[RTOS training camp] problems of evening students

1. ask ： What if I have to change my mobile phone number when I go to work in other cities ？

answer ： Mobile phone number change , Alone Headmaster Handle .

2. ask ： Today is live text ？/ Is there a live video now ？

answer ： Encounter difficult problems , Will use video live . There should be no live video today .

3. ask ： The bare metal course of single chip microcomputer is expected to have several classes ？

answer ： Bare metal courses 3、4 Second class .

4. ask ： Do you need to read the fundamentals of single chip microcomputer to learn this course ？

answer ： no need ,RTOS Focus on software , Single chip operation is not much ; The back can speak ARM framework .

5. ask ： If you have no time for training that night , Look back at the after-school materials , There should be no difference with what you miss ？

answer ： There is no interaction , There's no difference .

6. ask ： Why are all procedures from 0x80000000 Start ？

answer ： In this class, it's just a hypothetical address .

7. ask ： stm A series of code runs on the chip flash Do you ？

answer ： Let's assume that it is Flash Up operation , speak of ARM Architecture allows programs to RAM Running in .

8. ask ： CPU How to access the Flash、RAM、GPIO These memory or peripherals ？

answer ： CPU Uncle uses different addresses , visit RAM,GPIO,FLASH. Look at it this way ,GPIO、RAM、Flash Position the same .

​ a. RAM Its characteristic is ： Write something in , Read out what is still .

​ b. Flash Its characteristic is ： It is not easy to write data in , You can read .

​ c. GPIO Its characteristic is ： Writing data has special meaning , It may be that the pin becomes high 、 Low level .

​ These sons , The functions are different . But in my eyes , All sons , They all have addresses , All are accessed using addresses .

9. ask ： Can you write the address casually , For example, write an address 0？

answer ： Only when the corresponding device exists in this address , Wanted 7 A gourd baby , You want to call number 8 individual , It doesn't exist . in other words , The access address must be within the address range specified by the chip . You write an address , When it comes to chips , There is no device in this address ： Write it , It doesn't work .

10. ask ：a=123 How do you think this is the address of writing memory ？ The address is 123 Do you ？ Where did it go ？

answer ： a It's a variable. , Variable readable and writable , Only in RAM in .123 Is the value to be written , The address is assigned when linking the program , To check which address is assigned, debug and check when it can run .

11. ask ： “Flash Its characteristic is ： It is not easy to write data in , You can read ” I don't quite understand

answer ： When we develop programs , Burn the program , It's burning and writing Flash On , After power failure ,Flash The content of will not be lost ,Flash What is stored on it is a program , It's precious , You can't simply write data in , Special steps must be taken ： For example, write after erasing , For example, when writing, unlock it first . If Flash Can easily write data into , Your program is easy to break .

12. ask ： Variables of the previous routine a and p stay flash The address is also in the burning file + Does the data form exist , there “ Address ” Can I customize the allocation ？

answer ：a = 123; p =xxx; stay Flash This is how it is preserved in ： Suppose the compiler gives a、p The assigned addresses are addr1, addr2.a = 123; Into several assembly instructions , as follows ：

MOV R0, ADDR1
MOV R1, #123
STR R1, [R0]

It doesn't matter if you don't understand the compilation ,ARM The architecture will say . Translate ：

a = 123; Into several assembly instructions ：

• hold addr1 Put the value of CPU The internal register of R0;
• hold 123 Put the value of CPU The internal register of R1;
• hold R1 Write the value of to an address , Which address ？R0 Inside is the address value ;

Here comes the key ：a = 123:

• hold 123 The number of , Write to variable a Go to , Is to write addr1 Corresponding memory ;
• In assembly code , Implicit addr, Implicit 123;
• After executing the assembly instructions , come from Flash The numerical 123, Written to memory addr1 place ;

13. ask ：p = (volatile unsigned int *)(0x40010800 + 0x0c) Medium volatile unsigned in Don't write it ？

answer ： Don't write , There will be a warning .

14. ask ： cpu Not will flash Data read to RAM in , stay RAM Do you want to do it in the future ？ You can also read directly gpio The address? ？

answer ： It's not right .CPU It's a Flash The data of , Read in CPU Inside , stay CPU Internal implementation . Yes RAM Only 2 Kind of ： read 、 Write .

Look at this a = a + b：

• Read from memory a, Deposit in CPU;
• Read from memory b, Deposit in CPU;
• CPU Inside ：val = a + b;
• hold val Write to memory a It's about ;
• For memory RAM, Only reading 、 Write operations ; about Flash, There is only read operation here ,CPU from Flash Read to execute , stay CPU Internal execution instructions ：

15. ask ： Does the memory manager need to configure the address range of each peripheral ？ Or is it configurable ？ Do you need to configure ？

answer ：

• Generally, it cannot be configured ;
• Whether it's 103, Or other chips, such as IMX6ULL, Every chip manual has a chapter ：memory map, What it says is , Address range of each device on the chip ;

16. ask ： Variables and data are stored separately in two areas ？

answer ：

• Variable a, The address is 0x20000000, It is determined when linking the program , When the program runs, it is in memory , Note that Runtime ;

• a=123 This is an instruction , There is data in the instruction 123, It's in Flash On ;

• After execution a=123 after , Memory address 0x20000000 The value at is written as 123;

• 123 This value , come from Flash, It's spread RAM

17. ask ： Registers in the assembly , and GPIO What is the difference between these registers , Is the access method and address different ？

answer ：

register , This word is very bad .CPU Internal registers ,GPIO Register on , It's not the same thing ：

• CPU The registers inside , Use assembly instructions to read and write ;
• GPIO Register on , Like memory ,CPU Send address signal 、 Data signals , To read and write it ;

18. ask ： Is this messenger programmed by us ？ For example, send it an address , Then send a chip selection signal ？

answer ： Chip design , The hardware is designed , The software cannot be changed .

