51 MCU register group settings

Everyone knows that the 51 MCU has four groups of registers R0-R7. In the past year, when I was communicating with novice friends, I found that many friends often had problems using the register group. Although this is not a difficult problem, if an error occurs, it will also cause serious consequences.
    First, let me introduce the register group of 51:
    the register group can be switched arbitrarily by setting the 3rd and 4th bits of the PSW register. Before entering the interrupt, switching the register group can easily protect the data of the original register group from being destroyed by the statements in the interrupt, which is very convenient.
RS1 RS0 Byte address
 0 0 Register 00H~07H
 0 1 Register 1 08H~0FH
 1 0 Register 2 10H~17H
 1 1 Register 3 18H~1FH
 RS1=PSW.4 RS0=PSW.3

There are three common errors:
1. The 0th register group is specified for the interrupt function. 
    C program: void int0() interrupt 0 using 0
    The compiled assembly is as follows:
    PUSH ACC PUSH 
    B
    PUSH DPH
    PUSH DPL
    PUSH PSW
    MOV PSW,#0x00  
    . . . . . .
    Because the main() function uses the 0th register group, the interrupt program will change the data of the register group. When the main program is running, an interrupt may occur at any time. When the interrupt returns to the main program, the value of register R0-R7 has been changed. This is a very serious error. Moreover, the fault sometimes occurs and sometimes does not, and the error is inexplicable.
2. Interrupt priorities are different, but register group numbers are the same
    C program: void int0() interrupt 0 using 1 // low priority interrupt
             void T0() interrupt 1 using 1 // high priority interrupt 
    Because high priority interrupts can interrupt the low-level interrupt being executed and switch to high-level interrupts. This is called interrupt of interrupt. Like the first error, the program of high-level interrupt will change the register being used by the low-level interrupt.
3. Do not write using. Strictly speaking, this is not an error. But it is a very bad habit.
    C program: void int0() interrupt 0
    The compiled assembly is as follows:
    PUSH ACC 
    PUSH B
    PUSH DPH
    PUSH DPL
    PUSH PSW
    MOV PSW,#0x00  
    PUSH 0x00
    PUSH 0x01 PUSH 0x02     PUSH
    0x03     PUSH 0x04     PUSH 0x05     PUSH 0x06     PUSH 0x07     . . . . . .     Without using to specify the register group, the compiler will allocate the 0th group of registers by default, and then use 8 statements to save R0-R7 of the 0th group to the stack. When exiting the interrupt, 8 pop stacks are required. This "clumsy" writing method occupies 32 bytes of program space and 8 bytes of stack. Perhaps the advanced version of the compiler will be improved. It is better to be more disciplined when working on microcontrollers. Experience summary:     1. When writing interrupt programs, you must use the using statement to specify the register group. Groups 1, 2, and 3 are all OK, but not 0. 2.     The interrupt of the 51 microcontroller has two priorities. An interrupt will not interrupt another interrupt of the same priority. In this way, interrupts of the same level can use the same group. For example: low-priority interrupt functions all use using 1, and high-priority interrupts all use using 2. This will not conflict.     The following is a normal example:     C program: void int0() interrupt 0 using 1     The compiled assembly is as follows:     PUSH ACC      PUSH B     PUSH DPH     PUSH DPL     PUSH PSW     MOV PSW,#0x08           . . . It     's that simple. Although this is not a problem for experienced users, many newbies make this mistake. I have encountered no less than 15 of them. I wrote this post today to remind all newbies. It is better to prevent a problem before it happens than to put out fires everywhere.