最近在調試1768和外接的字節存取存儲器,涉及到了由於不瞭解結構體類型在ram中的存儲格式,出現bug也不知道是哪裏的問題.今天好好看了下,這裏做一總結.
lpc1768是小端存儲(註釋①),四字節對齊,下面舉例子說明結構體在ram中的存儲格式.
例子1:
結構體定義如下:
struct{
uint8 arg1;//arg1 =0x01
uint8 arg2;//arg2 =0x02
uint32 arg3;//arg3 =0x12345678
uint8 arg4;//arg4 =0x04
uint32 arg5;//arg5 =0x23456789
uint8 arg6;//arg6 =0x06 uint16 arg7;//arg7 =0x0755
uint32 arg8;//arg8 =0x456789ab
}TEST_STRUCT; 假設結構體首地址爲0x2007C124,則 其在存儲器中的存儲形式如下:
結構體中元素所佔的空間都要湊成4byte對齊...
註釋①:在 <<34.5 ARM Cortex-M3 User Guide: Glossary>>(參見1768手冊)有這麼一段話: 不知道可不可以認爲基於cortex-M3內核的存儲結構都是小端模式.望知情者告知,以免誤導他人.
34.3.1.5 Data types
The processor:
• supports the following data types:
– 32-bit words
– 16-bit halfwords
– 8-bit bytes
• supports 64-bit data transfer instructions.
• manages all data memory accesses as little-endian. See Section 34.3.2.1 for more
information.
