C51: Reentrant Functions and Variables

Information in this article applies to:

• C51 Version 5.50 and later

QUESTION

In the SMALL memory model (all RAM internal, all code in 8K), do reentrant functions use the push and pop instructions to put variables on the stack or do they overlay variables?

ANSWER

The reentrant stack is simulated using R0 and R1. Variables are "pushed" and "popped" using the MOV @R0/R1 instructions. The hardware stack is not used for these variables.

When you use the reentrant stack in internal memory, the hardware stack (accessed with SP) starts at the bottom of memory and grows up. The simulated reentrant stack starts at the top of memory (255) and grows down.

You may use a debugger watch and test for the collision of these two stacks.

SEE ALSO

• C51: Function Prototypes for Reentrant Functions
• C51: Functions That Are Reentrant
• C51: Using Non-reentrant Function in Main and Interrupts
• C51: When are Functions Re-Entrant?

Last Reviewed: Thursday, February 25, 2021