Arm pc=pc+8 is the most understandable explanation

https://www.cnblogs.com/douzi2/p/4934202.html

Why is “PC = PC + 8”;

“PC = PC + 8” In fact, it's easy to get confused by writing like this .“PC = PC + 8” The real meaning should be ：

Code address of execution place = PC - 8;

in other words ,”PC Address to “ leading “ Code address of execution place ”8 Bytes ;

in other words “PC = PC + 8” Second of PC, Actually, it means “ Code address of execution place ”

then , Understand why 8？

First ,arm The address width of is 4 Bytes , So every instruction executed ,PC It will add itself 4.

And an instruction must be completely arm absorption , It takes multiple steps , take arm7 For example, it adopts 3 Stage assembly line technology .

Assembly line , For our kind of play FPGA I can't be more familiar with , With 3 For the stage assembly line , Is to divide one thing into 3 A step , Each step is processed at the same time ,

The first two cycles are incubation period , Unable to produce results , After that, the results will continue to output .

Imagine , Instructions , Like products waiting to be processed , Running on the conveyor belt , and “FETCH”,“DECODE”,“EXECUTE”, Namely 3 A processing procedure .

1、 In the first cycle ,PC Point to the first program instruction , Carry out the value （FETCH）,DECODE, and EXECUTE step “ In the air ”

2、 In the second cycle , next PC + 4,PC Point to the second program instruction to take value （FETCH）,DECODE Processed in one cycle ,FETCH Instructions to ,EXECUTE Throw yourself into the air again .

3、 In the third cycle , still PC + 4,PC Point to the third program instruction to take value （FETCH）,DECODE Processed in two cycles ,FETCH Instructions to ,

EXECUTE At this time , Processing in one cycle FETCH Instructions to , This instruction is also in the second cycle DECODE Instructions .

4、 Then every clock cycle ,EXECUTE It can be processed twice code, however PC Always ahead of it , That is to say ： Code address of execution place = PC - 8;

  and ARM9 By increasing the number of assembly lines , It simplifies the logic at all levels of the pipeline , Further improve the performance of the processor . use 5 Stage assembly line technology , But it's also “PC = PC + 8”,

Why is that ？ Let's take a look at the picture ：

We found that EXECUTE Still in the third step , And conform to ： Code address of execution place = PC - 8; As long as you understand this, it's enough to read the code .