageSize
#if defined(V8_TARGET_ARCH_X64)
#define LUMP_OF_MEMORY (2 * MB)
code_range_size_(512*MB),
#else
#define LUMP_OF_MEMORY MB
code_range_size_(0),
#endif
#if defined(ANDROID)
reserved_semispace_size_(4 * Max(LUMP_OF_MEMORY, Page::kPageSize)),
max_semispace_size_(4 * Max(LUMP_OF_MEMORY, Page::kPageSize)),
initial_semispace_size_(Page::kPageSize),
max_old_generation_size_(192*MB),
max_executable_size_(max_old_generation_size_),
#else
reserved_semispace_size_(8 * Max(LUMP_OF_MEMORY, Page::kPageSize)),
max_semispace_size_(8 * Max(LUMP_OF_MEMORY, Page::kPageSize)),
initial_semispace_size_(Page::kPageSize),
max_old_generation_size_(700ul * LUMP_OF_MEMORY),
max_executable_size_(256l * LUMP_OF_MEMORY),
#endif

对于新生̼内代它由两个reserved_semispace_size_所构成。按机位数不同， reserved_semispace_size_在64位系统，32位系统上分别为16 MB֖ 8 MB。所以新生代内存的最大值在64位系统和32位系分别为32和16M