19. ask ： Why is the address length 4？

answer ： ① First , There is a premise here ： Is in 32 Under the machine .② this 32 Bits limit the length of the memory address to 32 Bit is 4 Bytes , So we use sizeof When getting the length of a pointer variable , You can only get one result , That's it 4 Byte length .③ about 32 position CPU,CPU Uncle can send 32 Address lines , So the value of the address , Just use 32bit To express .④ Various variables have different sizes , But its address , To be exact, the first address , All are 32 Bit .⑤ Pointer to the variable , Used to save the address , So any pointer variable ,sizeof(XXX *) All are 4 byte , That is to say 32 position .

20. ask ： char buf[1000] , Not initialized ,sizeof(buf) It's also 4 Well ？

** answer ：** yes 1000. No initialization means that the value inside is not set , But the occupied space CPU Keep it for you .

21. ask ：

ldr	r0, =0x20000000
ldr	r1, =123
str	r1, [r0] 

How to judge whether it is data or address ？
answer ： It is impossible to judge whether it is data or address , Only when it is used can we know whether it is data or address . In the 3 Orders ：STR R1, [R0] This is a write memory instruction ,R1 Is the data ,R0 It's the address . If you write it this way ：STR R0, [R1], that R0 Is the data ,R1 It's the address .STR yes store It means .

22. ask ： Can you understand the process of program operation in this way ：

​ Programs are stored in Flash in （ To be processed ） CPU Execute one sentence of code at a time （ Deal with a sentence of code ）, Store the results of each execution in RAM in （ Whether heap or stack ）, If necessary, just RAM Extract data from （ such as GPIOS－ODR register ）.
answer ： The results may not be saved to RAM, such as ：*p = 1;p Point to GPIO register , Then this instruction is To write GPIO register , The result is written GPIO Went to the . Of course ,p The value of is from RAM.“ The results of each execution are stored in RAM in ”： Wrong **“ Every time ”**

23. ask ： arm Instruction sets and thumb Instruction sets are 32 For addressing ？

answer ： Yes .Flash Upper each thumb When the command 16 Bit , however CPU To read Flash、 Reading and writing RAM、 Reading and writing GPIO when , The address line sent is 32 Bit .CPU Got it 16 Bit instruction , according to 16 Bit instruction ： such as STR R0, [R1], It is a handle. R1 Of 32 The digit value is sent to the address line . therefore thumb still arm Instructions , It does not affect the address line length of addressing .

24. ask ： stay Linux Run your own program in , You can delete your own executable , Why is the program in the single chip microcomputer Flash？

answer ： Linux Is to read the program into RAM, And then run ; SCM can also put Flash The program on reads RAM function , And then erase Flash, It's just a single chip computer RAM Not that big. , We can not put Flash Copy the code on to RAM.

25. ask ： Mentioned the first address , The teacher has time to talk about pack Instructions ？ When do I need to use alignment , How to use ：

struct {
    
 char c;
 int a;
};

answer ： First , How big is this structure ？ Logically speaking ,char yes 1 byte ,int yes 4 byte , So this structure is 5 byte . however , If you allocate space like this ,int a The address of is odd , Inefficient access . therefore , This structure is for char c, Still allocate 4 byte , Even if only 1 byte . Can we force it to allocate 5 byte ？ Sure , use pack Instructions , Specific usage can be Baidu search .

26. ask ： cortex-m Series I can understand as code located in Flash On , The variable is located in RAM On? ？ So from Flash Will the instruction fetch speed be limited ？ This is a cortex-m Features of the series ？

answer ： There is no problem with this understanding . In fact, you can M series Flash Put the code on it RAM Run in , If RAM If there's enough space . stay Flash The speed will be a little slower , Considering the cost, we can tolerate .

27. ask ： FreeRTOS Is the task stack of generally dynamically allocated ？ Dynamic allocation generally does not overflow ？

answer ： Dynamic allocation is more convenient , But some systems that pursue extreme stability do not allow dynamic allocation .

28. ask ： stay 32 Bits will be among them , Why is it more efficient to allocate four byte aligned access to the structure ？

answer ： First look at this picture ：

CPU The access bit width is 32 Of RAM when , The last time the hardware read and write was 32 Bit as a unit of , such as ： Even read one char c, Memory controller also goes RAM I read that 32 Bit data is 4 Bytes of data , Return one of the stanzas to CPU.① For the variables in the above figure a, It can be read at one time 、 finish writing sth. ;② For the variables in the above figure b, Read 2 Time ： The first 1 Read the following ellipse for the second time 4 byte ：

The first 2 Secondary reading , Get 4 byte ：

Then combine the contents of the ellipse in the figure below ：

What's the problem with such an operation ？ Especially inefficient , The speed is particularly slow ！ And the hardware does not necessarily support such addressing and reading .

29. ask ： A memory controller uses chip selection to address the corresponding devices , Since each device has its own fixed memory address , Why do you need to select a device for film selection

answer ： Because everyone shares the address line 、 cable , Address purely through the address line CPU It's impossible to tell which family is found .