Created attachment 12573 [details]
a.out

$ gdb --version
GNU gdb (GDB) 10.0.50.20200517-git
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Consider test-case:
...
$ cat small.c
char *pa;

void access (volatile char *ptr) {
  *ptr = 'x';
}

int main () {
  char a;
  pa = &a;
  access (pa);
  return 0;
}


...

When stepping through the program by using "stepi"
...
$ gcc -O1 -g small.c; gdb -q a.out
Reading symbols from a.out...
(gdb) b main
Breakpoint 1 at 0x401106: file small.c, line 9.
(gdb) r
Starting program: /home/yibiao/Debugger/a.out 

Breakpoint 1, main () at small.c:9
9	  pa = &a;
(gdb) stepi
0x000000000040110b	9	  pa = &a;
(gdb) 
access (ptr=0x7fffffffdf07 "") at small.c:4
4	  *ptr = 'x';
(gdb) 
main () at small.c:11
11	  return 0;
(gdb)
...

/**********************************************
As showed, when stepping with stepi, Line 10 is skipping over. While using step, Line 10 is hit by gdb as follow:
***********************************************/

When stepping through the program by using step:
...
$ gcc -O1 -g small.c
$ gdb -q a.out
Reading symbols from a.out...
(gdb) b main
Breakpoint 1 at 0x401106: file small.c, line 9.
(gdb) r
Starting program: /home/yibiao/Debugger/a.out 

Breakpoint 1, main () at small.c:9
9	  pa = &a;
(gdb) step
10	  access (pa);
(gdb) 
access (ptr=0x7fffffffdf07 "") at small.c:4
4	  *ptr = 'x';
(gdb) 
main () at small.c:11
11	  return 0;
(gdb)