Importance of StartUp.A51 in keil

I recently wanted to write a program using an STC microcontroller, but STC was not in the KEIL microcontroller library, so I just found a replacement. However, the Keil I found did not add the StartUp.A51 file. At first, I wrote and debugged the program normally. But later, it became more and more strange. There were frequent errors when powering on. After searching for a long time, I found that a variable was not initialized to 0. Suddenly I realized that Keil did not help me clear the memory space when powering on! I found that there was no StartUp.A51 file. After adding it, I set the internal and external space addresses, and everything was normal. It wasted a whole day of my time. Due to the importance of the CPU and program startup code file STARTUP.a51, some 8051-derived CPU products require the CPU to be initialized to meet the corresponding hardware in the design. Therefore, sometimes users need to modify STARTUP.a51, so here is a comment: ;--------------------------------------------------- ; startup.A51: User power-on initialization program; ---------------------------------------------------- ; ; Use the following EQU command to define the memory space that needs to be initialized with 0 when the CPU is reset; ; The absolute starting address of the IDATA memory space is always zero IDATALEN EQU 80H ; The number of bytes of the IDATA memory space that need to be initialized with 0; XDATASTART EQU 0H ; The absolute starting address of the XDATA memory space XDATALEN EQU 0H ; The number of bytes of the XDATA memory space that need to be initialized with 0; PDATASTART EQU 0H ; The absolute starting address of the PDATA memory space PDATALEN EQU 0H ;The number of bytes of PDATA memory space that need to be initialized with 0;Note: The IDATA memory space physically includes the DATA and BIT storage spaces of the 8051 microcontroller;At least the memory space related to the C51 compiler runtime library must be initialized with 0; ;Reentrant function simulation initialization;----------------------------------------------------------- ;The following uses the EQU instruction to define the initialization of the reentrant function simulation stack pointer; ;When using SMALL memory mode, the stack space of the reentrant function IBPSACK EQU 0 ;When using SMALL memory mode, set it to 1 when reentering the function IBPSTACKTOP EQU 0FFH+1 ;Set the top of the stack to the highest address plus 1; ;When using LARGE memory mode, the stack space of the reentrant function XBPSTACK EQU 0 ;When using LARGE memory mode, set it to 1 when reentering the function XBPSTACKTOP WQU 0FFFFH+1 ;Set the top of the stack to the highest address plus 1; ;When using COMPACT memory mode, the stack space of the reentrant function PBPSTACK EQU 0 ; Set it to 1 when reentering the function using COMPACT memory mode PBPSTACKTOP WQU 0FFFFH+1 ; Set the stack top to the highest address plus 1 ;;----------------------------------------------------; When using COMPACT memory mode, the paging definition of the 64KB XDATA memory space; ; The following uses the EQU instruction to define the page address of the PDATA type variable in the XDATA memory space; When using the EQU instruction to define PFAGE, it must be consistent with the control parameters of the L51 link locator PDATA instruction 
; PPAGEENABLE EQU 0 ; Set it to 1 when using PDATA type variables PPAGE EQU 0 ; Define the page number; ;------------------------------------------------ NAME ? C_STARTUP ; Module name? C_STARTUP ? C_51STARTUP SEGMENT CODE ; Code segment? STACK SEGMENT IDATA ; Stack segment RSEG ? STACK ; Stack DS 1 EXTRN COE(? C_START) ; Program start address PUBLIC ? C_STARTUP CSEG AT 0x8000 ;Define the starting address of the user program, which may be useful when using the MON51 emulator? C_STARTUP: LFMP STARTUP1 RSEG ? C_51STARTUP STARTUP1: ; ;Initialize the serial port MOV SCOM, #40H MOV TMOD, #20H MOV TH1, #0FDH SETB TR1 CLR T1 ;Clear the IDATA memory when the microcontroller is powered on. If you do not need to clear IDATA on power-on, you can cancel the ;statement between IF and IFEDN, or modify the length of IDTALEN. In order to make the CPU have power-off protection function, you need to determine the length of IDTALEN IF IDATALEN <> 0 MOV R0, # IDATALEN-1 CLR A IDATALOOP: MOV @R0,A DJNZ R0,IDATALOOP ENDIF ; ;Clear the XDATA memory when the microcontroller is powered on. If you do not need to clear XDATA on power-on, you can cancel the ;statement between IF and IFEDN, or modify the length of XDTALEN IF XDATALEN <> 0 MOV DPTR, #XDATASTART MOV R7,#LOW (XDATALEN) IF (LOW(XDATALEN)) <> 0 MOV R6, #(HIGH(XDATALEN))+1 ELSE MOV R6, #HIGH (XDATALEN) ENDIF CLR A XDATALOOP: MOVX @DPTR, A 
INC DPTR DJNZ R7, XDATALOOP DJNZ R6, XDATALOOP END IF ; ;Send the high address of the PDATA memory page IF PPAGEENABLE <> 0 MOV P2, #PPAGE ENDIF ; ;The PDATA memory is cleared when the MCU is powered on. If you do not need to clear XDATA on power-on, you can cancel the statements between IF and IFEDN or modify the length of PDATALEN IF PDATALEN <> 0 MOV R0, #PDATASTART MOV R7, #LOW (PDATALEN) CLR A PDATALOOP: MOV @R0, A INC R0 DJNZ R7,PDATALOOP ENDIF ; ;Set the stack space for reentry function when using SMALL memory mode IF IBPSTACK <> 0 EXTRN DATA(? C_IBP) MOV ? C_IBP, #LOW IBPSTACKTOP ENDIF ; ;Set the stack space for reentry function when using LARGE memory mode IF XBPSTACK <> 0 EXTRN DATA (? C_XBP) MOV ? C_XBP, #HIGH XBPSTACKTOP MOV ? C_XBP +1, #LOW XBPSTACKTOP ENDIF ; ;Set the stack space for reentry function when using COMPACT memory mode IF PBPSTACK <> 0 EXTRN DATA(? C_PBP) MOV ? C_PBP, #LOW PBPSTACKTOP END IF ; ;Set the starting address of the stack MOV SP, #? STACK-1 ;For example, MOV SP, #4FH ; ;If the program exceeds 64K, use program grouping technology to start the following program 
; EXTRN CODE(? B_SWITCH0) ;CALL ? B_SWITCH0 ;The program starts executing from the first group of bank 0 blocks;Jump to the user program MAIN function LJMP ? C_START END The role of startup.a51 is the same as that of assembly. The variables and arrays defined in C are initialized in startup.a51. If you define a global variable with a value, such as unsigned char data xxx="100";, then there will be a related assignment in startup.a51. If there is no =100, startup.a51 will clear it to 0. (startup.a51 == variable initialization). After these initializations are completed, the SP pointer will also be set. There will be no assignment or clearing action for non-variable areas, such as the stack area. Some people like to modify startup.a51 to satisfy their own self-righteous hobbies. This is unnecessary and may be wrong. For example, if you want to save some variables during power-off protection, it is a stupid method to modify startup.a51 to achieve this. In fact, you only need to use the characteristics of the non-variable area and define a pointer variable to point to the lower part of the stack: 0xff. Why do you still need to modify it? It can be said that you don't need to modify startup.a51 at any time if you understand its characteristics.