1. 汇编标号：f表示forward， b表示backward：

Here is an example:
1: branch 1f
2: branch 1b
1: branch 2f
2: branch 1b
Which is the equivalent of:
label_1: branch label_3
label_2: branch label_1
label_3: branch label_4
label_4: branch label_3

 The "post-indexed" forms mean that the memory address is the base register value, then base plus 
offset is written back to the base register. 

先用base，再base= base+offset
• The "pre-indexed" forms mean that the memory address is the base register value plus offset, then the 
computed address is written back to the base register.

           先base= base+offset， 再用base