How compilers handle procedure calls

This note describes how the high-level abstraction of the procedure call is implemented in Assembly, as handled by the compiler.

When a procedure is called, we jump to the section of the code which contains the procedure body.

We use a stack to hold activation records which hold:

• input arguments
• the return address of the caller
• the results of the procedure computation
• saved values of registers

Each time a procedure is called, a new activation record is pushed onto the stack.

The stack exists in memory. We reserve some registers to make procedure calls and using the stack easier:

• SP - the stack pointer points to the top of the stack
• LP - linkage pointer which is the location of the caller in the program
• BP - the base pointer points to the location of the current activation record. This is a convenience, we can do everything relative to the SP, but as the stack grows and shrinks, this can be hard to think about.

The caller:

• pushes arguments onto the stack
• saves the current program counter to the LP
• branches to the callee
• after the callee finishes, deallocate the activation record

The callee:

• does the computation, accessing the arguments in the activation record as needed, and allocating local storage on the stack as needed
• puts the result in register 0
• branches to the return address