C51: Stack Pointer Initialization in Startup Code

Information in this article applies to:

• C51 Version 5.50 and later

SYMPTOMS

The stack pointer is initialized at the end of the C startup code in STARTUP.A51. Why is this necessary?

CAUSE

The last few lines of the startup code appear as follows:

IF PBPSTACK <> 0
EXTRN DATA (?C_PBP)
                MOV     ?C_PBP,#LOW PBPSTACKTOP
ENDIF

                MOV     SP,#?STACK-1
                LJMP    ?C_START

After reset, the stack pointer is initialized to 7h. The stack will start growing up from address 8h.

The Keil C compiler uses internal DATA memory for your variables and also allows you to use register banks 1, 2, and 3. If the stack pointer were not adjusted, calls to functions and interrupts would overwrite your variables.

Therefore, the last thing the startup code does is to set the stack pointer to the end of all your internal DATA variables.

SEE ALSO

• C51: Modifying the Startup Routines
• C51: Setting Program Start Address
• C51: Avoiding Startup Initialization of Static Variables
• C51: Checking for Stack Underflow at Runtime
• C51: Stack and Reentrant Stack Symbolic Names
• C51: Calculating Stack Size
• C51: Locating the Stack after IDATA Variables
• C51: Stack Usage

Last Reviewed: Thursday, February 25, 2021