When you enter the procedure, in most cases you will want to set up a “stack frame.” This allows you to access parameters passed on the stack and to allocate local data on the stack. You do not need to set up the stack frame if your procedure accepts no arguments and does not use the stack.
To set up the stack frame in a 16-bit program, issue the instructions:
push bp
mov bp,sp
To set up the stack frame in a 32-bit program, issue the instructions:
push ebp
mov ebp,esp
This sequence establishes BP as the frame pointer. You cannot use SP for this purpose because it is not an index or base register. Also, the value of SP may change as more data are pushed onto the stack. However, the value of the base register BP remains constant for the life of the procedure unless your program changes it, so each parameter can be addressed as an offset from BP.
The instruction sequence above preserves the value of BP, since it will be needed in the calling procedure as soon as your assembly-language procedure returns. It then transfers the value in SP to BP to establish a stack frame on entry to the procedure.